Another great video James! I got rid of my Orion TOAG because I could get parfocus with an ASI120MM - a bit of a rookie mistake not doing enough homework ahead of time. The first time I set up the Celestron OAG and ASI174MM mini with my EDGE HD8 + FR , stars were there! Your videos discussing alignment and rotation in Stellarium are invaluable! Thank you.
Thanks, David. You were luckier than I thought I was. Nearly drop-kicked the OAG over the fence. Then came back in the house only to see I had guide stars all along. Stellarium is an invaluable planning tool. Thanks for watching!
Hi James, another interesting video, thanks. I use an OAG to guide my 9.25 & a couple of great tips I picked up on the PHD2 group was to Bin2 the guide cam and for the guide cam to be ever so slightly out of focus. This really aids finding guide stars to use. I get really good results using this method. My guide camera is a Lodestar x2 and I don’t think I’ve ever altered any gain settings, just left it at factory default and I use 2.5 sec exposure in PHD2
Thanks for watching! Glad to hear another C9.25er having success with off-axis guiding. I think I can get away without binning--the ASI174mini is pretty sensitive. I don't mind dialing up the gain if it lets me get away with a smaller arc-sec/pixel. I've heard the comment about out-of-focus guide stars being OK. I found focusing to be quite sensitive, so out-of-focus is almost guaranteed.
Thank you very much for posting this great video. I am saving it into my library for reference to understand the OAG system. Take care. I Just subscribed in your channel:)
Thanks, Luis! There are some more recent videos that address using the OAG, such as my most recent one. If you haven’t already, check it out too. Thanks for watching!
Hi James, ive had my celestron oag for a couple of years now, couldn't get the guide cam to see stars so its been thrown back in the box in disgust..now i think i understand the system abit better and eager to try it again..watched your video and picked up a few tips so thankyou..one thing that springs to mind that im not too sure that you're understanding is the light coming into your imaging camera is coming to focus at the sensor, its a light cone, not a straight line as you show.. not too sure in why you wasnt seeing the guidestar thou.. on the whole thou a very precise, informative video..much appreciated and hope theres some more
Hey, Derek thanks for watching! Good luck re-trying your OAG. I had mine for 2 years before I ever tested it out--nervous about the hassle of finding a guide star. After getting a heavier camera system (filter wheel + ASI1600), I decided to jump in and ditch the guide scope. That video is my first attempt and the lessons learned--not sure why I wasn't seeing the guide star either. Lately, I've been imaging with my 250mm and 700mm scopes, but galaxy season is here, so I plan to go to the C9.25 this weekend--I'm preparing for frustration...and more lessons learned.As to your point about light coming to focus at a single point on the sensor: I agree with your statement; however, that's not what I'm showing there. I'm showing all of the light that the scope can see (the light cone) and am just looking at the possibility that the OAG will clip the boundary of the cone and at a single ray that gets to the imaging sensor and the guide camera sensor. When we talk about the "field of view" for a given sensor/focal length, that's the angle shown in the CAD file. Hope that clears up the confusion.
Great info! I just got a new galaxy rig. 8” edge HD, cem70 on tri-pier, and a Celestron OAG with asi174mm mini for guiding. Knowing Stellarium can be used to frame up not only the main imaging cam, but also the OAG is going to be huge in planning for those targets in empty areas of the sky such as m51 & m101. Thanks so much for sharing! Hope you’ve gotten some great results since this vid!
@@Aero19612 hahaha that made my night! So true!! I ordered the OTA over two months ago and the only reply from Celestron is that they’re waiting for them to arrive from China and I’ve had so many clear nights during that time!! I just know the clouds will roll in the exact moment I have my rig complete LOL
Hi James. I finally had clear enough skies to try my new CEM70G mount out. So far it has been awesome. Seeing conditions were poor an an 80% moon did not help matters, but all I was hoping to accomplish was: polar alignment, star alignment, guide cam focus (ASI174), check PEC with PEMPro, get PHD2 working, and (if the gods were in my favor) take a few subs of M81. Polar alignment was even easier than with the Polemaster. Once I replaced the batteries in both my reddot finders, 3 star alignment was fine. The huge moon came in handy to focusing the guide camara. The PEMPro stuff did not work properly (wasted an hour at -3°C). I got a great calibration with PHD2 (with out of the box setting) at DEC 0 and west of southern meridian (guiding ~0.9arcsec). Took about 45 × 120 sec subs of M81 and still had similar guiding (I used your procedure to determine the GC alignment with stellarium in the warmth of my kitchen). No discarded subs for serious elongation or trailing stars (fwhm was 4+ due to poor very poor seeing). Then I decide to try M51. I spent 3 hours (at -6°C) trying to find a guide star without luck. M51 at 1645 fl is tricky for guide stars. I have a few questions that I would really appreciate your help with. 1) If you rotate the guide camera, do you have to perform a new PHD2 calibration? 2) Can I do a PHD2 calibration at say DEC 45 or 50°? 3) Where do you set the gain for the ASI174 for PHD2? Thanks in advance for any advice you are my Sensei:-) Bob
I looked up the CEM70G. VERY NICE, Bob! Is there built in software that uses the iGuider camera to perform polar alignment? I didn't think anything could be easier than the PoleMaster. I've been postponing my imaging of late because of the Moon. I think you found the one good thing it is useful for: guide camera focusing. Answers to your questions: 1. Yes. Whether you rotate the guide camera about its axis or the OAG about the telescope axis, you will have to recalibrate. 2. Yes, but the PHD folks suggest calibrating near the celestial equator. I would just avoid calibrating near the horizon. 3. I think I use 50% gain (I think that's what the slider means). The setting is on the camera tab in the "Brain" Advanced Settings/Camera Parameters. Keep me posted on your experience with your mount. The info I saw suggests a +/- 3 arc sec accuracy (periodic error < 3.5 arc-sec). My CGEM has 20+ arc-sec of periodic error. I'm also curious about your DEC backlash. So many questions. Once you get some good seeing and can image all night, I would like for you to send me a PHD2 guide log.
