Back-Focus 101: Celestron 8" EdgeHD SCT (no reducer)

The Celestron EdgeHD design corrects for much of the aberration and field curvature inherent in the standard Schmidt-Cassegrain. To achieve this correction, the camera sensor must be placed a precise distance away from the baffle-tube lens element. For the Celestron 8" EdgeHD, this distance is 133.35mm. In this episode, of Back-Focus 101 I demonstrate some basic and "advanced" ways to get the correct back-focus spacing using this telescope. It should be noted that when using the Celestron 0.7x Reducer Lens for the 8" EdgeHD, the required back-focus spacing changes. The back-focus requirement of this telescope with the Celestron 0.7x Reducer will be shown in the next episode of Back-Focus 101.
Link to filter effect on back-focus: • Back-Focus 101: How Fi...
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[@JonnyBravo0311]

I set my 8" EdgeHD up just a bit differently than what you show (and what is shown in the Celestron documentation). Instead of using the SCT adapter to OAG, I instead use the long portion of the T-adapter into the female OAG adapter. Then, on the other side of the OAG, I use the male adapter to the 11.55mm ring to the ZWO EFW to my 294MM Pro. This gets me 49.3 + 4.5 + 29 + 12.5 +11.55 + 20 + 6.5 = 133.35mm. I do this because I switch between imaging natively at f/10 and reduced at f/7. While you didn't go into detail in this video, the reducer for the 8" EdgeHD puts the back focus distance at 105mm. Thus, if I use the reducer, my imaging train becomes SCT adapter to OAG to male adapter to 11.55mm ring to EFW to 294MM Pro. This gets me 25.3 + 29 + 12.5 + 11.55 + 20 + 6.5 = 104.85mm. There are two primary benefits to my method. First is that going between f/7 and f/10 imaging just requires me to replace the T-adapter/female OAG adapter with the reducer and SCT adapter. Because I'm not adjusting anything behind the OAG, I don't need to adjust the position of the guide camera, either when switching between f/7 and f/10. The other benefit is that it moves the OAG closer to the main imaging sensor. With your (and Celestron's) method, the OAG prism is 93.55mm from the main imaging sensor (assuming the prism is set at the midpoint of the OAG body). With my setup, the prism is 65.05mm away. Being closer to the main imaging sensor means the guide camera is closer to the prism and gets a wide view of the pupil and less aberrated stars on which to guide.

[@Ronbo765]

These types of videos are so important for the astrophotography community. Thank you so much.

[@jayg2580]

Indeed! For the first time I have confidence of what to buy to start my journey. This kind of info can be illusive to a novice.

[@davidleejenkins]

Thank you! That was very helpful! I just received my Celestron OAG and can now achieve proper back focus with my Edge HD 8.

[@mattwier4824]

Best explanation I've seen. I've been doing it wrong for years.... not anymore! Clear skies!

[@AstroBlender]

Glad to help!

[@jgrimm1055]

Thank you Cody! Your videos help so much! About the to get reducer for my 8” EdgeHD! Need a back focus video 😊

[@raypace6981]

Very clear and informative video Cody. Thanks for sharing

[@AstroBlender]

Thanks Ray!

[@briankotak403]

Thanks for this Cody. I have a standard C8 and appreciate your previous videos on the BF using this scope and CMOS cameras. Itching to get my C8 outside. Been using my 110 mm and 61 mm refractors so far. Still waiting to do first light with the C8. Have a great Christmas! Dr B from Manitoba, Canada 🇨🇦

[@stephen.and.the.huskies3741]

Great Video, Cody! And I can really recommend your "filter effect on back-focus" one, too. Happy holiday season and CS, Stephen

[@AstroBlender]

Thanks Stephen, I’m glad you liked those!

[@ericwilkes1840]

Thanks Cody, great video

[@westcountryastronomer1910]

Cody I'm so grateful for this. I bought the Edge HD8, the ZWO filter drawer and I already have a 533MC Pro but I don't have the reducer. There are lots of videos explaining back focus with either the reducer and / or OAG but nothing for my configuration. You've saved me a whole lot of head scratching. I'm trying to get hold of the reducer too but they are impossible to find here in the UK right now. I'll be checking out the video on back focus with filters too. Thanks again. Subbed.

[@AstroBlender]

I'm so glad you found it helpful!

