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New Observations on Star Artifacts Created by Celestron's Dew Heater Ring
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- เผยแพร่เมื่อ 13 พ.ย. 2023
- I wanted to share some recent observations about the star artifacts produced by Celestron's dew heater ring. I also ran a test hoping to solve the problem - fail. Oh well, my DewNot dew strap had been working pretty well...
I had the same issue with my David levy comet hunter using a dew strap. I disassembled the entire scope, checked for pinched optics, flocked it, and re-collimated, issue remained. Only when I found this issue you discussed on Cloudy Nights was I made aware of this issue. Removed dew strap and the problem went away. Plus side is I fixed the pinched optics. :-)
Amazing what we go through to track down stupid little problems. Thanks for watching!
Since watching this video for the first time, I've had a couple of nights out with my EdgeHD 8 and the Celestron Dew Heater & Celestron Controller. Last night I had clear skies until about midnight and it was about 20-23 deg F with about 80% humidity. Needless to say, when I brought the equipment inside it was frosted over -- except, that is, for the corrector plate which stayed dew-free. What's relevant to this video is that prior to using the dew heater the past couple of nights, I turned down the aggressiveness on the Celestron Dew Controller to 3 (from 5) and I didn't see the star spiking in any of the 60 frames I took last night. When I had the controller set to 5, I definitely did see spikes. I knew the dew heater & controller were doing their jobs since it was very humid and cold and I'm sure I would have had frost/dew on the corrector plate if the dew heater wasn't working. (I also used the Celestron aluminum dew shield with the clips on the side and the cutout for the dew heater wires, but I've been using that since day one and still had spikes with higher aggressiveness settings, so I don't think that factors into anything.) Not sure exactly what an 'aggressiveness' setting on the Celestron Dew Controller means, though. Is it just the power setting, or does it factor in some type of change in heat intensity over time? So, I'm not sure how the setting of 3 on the Celestron Controller would correlate with a temperature setting on a controller without a thermistor. Anyway, I thought this was good information to share...
FOLLOW UP/EDIT-- After calibration and integration (but before any processing), when I zoom into the image I can still see some very, very small spikes around the largest stars. However, the spikes appear more symmetrical, i.e., I don't see the predominant 11 and 5 type spikes James showed on the video. In terms of star bloat, I'm afraid I just don't have anything to close enough to compare to this set of images to determine if the star bloat is any less. (I've only been using the EdgeHD 8 for a short while and have had limited nights to test different things). So, in conclusion, I'm afraid the only data I can speak to is to support James' conclusion that less power to the dew heater ring reduces (but doesn't eliminate) the spikes. In my hands, though, at aggressiveness of 3 with the Celestron Controller, I did see diminishment in the elongation of the 11 and 5 spikes. Wish I could have been more helpful. (If anyone want's to see my images, shoot me an email at my firstnamedotlastname gmail account.
Very good feedback, Greg. Thanks! Do you think you're getting any star bloat either at a setting of 5 or, now, 3?
@@Aero19612 I'm running my subs through PI right now, James. I'm going to check my previous images at higher aggressiveness to see if I can detect star bloat. Wish I had used the same target as one of the previous sessions, but hindsight...
@@Aero19612 I couldn't really find anything to compare with this data to determine the degree of star bloat as compared to higher power/aggressiveness settings, James, and I modified my post since when zoomed in to the calibrated/integrated images I still see very small spikes.
Ahh. That's too bad about the spikes. Will the ring prevent dew at your location at an even lower aggressiveness setting?
@@Aero19612 Yeah, I got too happy too fast! :-) The good news is that the spikes aren't visible unless I zoom in (and they are more symmetrical) so I don't think it will degrade the image processing too much, but undoubtedly, it's degrading our resolution a bit. The next time I can set up under similar conditions, I'm going to try several different aggressiveness levels since I'm curious as well whether dew/frost will form if I go even lower than 3. Of course, I want to confirm that the spikes reappear once I go up in aggressiveness and I want to be able to compare star bloat at different aggressiveness settings. (All this as long as my fingers don't frost over...)