@@Aero19612 Hi James. Thanks for the speedy reply. From what I can tell the CEM70 is rock solid. You can't wiggle the RA or DEC when engaged (unlike my AVX, which is a fun mount but hardly up to the task of my 34 lb setup). The iPolar is quicker but I don't think it is as accurate (I would typically get within 4 or 5 arc min with the Polemaster). The built in guide scope (iguider) has a very short focal length (120mm). I believe you can use PHD2, I will try it at some point with a smaller scope (maybe even with my C9.25 for kicks). As far as the PEC being +/- 3 arcsec, not a chance. I ran PEMPro over 10 cycles and got about +/- 12 arcsec. The only reason I did this was for instances when I wanted to try unguided subs. I couldn't get the PEC curve back into the mount successfully. Judging by my "limited" success, I am not sure I will be doing many unguided subs. I didn't do the DEC backlash step but my feeling is that it is pretty low. My AVX had so much backlash it was unworkable. One pleasant thing I found was that the CEM70 is very easy to balance (no stiction). I had to get a CW extension (no wts available yet) to get it balanced (the CW is not even on the CW extension, it is just there as an expensive wt. I will send you a PHD2 log once I have a chance to fool around with it. The only other attempt I made to use PHD2 was using my ASI533 as the guide camara (before putting on the Celestron OAG) on my AVX. It was not a great success. BTW the reason I want to calibrate at such a large DEC is because I only have a good southern view from my front yard and I am not crazy about leaving all my stuff out by the street (even though my neighborhood is very safe). Thanks again for all your help. I am pretty handy but your videos have saved me a huge amount of time on trial and error. Bob
Hi James. Another very detailed and informative video. I have the 8 inch Edge scope with .7 reducer. I’ve just bought the ZWO OAG for my Explore Scientific 102mm setup but hoping to try it with the Edge. Could be some bad language I suspect! Still it’s worth a try. I’ll be using the Lodestar x2 so very sensitive guide cam. One point I’d mention is that there is a good video by the guy who developed sharpcap where he talks about guide cam exposure times and if I remember correctly he states that you should use the longest time you can given your mount tolerances in order to not be chasing seeing/slight wind gusts etc. I usually go with 2 seconds but I may try to pec train the mount and go for longer and see what happens. This hobby gets more complicated everyday! Clear skies.
Thanks for watching, John! Good luck with the ZWO OAG. The prism is much smaller than in the Celestron OAG so guide stars will be difficult to find, but the focal reducer should help. Get your favorite cuss words ready. I used most of them last night. Yes, “chasing seeing” is a valid, but not the only, concern. In mounts with “high” mechanical gear noise, like my CGEM, the shorter the guide cam exposure, the better. I did a video way back, “Limitations of the Autoguiding Algorithm”, that looked at the role of the mount command update rate (aka exposure time). There are nights when the bad seeing is worse than the gear noise and then I use a 2 to 3-sec exposure time. Longer exposure times are also good for improving guide star SNR-which you may need with your OAG/SCT combo. PEC training may help some. The periodic error is most associated with the slow worm gear period (8 min for my CGEM) can be corrected by autoguiding-it’s the higher gear harmonics that are difficult for the guiding algorithm to control. I use the PEC correcting guiding algorithm in PHD2, which is pretty good. Good luck!
Another great video. Thanks for taking the time to make it ! I just learned I made the wrong purchase with the ZWO AOG for my 11"... I need the Celestron one to be able to rotate the guide camera, otherwise I will end up tossing the thing away.
Thanks for watching, Carlos! The off-axis guide is easy with a wide-angle scope--tons of stars to choose from. The long focal length SCTs are a different matter. The Celestron OAG is good because (a) you can independently rotate the guide camera and the imaging camera 360 deg and (b) it has a large prism--just need to find a guide camera with a large enough sensor to use that prism. Good luck!
You might have addressed it but for my OAG the main thing is focus. First I took flats till the edge shadow went away by adjusting the depth of the pick mirror. Then under stars I found a nice open cluster and put it on the same side of the chip. Then I took lots of short shots while focusing. It is very sensitive. I also tried to maintain a orthogonal rotation with the guide cam. Now I can use it very easily although the exposures are about 5 seconds to get a solid ADU count above the noise. It requires good polar alignment and smooth basic tracking. But my performance with OAG is much better once I got it sorted. Best of luck!
I don't think I discussed the focus issue. I did find it very sensitive. I first focused the imaging camera and the guide camera on Deneb before moving over to M101. When I finally did see that I had stars in view, they were C-shaped and I had to focus again. As for the shadow--I hope that can be dealt with via flats. I don't want to raise the prism any farther because I'm losing guide sensor area as it is and I need all I can get with the 2350mm focal length scope. Thanks for the comments!
It looks like the shadow is on the short side of the chip. I think you could leave the prism at the same depth but rotate it or the camera so that it is on the long side. I think the way the sensor is positioned in the camera you wouldn’t see as much shadow. Just a random thought. Looks like you’ve put a bunch of thought into it and I’m sure you will get it to work for you. Clear skies
Regarding the C shape. If I’m not mistaken this is the normal donut shaped diffraction pattern but since the prism is only intercepting one side of the light cone you only get half the donut. If it’s bright enough I find it actually helpful to focus. If the C goes to a dot, you’re spot on, excuse the pun.
Correct. I'm getting the shadow because the prism is in the worst possible location relative to the imaging sensor. In most cases, I can simply rotate the imaging sensor as you say. There may be some targets that require a specific orientation to fit on the imaging sensor. Just something else to think about. I'd prefer to do less thinking and more imaging for a change.