[@davidtavenner670]

Thanks Cody for these videos! Looks cold 🥶

[@AstroBlender]

Hi David, yep, you just can’t beat natural lighting though!

[@expatca4444]

Nice video man ! thanks !

[@rickmazzarella1911]

Excellent explanation and presentation.

[@AstroBlender]

Thanks, I’m glad you enjoyed it!

[@RaysAstrophotography]

Awesome Cody!

[@AstroBlender]

Thanks Ray!

[@mupangamwanakatwe1300]

Hi Cody. I am hoping that Santa will drop off an Edge-8 HD through the chimney on Christmas Day! 😊 Your excellent video is therefore most timely and so very well explained! Thanks and do keep up the great work and wishing you a Merry Christmas and Happy New Year …

[@AstroBlender]

I hope you’re on the nice list too! :D The 8” EdgeHD is one of my favorites, so well corrected and the perfect size!

[@EastSideAussieQuest]

Very informative video , thank you very much, learnt a lot.

[@AstroBlender]

Glad it was helpful!

[@carolayres4143]

Fantastic, thanks so much

[@GrundleStiltSkin]

i use 134mm as my edge hd 8 number because of the use of filters! great video. i personally flocked some of the off axis guider adapter, super shiny in there

[@AstroBlender]

Nice suggestion on the flocking! Glad you liked the video!

[@tornadoknut]

Excellent Job, thanks 🤓

[@AstroBlender]

I'm glad you liked it!

[@chris-xo2ry]

Thanks Cody, I literally just unboxed mine today and your video is extremely helpful. Was wondering how I was going to get the 133.35mm of backfocus and now I know. Wished I had found your video earlier, thank you!

[@AstroBlender]

Glad I could help!

[@petequintanilla4237]

Not a lot of Astro Photog youtubers these days covering equipment. It's all you cody - no pressure tho.

[@AstroBlender]

Haha I try!

[@lazylazyshark]

excellent video! When is your next one on the reducer coming out? Do you confirm the manufacturer info that back focus with the 0.7x reducer is 105 mm?

[@Microtonal_Cats]

Thank you! I have this scope and am learning my way around it. Oh, 8:52 this scope looks like plumbing right here. Maybe the bottom of a whole-house hot water heater.

[@silvanojs5148]

Sensacional nota dez equipamentos

[@DeereX748]

Outstanding tutorial. I'll have to look at the best way to connect my OAG in the train, as I have the ZWO OAG. I hope it will work okay, as it has a smaller prism than the Celestron, but my guide camera also has a smaller sensor (ASI120MM mini)

[@karcoms]

Love your videos. 🙏🙏🙏🙏

[@AstroBlender]

I appreciate that!

[@staceylloyd6236]

Cody, enjoyed your video as usual. I am looking forward to the 8" w/reducer and OAG as that is my current config. I am specifically interested in your thoughts on the Celestron OAG given the small image circle of the scope with a reducer. The Celestron OAG prism can't extend as close to the sensor as one would like with APC and smaller sensors. This appears to reduce the light from the stars getting to the guide camera. Maybe I am missing something?

[@Fatal_Inertia]

That carbon SCT has me foaming at the mouth

[@AstroBlender]

Me too!

[@silvanojs5148]

Sensacional nota mil

[@garysday]

This is awesome. It really helped me understand the back focus. I have a 662mc which includes no adapters and has a back focus of 12.5mm so getting this to 55mm (excluding the t adapater) is tough. Closest I’ve seen is 2x21mm from zwo. Does this make sense?

[@2badger2]

Great video! Did you make the video when you use the reducer? Thank you

[@MountainFisher]

I thought about getting the Evolution Edge HD mount and scope as I liked the built-in battery and weight, but I decided to get an iOptron AZ Pro mount with a double scope capability instead. The gps self-align kind of made up my mind for me, plus its weight capability of 43 lbs and its 10 hour battery is easily replaceable I can have a spare. Also, it was on sale for $1260 instead of $1618 really made me pull the trigger. Though I could do short exposure photos, I'm more into visual and double stars and its database is over 112,000 objects. I have the C8 on the right and a 102mm triplet on the left. I had to buy an ADM dual saddle for the left side as the little one they sell is too small.

[@nelsonrivera6575]

Hey Cody ,Merry Christmas ,im having trouble with my refractor and back focus , do you have a video on this? ive seen another ,but im not totally clear ,anyways ,Happy Holidays

[@wolfgangkuechle9085]

Great video, thank you! Since my laptop just died in a cold winter night it would be great if you could create a video concerning cold weather equipment.