I have the same SCT 9.25 and use the Astrozap Heated Dew Shield and works great for dew prevention and also a light shield. However, it was a little long for my taste, so I cut 5" off the length. I was thinking about getting the Celestron dew heater ring at first and now glad I passed on it. Thank you for your keen observations. Clear Skies! -Eddie
I agree. A dew strap and shortened dew shield will work just fine. The shortened Astrozap dew shield is even better I bet.
Great video - why is it you keep producing videos that make me go "wait, I think I have a similar issue" then start diving into similar tests to see if my issues are the same as yours!
Why? Probably because Misery loves Company.
the astrozap dew shield with heater strips built in are great been using one for a few years now works great ... great video very interesting i was not aware of this issue of the celestron new style dew heater ring i seen the product when they first come out thought it was a great idea at the time but had all the dew heater stuff i needed glad i did not try to upgrade to what i thought at the time was a great idea . cheers james
I thought it was a good idea too. My non-heated dew shield and dew strap work pretty well, so I guess I'm back to using that approach. Maybe add another dew strap. Thanks for watching, James!
I think you're on the right track with the gaps hypothesis. I have a C8 with heater ring with the same issue as you, but also have a C5 with the heater rings that I've been using for some years before upgrading to 8" and never saw any of those spikes. I've just checked and turns out that the ring for the C5 only has a small ~10mm gap at 3 o'clock and no more than that.
I had a C6 with the dew heater ring and no problems. I would even crank the thing to high in very wet high humidity. But fast forward to my RASA 8" and go anywhere over 10% power and the diffraction spikes.
Ahhh, I see that you saw this post. I wonder if your C6 is constructed with a single piece split ring like the C5?
@@Aero19612 I once had to remove the corrector to clean it and I do remember it having 2 gaps. They were much smaller than the larger scopes but they were there. I no longer have the C6 to double check. Also the C6 I used the hyperstar with a ASI2600MC Pro mostly so with such a big pixel scale I probably never noticed.
@@deep_space_dave Ok. Thanks, Dave
Great suggestion with only turning on the dew heater after taking pictures of the stars!
I wonder if the opposite problem from what Deep_Space_Dave describes could be the issue. The heat from the dew strap would warm the metal body of the SCT, but with less conductive cooling in the gap areas, this would lead to an increased thermal expansion in the more insulated areas. Presumably, this would cause slight bending in the corrector plate without needing to vary the plate temperature. Might be a good excuse to buy a cheap thermal camera. :)
Good idea about using a thermal camera! Since the tube on the OTA is thick aluminum, it would act like a heatsink and dissipate the heat away from the corrector especially when it gets cold. This is why dew forms because it actually cools the corrector below ambient temperature. So the hot element is basically radiating heat directly to the cold corrector in a very tight spot causing it to contract but the rest of the glass is on the giant heatsink (OTA) which is colder and expanding. So basically you got 2 extreme no matter what which is cold and hot = diffraction.
You may have mentioned this in the video and I may have missed this, James, but does the star spiking go away immediately after turning the dew heater off? In other words, is there any residual effect of the actual heat from the ring or, as you alluded to, is it just the power going through the ring?
Good question...
Yet again...an extremely helpful video. I was seeing the exact same shape in my guide stars. I have the same setup as you (SCT with Celestron OAG). As a scientist, I'll repeat your experiment in a different lab
...and now that I finished the video...nice further investigation into the star artifacts. Your observation into the rapid onset was very interesting. I have not reached out to Celestron...but I think I might. Have you?
No, I haven't brought this up with Celestron. I never come away from those interactions with more information than I went in with. If you happen to do some further investigation, let me know what you find out.
Will do...
Hmmm, very interesting, James.
The next chance I get to use the Edge11", I will look for this and also, crank down the wattage to the ring.