Thanks for posting James. I just purchased an Edge 8HD and am looking at various autoguiding options. I did not know Stellarium could show the Autoguider view and camera view at same time. I don't have the ASI174 but I do have the ASI178 which has a larger sensor than the ASI120 and 290. I wrote to ZWO to see if it would work as an autoguider but they haven't gotten back to me yet. I kind of like the the OAG idea but am worried about not finding stars. Your preplanning solution looks good. Cheers Kurt
Hey Kurt. My biggest worry was finding a guide star. Pre-planning is the key for long focal length scopes like your Edge 8HD (congratulations, BTW!). Not sure what I would do without Stellarium. Great tool (along with a digital angle gauge). I preset my OAG angle before last Galaxy season based on my first two targets, then picked following targets that left me with guide stars for that orientation for the whole "season". I will re-orient for the upcoming season. Loving the OAGs (I use one for all of my scopes, but I only pre-plan for the C9.25. No planning required for
@@Aero19612 Thanks James. I was thinking on just trying it out as well. It can't hurt and it is the mono version of the ASI178. Yes I was wondering if I can set the bin to 2x2 in PHD2. I have heard from a number of people that the Celestron OAG version is bigger and better - that will be my next purchase. Everything is on backorder now so I may actually end up missing galaxy season but who knows.
I have a C8 SCT and in the process of obtaining the equipment to start imaging. I've decided to go with the Celestron OAG. Given that in your setup the OAG prism top is not getting any light due to the diameter of your rear light tube (44.5mm), I measured my C8 rear light tube and its about 38mm. I'm planning to use my Nikon D5100 body at first. It's sensor size is 23.6mm x 15.6mm compared your ASI1600 17.6mm x 13.3mm. Using your drawing I'm thinking with my D5100 sensor being so large the prism will have to be raised even more than in your situation. With only 38mm light path and the prism having to be raised more (maybe 3 to 4 mm more) I'm wondering how my prism will be left for the guide camera in the OAG? I was thinking of getting the ASI 174mini like you but it doesn't all the light will get to its sensor. Would a different guide camera be better such as the ASI220 or AS120mm. I eventually will get a dedicated camera so would the ASI174mm mini be more future proof? Maybe the OAG route is not the way to go? I'm planning to get the ASIAir Plus to do the guiding. I don't own a PC (just a Mac). Thanks for any advice.
Thanks for watching! Please consider any "advice" I provide as just one of many potential points of view. I do think the OAG is the right way to go for an SCT and the Celestron OAG is a good product. I would definitely not recommend using an ASI120 with the OAG/C8 - sensor is way too small. The ASI220 has a nice sized sensor (I just bought one for my OAG on my refractor). The ASI220 gives you about 2x the sensor area than the ASI120 and the price scales accordingly. The ASI174 gives you 2.5x the area of the ASI220 but is only 33% more expensive - so a decent value. As to whether the whole sensor area of the ASI174 can be used, my experience may simply be a result of where I have the prism. I have reduced the prism height but have not yet investigated the effect on the usable illuminated area. You still get the near-full width, which is good. The prism can cast a shadow on the imaging sensor, but that shadow can be dealt with via FLATs. Hope that provided some perspective! Good Luck!
I am just finding out the problems of OAG after switching to a ZWO OAG/ASI120MM-Mini setup for my Edge HD11/.7 reducer. My problem has been the guide star going in and out of focus/round shape and the ASIAir Pro losing the guide star due to loss or change of star mass. I am on the verge of trying the Celestron OAG/ASI174MM-Mini setup. In any case, thank you for the great video.
Hey Bob. That's weird about the in/out focus. Silly question: Have tightened all of the set screws that hold the prism? Do you feel any looseness if you try to jiggle the guide camera? I'm surprised you got the ZWO OAG to even see a guide star with that small prism and large focal length SCT. You'll have no problem at all with the Celestron OAG and its large prism combined with the ASI174's large sensor. Good luck!
@@Aero19612 Yes I have triple checked that everything is secure. I just spoke with Steve at Starizona and he had one suggestion that I had not thought of. He said that the ASI6200MM requires a lot of power at times and he has seen some unusual problems when powered from the ASIAir Pro. I currently have the ASI120MM-Mini plugged into the 6200 USB. Steve suggested trying a separate power source for the 6200 and plugging the 120 into the AAP USB. I am going to try this.
Interesting. I don't understand how a power issue translates into the appearance of an out-of-focus star. But it's a cheap fix if it works. Let me know how it goes. I'll file it away as a "lesson learned".
HI James, great information as usual, many thanks. It's been 2 years now since this video was published, in a brief statement have you found OAGing more or less beneficial in the long run please?
Beneficial. Period. 1. At short focal lengths (< 800 mm, say), there is never a problem finding one or multiple guide stars - cheaper option than a guide scope 2. At long focal lengths (> 1500 mm, say), you can (need to) use Stellarium to plan out the orientation of the OAG so that you WILL have a guide star (I have several videos that address this). Make sure to get a guide camera with a large enough sensor to take advantage of the larger prism. 3. You get the guiding benefit of a smaller guide image scale 4. Your setup is more "compact" and, without the guide scope, your payload has MUCH LESS inertia for the RA motor to overcome Totally sold on OAGs. Not going back. Thanks for watching!
James love your videos, they really help to clear up things that have confused me for years. I'm a Windows user, have you heard of an alternative to pixinsight that runs on window?
Thanks Pat. A lot of people use Photoshop along with an add-in tool set for astrophotography. There is GIMP, which is free. And maybe StarTools. I don't anything about other software as I have always used PixInsight.
@@Aero19612 No, I see it is used by virtually every video on astrophotography but some how I got it in my head that it was only for Apple computers. I don't mind buying it because it really seems like a final solution after trying so many others. My trail period license is pending. I really like the idea that I can use it on all of my computers and OS's with one license.
In balancing this type of scope, rather than precariously moving it back and forth on the dovetail, I add a saddle at the front of the scope and attach weights, which typically is a C clamp.
could i ask how you achieve the proper 105mm back focus? I can't see what spacers you have used. the Celeston OAG seem deeper than the manual would suggest.