[@rickarmstrong2476]

Asiair! Total game changer with convenience and I’ve never had an issue in the cold. I live in Idaho so it gets down to well below freezing at night.

[@rickarmstrong2476]

I’ve imaged in 5 degrees Fahrenheit with no performance issues

[@alvisedorigo5650]

Very useful video, straight to the point! Well done. I have a question: do you think the rather small sensor of my ASI120MM Mini cuts too many stars for an acceptable guide?

[@AstroBlender]

It depends. If you're using an off-axis guider, then yes. If you're using a guide-scope, the ASI120MM mini should be fine.

[@sreeshab4093]

Excellent video cody. I love your demystifying complicated subjects. The first part of your video where you do backfocus of 133.35mm withour OAG, you added a 11mm for the camera first. I bought 2600mm and it doesnt come with 11mm spacer. Can you please point right link for me to buy one. Secondly, I am inkling to buy the latest 2600MC DUO. Do you think this will work well with Edge 8 HD with and without the 0.7x reducer? Also with L-pro filter do you think it will be sensitive enough to guide?

[@AstroBlender]

The 2600 cameras have 17.5mm of back-focus, so you don't need the 11mm spacer, it's already "built-in" to your camera. I think the duo cameras are more suited to faster, wide-field refractors. Finding a guide star at such long focal lengths can be tricky, but it certainly is possible.

[@alexrelexer]

Can I use this T-adapter 93644 also on my Celestron CPC800XLT with a dSLR in prime focus? I only bought a 93633 and used it with my dSLR without a focal reducer.... I didn't know about backfocus lol 🙂and this adapter is as I can see in your other video meant to be used WITH the F6.3 focal reducer. And what if I want to image f.e. Mars with my TV 2.5x (1.25") on my CPC800 without focal reducer. What is the backfocus distance?

[@petef.4361]

I watched this video, and the backfocus video too. I have the exact same scope (not carbon fiber), OAG, guide camera, zwo filter slider, and the 294MC Pro. I switch out between the Optolong UV/IR cut filter, Optolong L-Pro, Optolong L- Extreme, and my newly acquired Optolong L-Ultimate (which I have not been able to try yet.) So I always have a filter inserted. I never even thought to add an extra spacer to account for the glass in these filters which are about 2mm thick from my understanding. Since I am imaging at either F/7 or F/10 should I not worry about adding a small spacer, or should I do it? Just wondering if the difference is noticeable in the images, and if you bother to do it yourself with your setup when using filters.

[@AstroBlender]

Celestron says you should be within 0.5mm of the 133.35mm requirement. 2/3 = 0.66, so you could probably get away without it, but it certainly wouldn’t hurt to add one!

[@octflyer]

Hey Cody, love your channel! What is the back focus distance if you use the Celestron reducer with the OAG? Is it still 133 or 105?

[@AstroBlender]

Hey there! With Celestron’s 0.7x reducer for the 8” EdgeHD, the back-focus becomes 105mm.

[@octflyer]

Thanks!

[@albertmasella4385]

Great video. However i have noticed with dedicated astro cams with aps-c or full frame sensors, the m42 spacers create vignetting. I had to change all of mine to M48. Not sure why ZWO doesn't include both. Have you ran into this issue?

[@AstroBlender]

Yes, I'd definitely use 48mm spacers with larger camera sensors. I have a drawer full of M42 and M48 adapters so I never run into issues. Haha.

[@ebrotman]

Terrific video. You mentioned that mirrorless cameras require more back focus. I am using a Canon R. How much more back focus is required?

[@AstroBlender]

Canon R cameras I think only have 20mm of back-focus, so you'd need an additional 24mm, or buy a Canon R specific T-ring. I highly recommend the William Optics T-rings. They're extremely well machined.

[@philbivins7133]

Hello, if I have a ASI AIR PLUS can I still use a Star Sense or is it just a redundant piece of equipment. Thank you

[@Astropaesan]

Can you use the OAG with the Celestron focal reducer?

[@johnmorris3073]

Hi. Great video - very informative. Thank you for sharing it! Question though: simply screwing on the T-Adapter and the proper T-Ring achieves perfect focus? It seems to me you'd need some sort of fine-tunability in there, wouldn't you? Unless every copy of the scope, T-Adapter, T-Ring, and camera were machined to PRECISELY the 55mm (or whatever appropriate backfocus distance) tolerance?

