Celestron SCT Back Focus Study at Native and Reduced Focal Lengths

[@Ricklflyfisher]

I have an Antares SCT 0.63 reducer which has been described as being identical to the Celestron unit. It was described by the manufacturer as having an 86 mm backfocal distance . Less than a week ago I tested this on my C8 using a QHY 533M camera and found that a backfocus of 87mm gave me my best results. I have followed up on your excellent suggestion of swapping out the 25mm SCT adapter by purchasing the Fireball adapter in the hope that I could then add my filterwheel to the train. Great video. I enjoy and learn from your work. Thank you.

[@Aero19612]

Very interesting about the Antares focal reducer and its back focus. Thanks for watching!

[@ronlundgren4391]

Thank you James. I’m just moving to a OAG with my classic C8 and you’re videos are most helpful.🍻

[@Aero19612]

Great! I'm glad they're helping. Good luck!

[@DanPechiu]

James, I'm glad you were able to reduce your back focus and improve star quality. At this point, your Celestron OAG prevents you from reducing back focus any further. You happen to have the absolute thickest OAG available. I also have the same 1mm long SCT to M48 adapter and the ZWO filter drawer, but I went with the Orion Thin OAG. At only 10.5mm thickness it allows me to reach 86mm BF measured from flange/shoulder of the reducer (77mm from where you're measuring). Optical people will tell you back focus is measured from the middle of the FR lens, and that's where the shoulder is. The total thickness of the FR is 34mm. The scope side edge to glass distance is 13.5mm. The camera side edge to glass distance is 0.5mm. The thickness of the glass is 34-13.5-0.5 = 20mm. The shoulder is at 10.5mm from camera side edge. Therefore the shoulder is 10.5-0.5 = 10mm from the camera side glass edge, which makes it the middle point of the glass. Best of luck! :-)

[@starman6092]

Hey James, this video is coming at the perfect time for me. I have been sitting on my new 9.25 SCT for about 6 months now and finally decided to set it up this week and your analysis of back focus is just the info I needed as I was originally going for the 105mm. Now I'm going to aim for 80mm and see what stars I get. I also purchased the Blue Fireball SCT/48mm Adapter last week and expecting it any day now. Thanks for the video. Eddie

[@Aero19612]

Great! Let me know what you find if you're able to use a back focus less than 85 mm.

[@johnmacdougall4545]

James...you don't get more practical than "...awesome is not going to happen here...". I love your videos. Data driven analysis

[@Aero19612]

All I know is that “awesome” ain’t happen’n HERE. Perfect is the enemy of the Good.

[@mrpaille]

Hi James, nice analysis. I tried myself with an homemade optical testbench. I found it is easier to use for example a flashlight at infinity (so approx 1 meter of the focal reducer) and move a whitepaper until light is perfectly focused, then measure the distance between focal reducer and focal plane (backfocus). You can use this method without the scope so it is easier and quicker than doing it at night with the scope. But I could be wrong.

[@Aero19612]

Sounds like a great idea...especially with it cloudy outside. Thanks

[@michael.a.covington]

The focal length of the focal reducer is NOT the distance that it should be from the sensor when used as a reducer. If it were, it would not form an image at all! There are a few pages about the optics of focal reducers in the first edition of my _Digital SLR Astrophotography_ (not kept in the second edition). Key idea: A focal reducer does not take an existing image and make it smaller. It intercepts light that is converging to form an image and makes it converge more, to form an image in a different place.

[@davidpendley5224]

Excellent video. Thank you for testing the SCT back focus. I have been trying to improve my astrophotography. Based on Celestron data and your test results, my native back focus is way off (55mm). Stars aren't too bad but I can never get a super crisp image of the target. I suspect that Pixinsight stacking is aiding the star shape. I am going to shift to your back focus length and give it a try. Thanks again.

[@anata5127]

Have you tried Starizona version of reducer for conventional SCT? When I had NexStar 8SE, Starizona reducer gave massive improvement over Celestron SCT.

[@Aero19612]

No. I don't have that reducer. I'm sure it's better.

[@wadeprunty]

Great information! I've got an EdgeHD 9.25 and then two C8s, and I've done a lot of this myself. I also use an OAG with my setups, but I use the ZWO OAG. The ZWO-L gives you a 12mmx12mm prism, but takes up less space in the image train. To state the obvious, less is better, because you can always add more spacers. With the ZWO-L you could probably get down to your 77mm on the reducer setup and then add spacers to work your way back towards the 85.3mm to find the optimal distance...if you wanted to go through the trouble. But, no matter what, your stars looked pretty good for a non-corrected SCT. Now on to the headache of removing all the tilt!

[@Aero19612]

Haha. The SCT is a balance of image quality and the amount of money/time to get there. I find that I’m more on the “I’ll take what I get” for my current level of investment. I’ll be lazy and BlurXterminator do the rest! Thanks for watching, Wade!

[@claytonwilson4730]

Thanks for the video James. I believe you’ve answered some big questions for me. I'm using the standard 8-in Celestron and the COAG. I found when I started using the OAG and precisely calculated my back focus to 105mm, that my edge stars got worse. This has been baffling me for quite a while. I never assumed that the published back focus spec could be wrong. I'm going to try some of the techniques you show here to reduce the back focus and see if I can get those corner stars looking better. Thanks for the work you did.

[@Aero19612]

Great, Clayton! A problem is there is no "published" back focus number. Seems logical to assume Celestron means 105 mm because they sell a T-adapter and T-Ring for DSLRs that's equal to 105 mm. Good luck!

