Backfocus

Finding the proper backfocus with the parts you already have and the accessories you want to use isn't hard, but with so many combinations, it can start to make your head spin. Use this free google spreadsheet to quickly try out different combinations and find the right solution for your next astrophotography shot:
docs.google.co...

[@RobPauza]

Thanks so much. I just got an EdgeHD 9.25, which is my first reflector. It's definitely a different breed than my refractors and I was getting super frustrated by not being able to get sharp focus. The collimation seems dead on from the factory, so I was scared to touch it. Your spreadsheet helped me confirm that my back focus was indeed a bit off. Now to wait for a clear night to test. Thanks again!

[@mosesrodriguez1494]

This video is so much help for an astronomy newbie. Thanks

[@engineerncook6138]

Good idea, just went through this exercise myself using a spreadsheet but didn't write it to include all my parts and use checkboxes to include/exclude. Thx.

[@scericat]

Appreciate the info in this video.

[@user-dr8ug3gs3c]

Thanks for the lesson & spreadsheet. Very helpful.👍

[@longhornastro]

Very cool video and thanks for sharing the spreadsheet.

[@rainerehlert]

Hi, There is one think missing and that is that a filter does lengthen the Backfocus by a third of its thickness, eg a 3mm thick filter increases the backfocus by 1mm... and so you need to substract this to the equation.

[@MrWilsonsChannel]

That's interesting... I haven't heard that before. Thanks for the tip 👍🙂

[@Oamaruastro]

This video speaks to me love of data and spreadsheets. Love it.

[@marshbum]

Hi, thanks for the video and the spreadsheet! I wanted to ask about focusers and back focus. I have an 8" LX200 Classic, now outfitted with a Moonlite focuser attached (the course focus knob on the LX 200 does not do fine focus well enough). One configuration is (A) Scope + Focuser + Spacers + camera. A second configuration is (B) Scope + Reducer (which sits inside the focuser attached to the draw tube 😁, which is what I like about the Moonlite focuser) + spacers + camera. My assumption is that in any configuration, the focuser length (body + draw tube) needs to be taken into account for the back focus calculation, AND that as the focuser draw tube moves, it can change the back focus in my configuration (A). For configuration (B), since the Reducer is attached to the draw tube, the back focus stays constant over the travel of the draw tube. I just wanted to check and be sure that my assumption is correct that for my SCT, the length of the focuser + draw tube would need to be part of the back focus calculation. If my assumptions are incorrect, please be free to educate me :). Thanks in advance for your help.

[@drdentin3215]

Thanks for the explanation, succinct and informative.

[@Kelli.Hicks.5]

Congratulations on 1,000 subscribers. 🎉 That's amazing! 🤩 That spreadsheet is VERY handy. 📊 Thanks for the link. 👍

[@angelo327able]

Thank you for sharing a great worksheet. A question I have is from what point do I begin measuring from? The back of the Visual Back or the flat flange of the SCT the Visual Back screws on to. Back Focus is a difficult challenge for a newby like me. I've read a focal reducer does and does not alter back focus measurements.

[@MrWilsonsChannel]

The rule of thumb is you measure from the last piece of glass (prisims don't count). So if you use a reducer, you start the measurement after that. the reducer adds nothing to the measurement, but may alter the back focus value you're trying to reach. For my C8, for example, the back focus if 105mm with the reducer and 127 without it. Without the reducer, you start the measurement from the back of the scope (nothing attached). The very first piece I screw on is an adapter that changes the threads from whatever pitch the sct uses to the standard m48 pitch. That part adds 1.5mm and so that is my first part of the back focus calculation. Hope that helps 😊

[@rauljimenez8132]

Excellent video, does having a filter inside a flattener make any difference in the 55 mm backfocus measurement?

[@MrWilsonsChannel]

Excellent question. My understanding is that if a filter is inside another piece, like a filter drawer, it has no effect on the back focused value. Just the width of the filter drawer itself would need to be taken into account. When it comes to flatteners, you take the measurement from the last piece of glass. So the width of the flattener itself is not factored in. However, usually flatteners will change your telescopes natural back focus value. For example, my telescope has a back focus of 127 mm. But with the flattener, it changes to 105 mm. I don't take into account the width of the flattener itself, only the parts that come after it

[@damienfogassy2369]

Thanks for the video. I'm m interested in eyepiece projection astrophotography. I don't understand the backfocus length of different focal length eyepieces and where to place the camera with various eyepieces.