(Lately, I've been using it for planetary imaging and also using my time with the EON 130mm refractor )
- Patrick -
Let me know if you see any of these effects if you get a chance to play with the power level.
A couple of data points. When I got this effect the other night, I had the Celestron dew ring turned on to 60% duty cycle at 13.5 volts. It turns out to have a resistance of 8 ohms (less than most dew heaters, so more power), meaning it was putting out 60% x (13.5^2 / 8) = 13.7 watts, which may have concentrated too much heat in a small area.
Last night I kept it turned off and instead used an Astrozap strap installed in the traditional position around the tube just behind the corrector plate. No streaks on stars. The straps' resistance is 12 ohms and I had to turn up the duty cycle to 100% to completely get rid of the dew. That was 15 watts, but spread over a much larger area. I think the main effect is actually to warm the air inside the telescope slightly, keeping the corrector plate warm via the air behind it.
Bear in mind that by "warm" we actually mean "about the same as the surrounding air." Dew is a problem because some surfaces exposed to the sky will radiate away so much of their heat that they become colder than their surroundings. So, used correctly, a dew heater should probably improve seeing rather than worsen it.
I'm guessing the Celestron heater is prone to creating small hot spots, but I haven't investigated.
Hi Michael,
There’s no doubt that the Celestron dew ring puts out much more heat than my DewNot dew strap. Somehow, that dew strap is able to keep the dew off of the corrector plate most (all?) of the time. There’s no way it’s heating the corrector plate. In my case, I mount the strap on the front of the scope with it overhanging the end. It must be heating the air in front of the corrector plate just enough. I wish I understood this process better. But I’ll settle for no dew and ignorance, but don’t leave me with bloated stars.
Based on what I've read so far, I think the leading possibilities are (1) the corrector plate gets warmer over the gap, causing it to flex and introducing astigmatism; and/or (2) the warmth sets up an air current that is primarily through or near the gaps.
I don't think the dew heater glows. There is just not enough heat. Visible glow requires a temperature hot enough that you would burn your hand touching it. Dew heaters, even this one, rise only a few degrees above ambient, normally. They glow only in the far infrared.
I don't think magnetic fields affect light. Conceivably, there could be some vibration caused by the DC power pulsing on and off, but you'd hear a whine. People have seen this effect with several different power supplies (mine is a PWM of my own design, frequency about 450 Hz, which would be quite audible).
Some good thoughts, Michael. There could be some local warping of the corrector plate. I doubt that air currents are the issue and certainly don't think any form of vibration is the source.
As for heat glow, the temperature increases the corrector plate temperature by 12 deg C about 1" from the dew heater ring. See 7:20 in
th-cam.com/video/gU43rhYAoGA/w-d-xo.htmlsi=q0lw5m9yXbYd_5Pz
@@Aero19612 My impression is that 12 C is a very large temperature increase for dew control purposes -- I was under the impression that a couple of degrees were usually sufficient.
Metal and glass expand differently, so there might be pinching.
The chart at 8:08 in that video is alarming. Don't we want a uniform temperature, as far as possible? Also, those temperature increases look way too big. And, looking at the earlier chart, 100% power is 30 watts! With that setup I would run that dew heater at maybe 15% power.
The root of the problem may be that this dew heater easily gets too hot.
My Astrozap dew heater strap draws 1 amp (12 watts) at 100% power. I normally run it at half that or less. And it's on the outside of the tube, with plenty of metal to distribute the heat evenly.
@@michael.a.covington Yeah, very hot. The "his video" is "my video", haha. Absolutely, we would prefer to have a small uniform temperature across the corrector plate. But we can't get a uniform temperature because the source of the heat is on the perimeter and has to diffuse through the glass. The max power is 24 watts (2 amps at 12 v) and is probably way too high. But the 25% power does not seem to heat the corrector plate adjacent to the secondary mirror.
Just a thought... Since the "disruption" begins immediately when you turn on the heater and the heater has no moving parts, it might be an interaction between flow-induced vibration (FIV) and forced convection heat transfer. If you do a search for FIV you get a bunch of results. Since you don't have suprots the full 360 degrees around the heater could it be producing a vibration at the missing support points?