[@AstroBlender]

Yes, you’re certainly right. Not every system is identical. If you notice any slight issues, you can always pick up a variable thickness spacer set from AgenaAstro or other retailer to fine tune it if necessary.

[@mathewgangl6064]

Where did your backfocus video go using the .7 reducer cand celestron oag can't find anywhere?

[@davidheflin5847]

So if I am using the Meade 8” ACF and the zwo 071 what back focus is required?

[@Astropaesan]

Cody, question. Why did you replace your Celestron motorized focused with the ZWO?

[@AstroBlender]

The ZWO EAF gives me the ability to auto-focus during imaging runs with the ASI Air. I can control everything with the ASI Air that way. I actually prefer the Celestron Focus Motor but it isn't compatible with the ZWO ecosystem.

[@noahhaverkamp]

Pretty much exactly what I needed! I plan on purchasing the ZWO OAG with helical focuser instead of the Celestron one because of the price, but I think that will also come with the back focus required extensions and adapters as well...please someone let me know if you think otherwise!

[@circuitsbycode5443]

I have the ZWO OAG on my EDGE HD 8. While the Celestron is 'fatter' than the ZWO and takes up more backfocus space, I think it is superior in that it has the ability to fully rotate both the OAG and the main camera independently. The ZWO can only rotate the main camera by a small amount. The reason this is important is that the guide camera may not be able to find a guide star (because there aren't any where it is pointed), so being able to rotate the OAG fully means you can do a 360 degree sweep of the OAG to find a guide star. Then, because you can also rotate the main camera independently, you can also frame the image exactly as you wish. You can't do this with either the standard or L ZWO OAG. Stellarium (which is free) allows you to enter your OAG dimensions and see both the main sensor view and the guide camera's view exactly as they appear in the night sky. In other words, you get to see exactly where the guide stars are relative to your guide camera. I think this is essential to visual how the OAG works in relation to the main sensor.

[@noahhaverkamp]

@@circuitsbycode5443 Thank you for that info!

[@TheZerocool3312]

I have the Edge HD 8", but I want to start out with a ZWO planetary camera, but it doesn't come with additional spacers. Does the planetary cameras use the 133. 35

[@rudiroothooft2414]

I have an old Celestron cpc 9.25 with a zwo asi462 mc,no filters, no reduces atm, and i cannot get it in focus. Do i need also to put it in backfocus and if yes, how much is that ?

[@ngjunkai4707]

does the 133.25 mm back focus also applies to visual observation case ?

[@neiltonks]

A big question I have is how accurate do you have to be? The celestron T adapter must have a machining tolerance, as does the camera adapter. Have you measured the overall length using a Vernier? (Or other accurate device)

[@AstroBlender]

You can see the error tolerances on the Celestron EdgeHD White pages. :)

[@charlesowillford2474]

Where are you finding the various BF distances for the Celestron family of SCT; Edge and Std???? I have a C6, for example.

[@AstroBlender]

Hi Charles, if you haven’t read the Celestron EdgeHD white paper, I’d highly recommend it! Additionally Celestron posts the standard back-focus for the SCTs on their website.

[@Swedishoyster]

You mention a video of how to attach a camera when using the reducer, do you have a link to that? I can't seem to fit my Zwo camera to the reducer + T2-adapter since I have to remove half of the T2 adapter to get the right backfocus distance...

[@AstroBlender]

That will probably be a future video. For your setup, you’d use the Celestron Edge T-Adapter (as shown in the video), but remove the extension tube from the end of it. That will give you 50mm. Your camera and extension tubes should give you the other 55mm when using the Celestron 0.7x reducer.

[@brucepeters4274]

If you have a guide camera and a program for guiding, what do you use the StarSense for?

[@AstroBlender]

For visual use! :) The 8” EdgeHD always blows me away with how sharp it is. The StarSense is nice for winter viewing, I just set it and it aligns itself!

[@brucepeters4274]

Ahh, should have known, thanks!

[@Dudleydogg]

th eother videos always have the spacers in front the the OAG what would be the advantage / disadvantage of doing that. I know that if I keep my OAG/filterdrawer/camera all connected together its always in focus. This is different strategy, but looks sound.