[@MrWilsonsChannel]

That's an excellent question. One I'd have the research for sure. I did a little eyepiece projection in the beginning of my astrophotography journey but made the jump to dedicated astro camera soon afterward. The eyepieces I have aren't the best quality so the decision was to spend money on better eyepieces or even more money on a camera. I decided to go the camera route because it seemed like it had the fewest variables, but the topic is still fascinating and one I'd like to explore one day. Maybe they'll be a future video on this topic. Thanks for the idea 👍

[@muhammadnajmibintajudinkpm4360]

Hi mate, thanks for the video and ur spreadsheet really help. Since i love ur video, i have a few simple(not really) questions here. In ur opinion: 1. UHC filter better put before or after flattener? 2. Or it is the same anywhere? 3. Or it is depends on what kind of filter I gonna use? Thanks again. Cheers

[@MrWilsonsChannel]

I believe you want any flatteners and reducers to come first in the image train, before any other accessories, extension tubes, oags, etc. You want to put filters as close to the cameras sensor as possible. If you're using an oag, put that between the flattener/reducer and the filters so the guide camera doesn't have to look through the filter to see a guide star.

[@naegeleh]

Hi thanks for your informative video, one thing I do not understand. When I have a mechanical focusser, in my case a diamond steeltrack on an edge HD11, do I have to adjust the backfocus exactly or only approximately (because the steeltrack alters the backfocus for best focus? Thanks

[@MrWilsonsChannel]

It looks like that device is accounting for the backfocus already. The one I'm looking at online has 30mm of travel which doesn't give you a whole lot of room, but you could still fit in a camera and filter wheel with that. Then you would adjust the focuser on the steel track until you got to the proper backfocus amount. Improper backfocus on an sct like the edge hd will affect the stars at the edge of the field of view. I've never seen this type of focuser on an sct before, but it seems like you would still use the focuser the telescope came with to focus and the steeltrack to achieve proper backfocus to make the stars at the edge look good.

[@raymondfrenette229]

Great spreadsheet, I have a C8SE and the back focus is said to be 5”/127mm, is this a true number? I tried a few millimeters longer and started to get fuzzy stars.

[@MrWilsonsChannel]

It sounds like you have the same scope I have. 127mm at native focal length, or 105mm if you're using the celestron reducer. You might be able to get away with +/- 2mm, but generally speaking you want to get it as close as possible. Also make sure your scope is collimated. My latest video will walk you through that.

[@dfinchermd]

Thanks for your exceptional tutorials and the spreadsheet. I have a 600 mm f7 refractor with an mfg stated back focus of 56mm. My confusion is measured from where? In prime focus I have an image train of 55 but a total back focus of 157mm factoring in the draw tube distance. This is what provides a daytime rough focus in prep for the night.

[@MrWilsonsChannel]

I suppose it could vary from manufacture to manufacture. However, most of them are measured from the point where your first accessory screws on. The draw distance of the focuser itself is just there to achieve focus once the proper amount of spacing has been applied. So if you add 56 mm of accessories and doodads, you should be good to go. Don't forget the camera itself adds a little bit of back focus. You can find that measurement on the camera manufacturer's website

[@drithen]

This was very helpful, thank you so much. One question, how do you get your spacers apart? I have a 5 mm and an 8 stuck together and I think I’ll have to replace them!

[@MrWilsonsChannel]

I haven't got any of mine stuck to the point of unremovable, but it's certainly a thing. I came close one time. When it inevitably happens to me, here are some things I'm going to try. My understanding is that it occurs due to temperature changes. When things get hot they expand, and when they get cold the contract. So when you take your parts from your relatively warm house into the cold outside and they contract and bind together. So perhaps one thing you might try is heating them up? Perhaps run them under some hot water for a few minutes? 5 and 8 mm is going to be pretty tough because there isn't a lot of surface area to grab onto. When I assemble my parts, I usually take them outside and let them sit there for a little while. As I put together the telescope, the optical train components are the last parts to go on. This way they've kind of climatized a little bit. I also make sure not to screw them on super tight in the first place, knowing that they will get tighter as they continue to cool. I know that's not a lot of help or in magic answer, but I hope it helps a little. Let me know if you get them apart. Clear skies 👍