I don't think vibration is a player here. To cause vibration of a large mass like this via FIV, there would have to be very high speed flow. The wind would produces MUCH larger effects than this little ring. I think there are simpler explanations
The problem with the gap is that the gap is present whether the dew ring is turned on or off so aberrant light is not the issue. So why is the problem immediately after power is turned on? We have not found the problem. I would suggest we pull out our hand held temp meters and see if the temp does rise rather quickly and could be the culprit. In other words take a photo that only last a couple seconds into the power on sequence. Similarly could you use a specifically aimed temp meter and point at the gap areas and the none gap areas....
I didn't mean external light. I meant light associated with the dew heater ring heating element. I'm temporarily done with testing the heater ring - I put the original (non heated) ring back onto the SCT. Maybe I'll do some tests with the ring alone... Thanks for watching!
@@Aero19612 OK - wow that makes sense! Thank you for what you have done so far. One question is that gap underneath the corrector plate?
Yes. the gap is in the ring that supports the corrector plate from below (inside the tube). I'm assuming they used a two-piece ring to make it easier to assemble the SCT.
warm air eddy currents were the first thing that jumped to my mind, but i guess that wouldn't produce such a giant spike, that's pretty wild though
No, I don't think eddy currents are the issue. Whatever eddy currents are present they have to be much much smaller than the effect of wind acting on the SCT body.
Interesting, that the gaps in the ring only lead to changes when the heating is turned on. Maybe they expand due to the heat and start to cover areas in the warm state that are not covered when the heating is off?
I doubt it. The thermal expansion coefficient is very small. For a 10 deg increase (that's a lot), the support ring will only expand by 0.0001". Pretty small.
This appears to be electrical noise. It appears immediately when powered on, and reduces when powered down.
Use a separate isolated power supply.
Provide an electrical ground to the dew ring circuit.
Turn off heater during exposures.
Electrical noise could be a problem, but I don't think that's what's going on here. Two reasons: (1) I don't see "bloomed" stars when using the dew strap at the same power level (same power source) and (2) the "streaks" are not consistent with general noise, i.e., no reason for directionality, plus the streak orientation is correlated with the gap in the corrector plate support shelf.
I wonder how this would affect planetary imaging?
Really bumming, just bought the ring and on top of it, had to buy the Ecco to make it work with the Eagle 🙄
Just try to operate at the lowest possible power level. I only rarely do planetary imaging, but I think you should be OK. Turn the ring on before imaging to pre-heat the corrector plate, then turn it off while collecting data. Rinse and repeat. Good luck!
This is a very interesting find James. One thing I'm wondering is, is celestron's do heater controller using a PWM signal to send the power/current to the do heater ring or is it a continuous controller because if you're using a Pegasus power box that is PWM that might be the difference between what they're using. On top of that, I'm wondering if the PWM frequency is actually creating some type of resonance frequency that could be also affecting the FWHM?? One last note, I know celestrons controller unit measures ambient temperature and humidity so I think they're automatically calculating what the dew heater ring temperature needs to be to properly heat the corrector plate so that it doesn't get dew, that amount of power may be insignificant compared to what the Pegasus power box might be outputting which could possibly be distorting the corrector plate slightly. If you could get your hands on the Celestron controller that my answer your question somewhat
Good points. I was only willing to buy the dew heater ring if I could power it through my Pegasus Astro UPB. The Celestron controller is around $300. I did some simple heating measurements of the corrector plate heating (earlier vid). Seemed like 25% power wouldn't heat the corrector plate enough. 100% is probably way too high.
@@Aero19612 I really need to test this with my dew heater ring as well, as you know I have my edge 11 set up with a hyperstar and now I'm curious if I will have different results if I have the dew heater off. I'm definitely going to do some testing on this. It's crazy how just when we think we fix the problem with the ZWO camera fan issue now we have a dew heater ring issue, lol
@@Aero19612the Same issues occur when using the celestron Controller.