[@noahhaverkamp]

I don't understand why it's so important to get exactly 133.35 mm back focus. Can't we just focus the scope a few mm or cm forward or backward to make up the difference? I don't know what the focus range is, but typically I am able to get my cameras to focus. However, whether or not they are capturing the entire FOV, etc. is a different matter. I am going to go ahead and order the celestron T-adapter anyway - I just mean theoretically.

[@davegibson6604]

That 44mm distance from the image sensor to the flange is Canon specific. I have a Nikon dslr that has a 46.5mm distance from the image sensor to the flange.

[@AstroBlender]

Yes, that's correct. I should assume most people don't realize this, and include that info.

[@chartspresents]

can i go 2 or 3 mm over proper back focus?

[@mileshuxley5462]

Would this work on a c6?

[@EugeneRu777]

👍👍👍👍👍👍👍

[@andrebremer7772]

You'll have a hard time focusing your guide cam given the large distance of the imaging camera from the OAG at 13:20. In general, it works better to bring in the camera to less than 60mm behind the center of the OAG to ease focusing of the guide cam - and then extend the distance accordingly on the telescope side of the OAG to reach the correct backfocus.

[@timothymaurer6720]

When shooting at f/10 I used the Celestron T-Adapter instead of the included SCT adapter. Otherwise, you'll need to get some sort of extension for your guide camera to reach focus.

[@EastSideAussieQuest]

Have just checked my 8" edge hd and found out it comes with a 50mm adapter which is threaded inside . Not sure what thread If I knew how to add photos to this , I could show you ..

[@screwyouyoutube5493]

Is the distances the same for the 11" EdgeHD?

[@AstroBlender]

No, see my C9.25, C11, C14 EdgeHD back focus video(s) for that setup.

[@expatca4444]

Do you think the ASI 2600 MC duo can aliviate the use of OAG ?

[@AstroBlender]

Yes, especially with the 0.7x reducer. I’m actually eyeing that camera for mine.

[@expatca4444]

@@AstroBlender Awsome ! Do a video please, that will be very appreciated.

[@gspinin]

M48 threads?

[@AshA-ww8hc]

Why won't they just sell adjustabe back focus rings, that can be screwed / unscrewed and locked to a certain length? Should be really easy.

[@AstroBlender]

I believe there are some out there that use a grub screw to lock in the length. The challenging part is millimeters are pretty small, so getting it locked down just right can be challenging.

[@Ronbo765]

Sadly, From what I've seen, you can't get the proper back focus with the reducer attached (105mm) + off axis guider, + a DSLR on a Celestron 8" SCT, but if using a dedicated Astro camera it isn't a problem.

[@AstroBlender]

From my talks with Celestron, the Celestron OAG shouldn’t be used with the 0.7x reducer. It was designed for 133 or 146mm of back-focus. Using it at 105mm will cause the prism to leave a shadow on the sensor, unless you use a small sensor. The ZWO OAG is recommended for use with the Celestron 0.7x reducer and 8” EdgeHD. This isn’t the case with the larger EdgeHD SCTs though, as they have the same back-focus with and without the reducer.

[@Ronbo765]

@@AstroBlender That advice seems erroneous as I've seen the reducer used with the Celestron OAG on Joes Astro "EdgeHD 8 Backfocus made Easy" and other places using the ASI 174 mini and the 294mc pro

[@AstroBlender]

I think the 294 would be fine. APS-C, probably not.

[@RedsRhetoric]

Hello AstroBlender. I just started watching your videos because I am about to drop a ton of money on a new set up. After some thought, I figured I could make an event out of it. So if you have time over the next few months, would you be willing to join me for a live-stream so we can "build" (order) my new set-up? I see you have an affiliate link, so that may come in handy. Let me know if this sounds like something you would like to do. Thanks.

[@clearskiesastro1028]

Please never use background music like that when talking. I thought I had a 2nd video playing somewhere in another tab lol

[@AstroBlender]

Funny you mention that! I’ve had several people in the past think the videos are too boring without music, so I’ll keep this in mind, thanks!

[@millsbobster]

100% agree -- love your stuff, Cody, but could not listen to this. your stuff isn't boring, and only boring pointless videos need background music. (musician & recording engineer here)

[@Goldiney]

I did this EXACT set up. Can’t reach focus (run out of play) and the distance to the Guide sensor is far, far off from the camera sensor.