I'm actually going to address that issue in a video. Celestron is anything but clear (I must have done this incorrectly 6 times). If you're using the focal reducer (the 105 mm back focus is for the focal reducer + 1/3 filter thickness), then you need to measure from the back-most edge/face of the focal reducer (in other words, not including the threads). The OAG includes the SCT adapter (about 32.3 mm - the Celestron manual has this as 25.3 mm) + the OAG body (30.3 mm, Celestron manual has this as 29 mm) + the external thread adapter (about 12 mm) = about 74.6 mm. You're left with about 30.4 mm (or 31 mm if you account for a 2-mm thick filter). For my system, I have a filter wheel (20 mm) and a 6.5-mm distance from the camera face to the sensor (ASI1600), which leaves me with a 5 mm spacer between the OAG and the filter wheel. So, it is close but possible. Good luck!
@@Aero19612 Thanks for this James. These are roughly my findings as well. I find the additional 0.3 on the SCT adapter a strange but very specific dimension? unless this allows for a filter? The manual clearly states it is compatible with the C* XLT Class scopes as well so i am hopeful. I have the f6.3, Celestron OAG (29, 25.3, 12.5 total 66.8mm) , ZWO Filter Draw(21mm), Zwo spacer ring (11mm) and 6.5mm for the camera back focus which all adds up to 105.3 according to the OAG manuals provided BF dims. i can achieve focus for the guide cam and main cam but compared to the images i was getting when using the ZWO OAG my images look softer and stars more bloated. with the ZWO OAG i could get 105mm bang on but the larger prism, the promise of more & better guide stars plus the fact it was made for the scope made the celestron effort hard to resist.
Technically, you would not have to take new flats if you rotate the guide camera and NOT rotate the imaging camera. However, it is almost impossible to rotate the guide camera without slightly rotating the imaging camera no matter how hard you try so you're better off taking new flats if you adjust the OAG at all.
It’s a Celestron CGEM and has a stated payload capacity of 40 lb. I have not noticed any effect of payload weight on performance though. The behaves the same whether it’s a heavy C9.25 or a lightweight Redcat 51. I don’t mean to imply that it’s awesome (it’s not. Just barely adequate for high focal length imaging). Thanks for watching, Michael!
Thanks for the great explanations ! I have a question regarding the scope (or this type of scope) I am hesitating between the Celestron C6 and a Ritchey-Chrétien 6 inch. The RC is cheaper, but the C6 is lighter. (I am actullay looking at the C8 and a RC8, with same dilemma ) So, my question is, how does the 9,25 deal with coma ? with and without the reducer ? Thanks a lot !
Hey, Julien. I have not had too many complaints with coma on my C9.25 with or without the focal reducer. I use it without the focal reducer most of the time. Of course, if you go the EdgeHD route, that will handle the coma correction. As for the weight issue, it certainly depends on your mount. I have not seen guiding performance be affected much on my CGEM. I thought I would see a huge improvement when I replaced the heavy guidescope with the OAG. Nope. Hope that helps a little! Clear Skies!
@@Aero19612 Are you using the Celestron #94175 flattener/corrector, James? If so, are you satisficed with your results? I have an Antares equivalent that just doesn't cut it for me for AP. Maybe I am doing something wrong. Allegedly they are the same thing with different labeling. Trying not to splurge on the Starizona offering (if/when it becomes available again).
Hey! Thank you so much for this video ....I have exactly the same setup scope wise except on an advanced vx mount. Have been struggling w my orion pro mono ssag which is not working on windows 10 now so looking at changing that to a zwo. Curious to know how good the tracking and guiding finally worked on this setup. Do let me know
Hey Raghav. Tracking is ok as long as I'm guiding. This mount has fairly large periodic error that PHD2 handles fine. There is also high frequency RA gear harmonics that PHD2 has more difficulty with. The DEC backlash is very high. I'm experimenting with options for dealing with it. I'm liking the PHD2 DEC backlash compensation. On good seeing nights, I get guiding performance between 0.7 arc-sec RMS and 0.9 arc-sec RMS which is tolerable for my SCT. With bad seeing, I'm around 1.3 arc-sec RMS. The ZWOs are good cameras. I'm using the ASI290MM mini for my small OAG and the ASI174MM mini for my SCT OAG.
I would use the Celestron OAG for the 11" edge because (1) The Celestron OAG has a larger prism (bigger guide cam field of view), (2) The Celestron is easier to focus since it has a helical focuser, and (3) The Celestron allows for independent orientation of the imaging and guide cameras. Thanks for watching!
@James Lamb do you have an email ID or message box where I can chat with you? your videos are very impressive. I need some help with OAG, thought will discuss offline please.
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this is for far the most comprehensive and yet user friendly explanation video. thank you for this great contribution to education.
Thanks! I’m glad it was helpful. Clear Skies!
Totally agree. James is a great educator and presenter.
Another great video James! I got rid of my Orion TOAG because I could get parfocus with an ASI120MM - a bit of a rookie mistake not doing enough homework ahead of time. The first time I set up the Celestron OAG and ASI174MM mini with my EDGE HD8 + FR , stars were there! Your videos discussing alignment and rotation in Stellarium are invaluable! Thank you.
Thanks, David. You were luckier than I thought I was. Nearly drop-kicked the OAG over the fence. Then came back in the house only to see I had guide stars all along. Stellarium is an invaluable planning tool. Thanks for watching!
Fantastic post! Very helpful and well done!
Glad you found it useful, Gene! Be sure to check out some of the more recent videos on this topic. They may be useful as well. Clear skies!
Thank you for doing these. I am starting to understand what you are teaching even if I cannot put it to practice yet.
Thanks for suffering through these "lectures", Ron! Hope they help in the long run. Clear skies!
Hi James, another interesting video, thanks.
I use an OAG to guide my 9.25 & a couple of great tips I picked up on the PHD2 group was to Bin2 the guide cam and for the guide cam to be ever so slightly out of focus. This really aids finding guide stars to use. I get really good results using this method.