I have a Celestron controller, Bill, and have the same issues. I bought it specifically to see if it would remedy the situation since I had the Dew Heater plugged into a Kendrick adjustable controller, but it didn't fix the issue. Still spikey stars. I tried loosening the retaining ring just a bit (thinking I overtightened it), and I also reduced the aggressiveness of the Celestron Controller (to 5), but neither helped. (After watching the video, I'm going to try reducing the aggressiveness for the dew heater ring in the Celestron Controller to 3 and see what happens.)
@@gregerianne3880Thanks Greg, that's really good to know. Please share if you find a remedy 😀
Having exactly similar star spikes on my RASA 8" with installed dew heater ring; how does Celestron not know of such an issue and continue to sell this product that causes such optical issues?
Yeah. I see this kind of thing over and over with them. I think they have an idea and push it to manufacturing and let the customer identify the design deficiencies. Oh well. Thanks for watching!
Always enjoy your videos. Question: Isn’t the Gap present when the heater is off? Why would the gap be the cause only when the heater is on and not when the heater is off?
My guesses were (a) that the heating element produces a glow that is block everywhere except at the gaps and/or (b) more heat is conducted out of the corrector plate where it is in contact with the supports, so the unsupported are (at the gaps) is hotter and possibly affecting the optical path.
Sorry, I feel like I pointed out a crack in your foundation or something 🙂 But it is what it is. The dew heater ring worked perfect on my C6 and the thing got hot in the winter to the point I had to turn it down because steam was coming off the corrector but no star aberrations 🤷 But C8 and above it is a crap shoot and even with the super expensive Celestron dew heater controller with temperature probe. I have been happily using an AstroZap 8" dew heater band on my RASA 8". Someone on IG pointed out my scope was covered in frost but I was like yep but my corrector is nice and clear with perfectly round stars. Thanks for this very detailed explanation and testing! I do think the gap is there for a good reason though, possibly to prevent too much pressure on the corrector when the glass expands in the cold? I am also curious is this is a common design "feature" with other brands like Meade? Actually Meade telescopes have gaps too but they don't use dew heater rings 🙂
It is interesting that @ldipenti commented he had no problems with his C5 and that scope has a single piece support ring with a small 10 mm gap. Perhaps your C6 had a similar design. I suspect Celestron is using a two-piece ring with a gap to make it easier to install when they're putting the scope together.
I'm confused how that gap could be causing issues only when power is applied to the dew heater ring. Glad I never bought one of these rings. I considered it pretty seriously when it was first announced, but decided to keep my strap/shield setup. I've got enough issues with star shapes as it is. No need to add this ring into the mix :).
The gap is in the support behind the corrector plate. When you apply heat to the ring, any glow from the heating elements has a direct line of sight toward the primary mirror. Yep - stay with the dew strap and dew shield. Save yourself some grief.
@@Aero19612 the heating elements actually glow? Bright enough to cause reflections? Further, even when James wrapped the whole thing in tape, it _still_ glowed brightly enough to cause some reflection artifacts?
Yeah, I definitely don't need that kind of grief in my life LOL!
@@JonnyBravo0311 I very much doubt that the heating elements *glow*. They would have to get to a temperature something like 800 F to do that. We would burn our hands handling them.
@@michael.a.covington I completely agree, which is why my questions sounded rather incredulous. I mean, it's clear _something_ is happening when power is applied to the heater ring. The before/after images clearly show this. But glowing elements? No, that I don't accept.
Yes, the heater will glow, at least in the infrared range. I think this is very likely the cause of the spike phenomenon. All objects emit black body radiation. At temperatures that humans normally experience, black body radiation is in the IR spectrum. At much higher temperatures, black body radiation occurs in the visible spectrum. FLIR sensors use this phenomenon to image temperature differences. Our telescopes generally also detect near-infrared because we want to capture Ha and it is right on the edge of the IR spectrum.