My guide camera is a Lodestar x2 and I don’t think I’ve ever altered any gain settings, just left it at factory default and I use 2.5 sec exposure in PHD2
Thanks for watching! Glad to hear another C9.25er having success with off-axis guiding. I think I can get away without binning--the ASI174mini is pretty sensitive. I don't mind dialing up the gain if it lets me get away with a smaller arc-sec/pixel. I've heard the comment about out-of-focus guide stars being OK. I found focusing to be quite sensitive, so out-of-focus is almost guaranteed.
Very good video thank you!
A good set of calipers are the OAG owners best friend 😁👍
A "must" I'd say. Thanks for watching, Dennis.
Thank you very much for posting this great video. I am saving it into my library for reference to understand the OAG system. Take care. I Just subscribed in your channel:)
Thanks, Luis! There are some more recent videos that address using the OAG, such as my most recent one. If you haven’t already, check it out too. Thanks for watching!
Hi James, ive had my celestron oag for a couple of years now, couldn't get the guide cam to see stars so its been thrown back in the box in disgust..now i think i understand the system abit better and eager to try it again..watched your video and picked up a few tips so thankyou..one thing that springs to mind that im not too sure that you're understanding is the light coming into your imaging camera is coming to focus at the sensor, its a light cone, not a straight line as you show.. not too sure in why you wasnt seeing the guidestar thou.. on the whole thou a very precise, informative video..much appreciated and hope theres some more
Hey, Derek thanks for watching! Good luck re-trying your OAG. I had mine for 2 years before I ever tested it out--nervous about the hassle of finding a guide star. After getting a heavier camera system (filter wheel + ASI1600), I decided to jump in and ditch the guide scope. That video is my first attempt and the lessons learned--not sure why I wasn't seeing the guide star either. Lately, I've been imaging with my 250mm and 700mm scopes, but galaxy season is here, so I plan to go to the C9.25 this weekend--I'm preparing for frustration...and more lessons learned.As to your point about light coming to focus at a single point on the sensor: I agree with your statement; however, that's not what I'm showing there. I'm showing all of the light that the scope can see (the light cone) and am just looking at the possibility that the OAG will clip the boundary of the cone and at a single ray that gets to the imaging sensor and the guide camera sensor. When we talk about the "field of view" for a given sensor/focal length, that's the angle shown in the CAD file. Hope that clears up the confusion.
Great info! I just got a new galaxy rig. 8” edge HD, cem70 on tri-pier, and a Celestron OAG with asi174mm mini for guiding. Knowing Stellarium can be used to frame up not only the main imaging cam, but also the OAG is going to be huge in planning for those targets in empty areas of the sky such as m51 & m101. Thanks so much for sharing! Hope you’ve gotten some great results since this vid!
Thanks for watching, Andrew. Stellarium is an AWESOME tool! Wouldn't want to try using my OAG without it. Good luck!
@@Aero19612 Thank you! All I need now is for the imaging scope to show up!
@@andrewlindenfeld6222 Well, keep in mind that scope are shipped with 3 weeks of clouds included at no charge.
@@Aero19612 hahaha that made my night! So true!! I ordered the OTA over two months ago and the only reply from Celestron is that they’re waiting for them to arrive from China and I’ve had so many clear nights during that time!! I just know the clouds will roll in the exact moment I have my rig complete LOL
Hi James. I finally had clear enough skies to try my new CEM70G mount out. So far it has been awesome. Seeing conditions were poor an an 80% moon did not help matters, but all I was hoping to accomplish was: polar alignment, star alignment, guide cam focus (ASI174), check PEC with PEMPro, get PHD2 working, and (if the gods were in my favor) take a few subs of M81. Polar alignment was even easier than with the Polemaster. Once I replaced the batteries in both my reddot finders, 3 star alignment was fine. The huge moon came in handy to focusing the guide camara. The PEMPro stuff did not work properly (wasted an hour at -3°C). I got a great calibration with PHD2 (with out of the box setting) at DEC 0 and west of southern meridian (guiding ~0.9arcsec). Took about 45 × 120 sec subs of M81 and still had similar guiding (I used your procedure to determine the GC alignment with stellarium in the warmth of my kitchen). No discarded subs for serious elongation or trailing stars (fwhm was 4+ due to poor very poor seeing).
Then I decide to try M51. I spent 3 hours (at -6°C) trying to find a guide star without luck. M51 at 1645 fl is tricky for guide stars. I have a few questions that I would really appreciate your help with.
1) If you rotate the guide camera, do you have to perform a new PHD2 calibration?
2) Can I do a PHD2 calibration at say DEC 45 or 50°?
3) Where do you set the gain for the ASI174 for PHD2?
Thanks in advance for any advice you are my Sensei:-)
Bob
I looked up the CEM70G. VERY NICE, Bob! Is there built in software that uses the iGuider camera to perform polar alignment? I didn't think anything could be easier than the PoleMaster. I've been postponing my imaging of late because of the Moon. I think you found the one good thing it is useful for: guide camera focusing.
Answers to your questions:
1. Yes. Whether you rotate the guide camera about its axis or the OAG about the telescope axis, you will have to recalibrate.
2. Yes, but the PHD folks suggest calibrating near the celestial equator. I would just avoid calibrating near the horizon.
3. I think I use 50% gain (I think that's what the slider means). The setting is on the camera tab in the "Brain" Advanced Settings/Camera Parameters.
Keep me posted on your experience with your mount. The info I saw suggests a +/- 3 arc sec accuracy (periodic error < 3.5 arc-sec). My CGEM has 20+ arc-sec of periodic error. I'm also curious about your DEC backlash. So many questions. Once you get some good seeing and can image all night, I would like for you to send me a PHD2 guide log.
@@Aero19612 Hi James. Thanks for the speedy reply. From what I can tell the CEM70 is rock solid. You can't wiggle the RA or DEC when engaged (unlike my AVX, which is a fun mount but hardly up to the task of my 34 lb setup). The iPolar is quicker but I don't think it is as accurate (I would typically get within 4 or 5 arc min with the Polemaster).
The built in guide scope (iguider) has a very short focal length (120mm). I believe you can use PHD2, I will try it at some point with a smaller scope (maybe even with my C9.25 for kicks). As far as the PEC being +/- 3 arcsec, not a chance. I ran PEMPro over 10 cycles and got about +/- 12 arcsec. The only reason I did this was for instances when I wanted to try unguided subs. I couldn't get the PEC curve back into the mount successfully. Judging by my "limited" success, I am not sure I will be doing many unguided subs. I didn't do the DEC backlash step but my feeling is that it is pretty low. My AVX had so much backlash it was unworkable. One pleasant thing I found was that the CEM70 is very easy to balance (no stiction). I had to get a CW extension (no wts available yet) to get it balanced (the CW is not even on the CW extension, it is just there as an expensive wt. I will send you a PHD2 log once I have a chance to fool around with it. The only other attempt I made to use PHD2 was using my ASI533 as the guide camara (before putting on the Celestron OAG) on my AVX. It was not a great success. BTW the reason I want to calibrate at such a large DEC is because I only have a good southern view from my front yard and I am not crazy about leaving all my stuff out by the street (even though my neighborhood is very safe).
Thanks again for all your help. I am pretty handy but your videos have saved me a huge amount of time on trial and error. Bob
Hi James. Another very detailed and informative video. I have the 8 inch Edge scope with .7 reducer. I’ve just bought the ZWO OAG for my Explore Scientific 102mm setup but hoping to try it with the Edge. Could be some bad language I suspect! Still it’s worth a try. I’ll be using the Lodestar x2 so very sensitive guide cam. One point I’d mention is that there is a good video by the guy who developed sharpcap where he talks about guide cam exposure times and if I remember correctly he states that you should use the longest time you can given your mount tolerances in order to not be chasing seeing/slight wind gusts etc. I usually go with 2 seconds but I may try to pec train the mount and go for longer and see what happens. This hobby gets more complicated everyday! Clear skies.
Thanks for watching, John! Good luck with the ZWO OAG. The prism is much smaller than in the Celestron OAG so guide stars will be difficult to find, but the focal reducer should help. Get your favorite cuss words ready. I used most of them last night. Yes, “chasing seeing” is a valid, but not the only, concern. In mounts with “high” mechanical gear noise, like my CGEM, the shorter the guide cam exposure, the better. I did a video way back, “Limitations of the Autoguiding Algorithm”, that looked at the role of the mount command update rate (aka exposure time). There are nights when the bad seeing is worse than the gear noise and then I use a 2 to 3-sec exposure time. Longer exposure times are also good for improving guide star SNR-which you may need with your OAG/SCT combo. PEC training may help some. The periodic error is most associated with the slow worm gear period (8 min for my CGEM) can be corrected by autoguiding-it’s the higher gear harmonics that are difficult for the guiding algorithm to control. I use the PEC correcting guiding algorithm in PHD2, which is pretty good. Good luck!
Another great video. Thanks for taking the time to make it ! I just learned I made the wrong purchase with the ZWO AOG for my 11"... I need the Celestron one to be able to rotate the guide camera, otherwise I will end up tossing the thing away.
Thanks for watching, Carlos! The off-axis guide is easy with a wide-angle scope--tons of stars to choose from. The long focal length SCTs are a different matter. The Celestron OAG is good because (a) you can independently rotate the guide camera and the imaging camera 360 deg and (b) it has a large prism--just need to find a guide camera with a large enough sensor to use that prism. Good luck!
You might have addressed it but for my OAG the main thing is focus. First I took flats till the edge shadow went away by adjusting the depth of the pick mirror. Then under stars I found a nice open cluster and put it on the same side of the chip. Then I took lots of short shots while focusing. It is very sensitive. I also tried to maintain a orthogonal rotation with the guide cam. Now I can use it very easily although the exposures are about 5 seconds to get a solid ADU count above the noise. It requires good polar alignment and smooth basic tracking. But my performance with OAG is much better once I got it sorted.
Best of luck!
I don't think I discussed the focus issue. I did find it very sensitive. I first focused the imaging camera and the guide camera on Deneb before moving over to M101. When I finally did see that I had stars in view, they were C-shaped and I had to focus again. As for the shadow--I hope that can be dealt with via flats. I don't want to raise the prism any farther because I'm losing guide sensor area as it is and I need all I can get with the 2350mm focal length scope. Thanks for the comments!
It looks like the shadow is on the short side of the chip. I think you could leave the prism at the same depth but rotate it or the camera so that it is on the long side. I think the way the sensor is positioned in the camera you wouldn’t see as much shadow.
Just a random thought.
Looks like you’ve put a bunch of thought into it and I’m sure you will get it to work for you. Clear skies
Regarding the C shape. If I’m not mistaken this is the normal donut shaped diffraction pattern but since the prism is only intercepting one side of the light cone you only get half the donut. If it’s bright enough I find it actually helpful to focus. If the C goes to a dot, you’re spot on, excuse the pun.
Correct. I'm getting the shadow because the prism is in the worst possible location relative to the imaging sensor. In most cases, I can simply rotate the imaging sensor as you say. There may be some targets that require a specific orientation to fit on the imaging sensor. Just something else to think about. I'd prefer to do less thinking and more imaging for a change.
Excellent video and simply explained . Thank you so much.
Thanks, Dennis! Glad it was useful.