We know the spikes are not a thermal effect because it happens immediately when the dew heater is turned on and stops when the dew heater is turned off. It’s also probably not a mechanical/vibration effect. It seems to be correlated to the gaps in the SCT support flange, so it seems logical to me that the support flange is obstructing the IR emissions, but the gaps allow it through to the optics. James’ tape would do little to block IR, so it made no difference.
Just before 12:00, it is not clear to me why fwhm would increase with exposure time. This might be due to saturation?
atmospheric seeing over a greater amount of time is going to cause a larger fwhm, unless jetstreams and stuff are actually calming down and the atmosphere is getting significantly clearer as imaging progresses
I have not seen this effect on my EdgeC8. I use a Pegasus controller with humidity sensor on auto. I am near the ocean so humidity is a big problem. Perhaps rotating the dew ring to see if spikes rotate? As someone here said, it could be an effect from the Celestron controller since the effect is so sudden. Perhaps those gaps are allowing light into the scope? Is the compliance graphics in the right location per Celestron instruction (at dovetail) ? Did you over tighten the mounting screws and make sure to tighten in a criss cross pattern? As I recall, the original gasket was discarded during installation. Did you remove it?
All good comments, Ron. I used the Pegasus Astro Power Box to power the ring. Others see the same effect when powered by the Celestron controller. I did remove the original gaskets and I didn't over tighten the screws.
BUT
I must have missed the instruction on placement of the compliance label. Just re-read the instructions. It's clear as day. I rotated the ring so that the power cable is at the top.
@@Aero19612 Has rotating this made a difference in your stars?
@@SKYST0RY good question. Simple answer: “No”. I should have. I tried a different approach that didn’t work and just decided to remove the ring. I would be VERY interested if there is actually a difference. I feel stupid for not seeing that instruction in the first place. Some days I just want to get out of my own way!
Hi James , could you 3D print a better 360 degree solution for this ? Might be something people would buy .
I think the ring would have to be aluminum since it's going to get hot and it plays a structural role - this is the part that has the threaded holes you use to attach the dew ring (or the original ring). But, yes it would be nice to at least close those gaps.
I'm curious if this effects the edge series scopes as well. I had a hell of a time with star shapes with mine.. I wonder if this is the real issue.
I have had it with an EdgeHD. Also note that the Edge is fairly picky about collimation and about back focus (focal plane position).
@@michael.a.covington Don't I know it haha
There you go! Michael to the rescue. Thanks Michael.
In your videos it looks like the dew heater protrudes to the light path and therefore the gaps may be visible and cause problems. In my setting (EdgeHD 1100) the dew heater is completely covered by the front ring. Could it be you mounted a wrong system? As I can say, I never observed those artefacts by turning on the heating. Greetings from Hamburg Herbert
Hi Herbert,
No, it's the correct dew heater ring. Must be an optical "trick". The outer circumference of the ring is right next to the tube. Nothing protrudes into the light path. That's interesting that you don't see any artifacts. Are you sure it's working, haha?
Sorry for example at 8:12 you see clearly that the inner ring with the gaps is narrower than the outer cover. In my Settings this is not the case (I have C6, RASA11 and EdgeHD1100 working with the dew heater ring…
My SCT is only 8” and a dew strap run with Pegasus keeps all the dew out even here in 100% humidity Wisconsin. As the problem begins upon powering up the ring and gets worse with increase in power the question arises does the power application cause a vibration in the dew heater ring? It is stiff plastic and not soft like a strap. I think we have all heard various heated elements making noises as low to high pitched hums when power is supplied. Could these vibrations change the width of the gap on a very small scale, but enough to cause this diffraction pattern?
@cjmenagh882 I don't sense any vibration as there are no moving parts. If power is applied in a cycle manner, it could cause motion, but I don't think vibration is a player here.
That's an optical illusion. The support behind the corrector plate is the same width as the dew heater ring and the dew heater ring is the same width as the original non-heated ring.