Thanks for posting James. I just purchased an Edge 8HD and am looking at various autoguiding options. I did not know Stellarium could show the Autoguider view and camera view at same time. I don't have the ASI174 but I do have the ASI178 which has a larger sensor than the ASI120 and 290. I wrote to ZWO to see if it would work as an autoguider but they haven't gotten back to me yet. I kind of like the the OAG idea but am worried about not finding stars. Your preplanning solution looks good. Cheers Kurt
Hey Kurt. My biggest worry was finding a guide star. Pre-planning is the key for long focal length scopes like your Edge 8HD (congratulations, BTW!). Not sure what I would do without Stellarium. Great tool (along with a digital angle gauge). I preset my OAG angle before last Galaxy season based on my first two targets, then picked following targets that left me with guide stars for that orientation for the whole "season". I will re-orient for the upcoming season. Loving the OAGs (I use one for all of my scopes, but I only pre-plan for the C9.25. No planning required for
@@Aero19612 Thanks James. I was thinking on just trying it out as well. It can't hurt and it is the mono version of the ASI178. Yes I was wondering if I can set the bin to 2x2 in PHD2. I have heard from a number of people that the Celestron OAG version is bigger and better - that will be my next purchase. Everything is on backorder now so I may actually end up missing galaxy season but who knows.
I have a C8 SCT and in the process of obtaining the equipment to start imaging. I've decided to go with the Celestron OAG. Given that in your setup the OAG prism top is not getting any light due to the diameter of your rear light tube (44.5mm), I measured my C8 rear light tube and its about 38mm. I'm planning to use my Nikon D5100 body at first. It's sensor size is 23.6mm x 15.6mm compared your ASI1600 17.6mm x 13.3mm. Using your drawing I'm thinking with my D5100 sensor being so large the prism will have to be raised even more than in your situation. With only 38mm light path and the prism having to be raised more (maybe 3 to 4 mm more) I'm wondering how my prism will be left for the guide camera in the OAG? I was thinking of getting the ASI 174mini like you but it doesn't all the light will get to its sensor. Would a different guide camera be better such as the ASI220 or AS120mm. I eventually will get a dedicated camera so would the ASI174mm mini be more future proof? Maybe the OAG route is not the way to go? I'm planning to get the ASIAir Plus to do the guiding. I don't own a PC (just a Mac). Thanks for any advice.
Thanks for watching! Please consider any "advice" I provide as just one of many potential points of view. I do think the OAG is the right way to go for an SCT and the Celestron OAG is a good product. I would definitely not recommend using an ASI120 with the OAG/C8 - sensor is way too small. The ASI220 has a nice sized sensor (I just bought one for my OAG on my refractor). The ASI220 gives you about 2x the sensor area than the ASI120 and the price scales accordingly. The ASI174 gives you 2.5x the area of the ASI220 but is only 33% more expensive - so a decent value. As to whether the whole sensor area of the ASI174 can be used, my experience may simply be a result of where I have the prism. I have reduced the prism height but have not yet investigated the effect on the usable illuminated area. You still get the near-full width, which is good. The prism can cast a shadow on the imaging sensor, but that shadow can be dealt with via FLATs. Hope that provided some perspective! Good Luck!
I am just finding out the problems of OAG after switching to a ZWO OAG/ASI120MM-Mini setup for my Edge HD11/.7 reducer. My problem has been the guide star going in and out of focus/round shape and the ASIAir Pro losing the guide star due to loss or change of star mass. I am on the verge of trying the Celestron OAG/ASI174MM-Mini setup. In any case, thank you for the great video.
Hey Bob. That's weird about the in/out focus. Silly question: Have tightened all of the set screws that hold the prism? Do you feel any looseness if you try to jiggle the guide camera? I'm surprised you got the ZWO OAG to even see a guide star with that small prism and large focal length SCT. You'll have no problem at all with the Celestron OAG and its large prism combined with the ASI174's large sensor. Good luck!
@@Aero19612 Yes I have triple checked that everything is secure. I just spoke with Steve at Starizona and he had one suggestion that I had not thought of. He said that the ASI6200MM requires a lot of power at times and he has seen some unusual problems when powered from the ASIAir Pro. I currently have the ASI120MM-Mini plugged into the 6200 USB. Steve suggested trying a separate power source for the 6200 and plugging the 120 into the AAP USB. I am going to try this.
Interesting. I don't understand how a power issue translates into the appearance of an out-of-focus star. But it's a cheap fix if it works. Let me know how it goes. I'll file it away as a "lesson learned".
HI James, great information as usual, many thanks. It's been 2 years now since this video was published, in a brief statement have you found OAGing more or less beneficial in the long run please?
Beneficial. Period.
1. At short focal lengths (< 800 mm, say), there is never a problem finding one or multiple guide stars - cheaper option than a guide scope
2. At long focal lengths (> 1500 mm, say), you can (need to) use Stellarium to plan out the orientation of the OAG so that you WILL have a guide star (I have several videos that address this). Make sure to get a guide camera with a large enough sensor to take advantage of the larger prism.
3. You get the guiding benefit of a smaller guide image scale
4. Your setup is more "compact" and, without the guide scope, your payload has MUCH LESS inertia for the RA motor to overcome
Totally sold on OAGs. Not going back.
Thanks for watching!
James love your videos, they really help to clear up things that have confused me for years. I'm a Windows user, have you heard of an alternative to pixinsight that runs on window?
Thanks Pat. A lot of people use Photoshop along with an add-in tool set for astrophotography. There is GIMP, which is free. And maybe StarTools. I don't anything about other software as I have always used PixInsight.
@@Aero19612 I'm an idiot, I just checked the pixinsight web page and I see they have a Windows version. Thank you for responding.
Ha. I thought you just hated PixInsight. It's a great piece of software. Highly recommended.
@@Aero19612 No, I see it is used by virtually every video on astrophotography but some how I got it in my head that it was only for Apple computers. I don't mind buying it because it really seems like a final solution after trying so many others. My trail period license is pending. I really like the idea that I can use it on all of my computers and OS's with one license.
In balancing this type of scope, rather than precariously moving it back and forth on the dovetail, I add a saddle at the front of the scope and attach weights, which typically is a C clamp.
Yep. I do something like that with my little Redcat 51. I may do that here as well. Thanks for watching, Peter!
Your videos are excellent and very helpful!
could i ask how you achieve the proper 105mm back focus? I can't see what spacers you have used. the Celeston OAG seem deeper than the manual would suggest.
I'm actually going to address that issue in a video. Celestron is anything but clear (I must have done this incorrectly 6 times). If you're using the focal reducer (the 105 mm back focus is for the focal reducer + 1/3 filter thickness), then you need to measure from the back-most edge/face of the focal reducer (in other words, not including the threads). The OAG includes the SCT adapter (about 32.3 mm - the Celestron manual has this as 25.3 mm) + the OAG body (30.3 mm, Celestron manual has this as 29 mm) + the external thread adapter (about 12 mm) = about 74.6 mm. You're left with about 30.4 mm (or 31 mm if you account for a 2-mm thick filter). For my system, I have a filter wheel (20 mm) and a 6.5-mm distance from the camera face to the sensor (ASI1600), which leaves me with a 5 mm spacer between the OAG and the filter wheel. So, it is close but possible. Good luck!
@@Aero19612 Thanks for this James. These are roughly my findings as well. I find the additional 0.3 on the SCT adapter a strange but very specific dimension? unless this allows for a filter? The manual clearly states it is compatible with the C* XLT Class scopes as well so i am hopeful.
I have the f6.3, Celestron OAG (29, 25.3, 12.5 total 66.8mm) , ZWO Filter Draw(21mm), Zwo spacer ring (11mm) and 6.5mm for the camera back focus which all adds up to 105.3 according to the OAG manuals provided BF dims.
i can achieve focus for the guide cam and main cam but compared to the images i was getting when using the ZWO OAG my images look softer and stars more bloated. with the ZWO OAG i could get 105mm bang on but the larger prism, the promise of more & better guide stars plus the fact it was made for the scope made the celestron effort hard to resist.
@@Aero19612 I look forward to the video :)
great video, i have maybe a silly question, but do i need to make new flats if i rotate the imagining or the guiding camera?
Technically, you would not have to take new flats if you rotate the guide camera and NOT rotate the imaging camera. However, it is almost impossible to rotate the guide camera without slightly rotating the imaging camera no matter how hard you try so you're better off taking new flats if you adjust the OAG at all.
@@Aero19612 thanks, i actually hate taking flats, but i have to get used to it.
What mount are you using and what is its maximum payload capacity according to the manufacturer?
It’s a Celestron CGEM and has a stated payload capacity of 40 lb. I have not noticed any effect of payload weight on performance though. The behaves the same whether it’s a heavy C9.25 or a lightweight Redcat 51. I don’t mean to imply that it’s awesome (it’s not. Just barely adequate for high focal length imaging). Thanks for watching, Michael!
Thanks for the great explanations ! I have a question regarding the scope (or this type of scope) I am hesitating between the Celestron C6 and a Ritchey-Chrétien 6 inch. The RC is cheaper, but the C6 is lighter. (I am actullay looking at the C8 and a RC8, with same dilemma ) So, my question is, how does the 9,25 deal with coma ? with and without the reducer ? Thanks a lot !
Hey, Julien. I have not had too many complaints with coma on my C9.25 with or without the focal reducer. I use it without the focal reducer most of the time. Of course, if you go the EdgeHD route, that will handle the coma correction. As for the weight issue, it certainly depends on your mount. I have not seen guiding performance be affected much on my CGEM. I thought I would see a huge improvement when I replaced the heavy guidescope with the OAG. Nope. Hope that helps a little! Clear Skies!
@@Aero19612 Thanks a lot James !
@@Aero19612 Are you using the Celestron #94175 flattener/corrector, James? If so, are you satisficed with your results? I have an Antares equivalent that just doesn't cut it for me for AP. Maybe I am doing something wrong. Allegedly they are the same thing with different labeling. Trying not to splurge on the Starizona offering (if/when it becomes available again).
@@AzCrawler00 That's the one. I use it maybe 10% of the time, but I do like it and I think it works very well. No complaints from me!
Hey! Thank you so much for this video ....I have exactly the same setup scope wise except on an advanced vx mount. Have been struggling w my orion pro mono ssag which is not working on windows 10 now so looking at changing that to a zwo. Curious to know how good the tracking and guiding finally worked on this setup. Do let me know
Hey Raghav. Tracking is ok as long as I'm guiding. This mount has fairly large periodic error that PHD2 handles fine. There is also high frequency RA gear harmonics that PHD2 has more difficulty with. The DEC backlash is very high. I'm experimenting with options for dealing with it. I'm liking the PHD2 DEC backlash compensation. On good seeing nights, I get guiding performance between 0.7 arc-sec RMS and 0.9 arc-sec RMS which is tolerable for my SCT. With bad seeing, I'm around 1.3 arc-sec RMS. The ZWOs are good cameras. I'm using the ASI290MM mini for my small OAG and the ASI174MM mini for my SCT OAG.
@@Aero19612 thank you for the reply James. Is there an email id I can contact you on in case I have more queries??
Try this one: james.lamb@trispectral.com
How does the celestron compare to the zwo offering.. does this combo work well with an 11" edge as well?
I would use the Celestron OAG for the 11" edge because (1) The Celestron OAG has a larger prism (bigger guide cam field of view), (2) The Celestron is easier to focus since it has a helical focuser, and (3) The Celestron allows for independent orientation of the imaging and guide cameras. Thanks for watching!
I feel your pain.
Ha. Yeah, the secret is to plan ahead with Stellarium so you KNOW there'll be a star in view. Thanks for watching!
@James Lamb do you have an email ID or message box where I can chat with you?
your videos are very impressive. I need some help with OAG, thought will discuss offline please.
Sure Sreesha. See the 'About' tab on the channel page for the email address.
Shreesha! Where are you? Drop me a line at James.lamb@trispectral.com if you still have questions. I’d be glad to help out.
@@Aero19612 thanks. sure will do.
Ty for the video. Please Invest in a better chair
Thanks for watching. Celestron chairs are the worst
Excellent video
Can you tell the model of dovetail you use to put the tube on the mount?
Thanks