[@andrewoler1]

Great video. @@Aero19612 I'm running into this same problem and I'm actually using the 105mm T-adapter and T-ring with a DSLR (as you mention). I get horrible stars in the corners. Is there any other option of options with a DSLR to get a back focus of 80-85mm that you're aware of? Thanks.

[@Aero19612]

@@andrewoler1 I think you can unscrew your T-adapter (tube) from the T-ring (the T-ring has the lens interface for your DSLR). Then you’ll have about 44 mm from the camera mounting surface back to the sensor + 11 mm (or is it 10 mm?) for the t-ring. Now you just need about 30 mm to get in the ballpark. You’ll need an SCT internal to M42 external thread adapter and probably another M42 adapter. Welcome to adapter hell.

[@andrewoler1]

@@Aero19612 Thanks, I got the SCT to M42 and some M42 spacers. Just waiting for a clear night to try it out! Thanks for the advice!

[@Aero19612]

@@andrewoler1 have you tested different back focus distances yet? If so, what did you conclude?

[@HeavenlyBackyardAstronomy]

Hi James, The back focus for my EdgeHD 11" drives me crazy. With the 0.7x reducer, the back focus is 146mm. With that, I am just about all the way back with the focus motor value of 2990, and the stars in the corners are still fuzzy. After watching your video, I am going to dedicate a night to work on this.

[@Aero19612]

I would have hoped the EdgeHD series would be better behaved. Sounds odd that you have to move the mirror all the way back. Certainly worth dedicating full Moon night to experiments. Let me know what you learn - I have no experience with the EdgeHD scopes.

[@chrislee8886]

Excellent work!

[@Aero19612]

Thanks for watching!

[@spz5687]

Hello James - I've seen similar trends with my C925 over two Celestron reducers. I've settled on a back focus of 92.5mm and a FL=1400mm. My recent purchase of the Starizona SCT FR gives a FL=1481 (F6.3) and better star shapes at their specified back focus of 90.3mm.

[@Aero19612]

Thanks for watching! I’ve heard great things about the Starizona FR. Who knows, maybe I’ll give in and buy it one day. I’m trying to resist…but the Force is strong with that one.

[@OlliesSpace]

Must admit I always found the basic Celestron reducer really not great but I also never did this level of testing. I eventually got the Starizona one which is very reliable and gives very good stars. I appreciate however the price tag between the two is substantial which could push someone just to move towards the Edge version. Very thorough video.

[@Aero19612]

Yep. I'm definitely trying to get blood from a turnip with this thing. The Starizona option is the way to go. Thanks for watching!

[@abmscopes]

Fascinating! I don't have your mathematics brain, but I came to the same conclusion and back focus as you have through trial and error. Interestingly though I long ago gave up with the Celestron reducer and my results apply to the starizona 0.63x reducer. At the starizona recommended back focus of 95mm I was getting the same star shapes as you do. After many lost nights fiddling with spacers I eventually found that I got pretty much perfect round stars at all edges by removing every spacer I possibly could and I ended up at around 85mm. A full 10mm less than it 'should' be. So maybe this is more of a c9.25 issue than a reducer issue? I feel fully vindicated now anyway after your video! Even though I'm happy with my results, I've been assuming I'm doing something wrong by settling on such a significantly different back focus than the 95mm it should be!

[@Aero19612]

Thanks for watching! I’ve heard a lot of people saying great things about the Starizona reducer/corrector, so you should do very well wit( it. Like you say, the C9.25 might have a good bit to do with the back focus issues people encounter.

[@MrGuilletv]

Hi james,thanks for the video. back focus with SCT standard + celestron reducer + celestron aog + reflex is really hard... 55 mm of backfocus from the canon makes its imposible, i have the agena astro adapter

[@Aero19612]

What is the 55 mm you mention? Back focus is determined by the lens, which in this case is the focal reducer.

[@MrGuilletv]

@@Aero19612 sorry I mean 55 mm between chip and t ring of the camera

[@yvangarcia3535]

Great job, Now I need to do it in my C8 (along with Askar M54 AOG + Canon 600D not modified) as I am using the 105mm backfocus and of course, I have alongated stars !!! I will make some modifications, if I can. Clear skies.

[@Aero19612]

@@yvangarcia3535 Good luck! Remember, don’t let “the perfect” be the enemy of “the good.” Sometimes we get too zeroed in on the star shapes and forget to look at the image as a whole.

[@DrunkferretKG]

Great work! I love BlueFireball stuff too! I have the 8" non-HD and will be looking into this with it. Yes i fell in love with the simplicity of refractor too. I wonder if the manufacturers will get to 200mm refractors in the consumer market. 😂 it would be soooo heavy...

[@Aero19612]

I do wish the Blue Fireball spacers had a thicker wall, but hey are very useful. Thanks for watching!

[@earthling-fh2mg]

Very interesting and useful video. I have an 8 inch Lx10 meade sct and an svbony 305c camera. I have been exceptionally frustrated with very soft focus capturing jupiter for my early attempts at doing so either at native or with my 0.63x reducer/corrector. I have been on the forums and people have repeatedly said “seeing” and collimation but do you know what I mean when I say one knows when it is neither? You know your collimation is good and you know your seeing should be allowling for better images. So, recently, I was capturing again and, while I was as well focused as I could be, on a night of good seeing, I happened to remove the camera for some reason and, when I did so, I caught a glimpse of the laptop screen and noticed, during removal, that I hit a point of even better focus than I was getting while the camera was fully inserted in the focus tube. It was somewhat of a shock and surprise but it was real. So, on pushing it back in, I didn’t fully push the nosepiece all the way and I found, on processing later, the focus, clarity and detail I’d achieved was improved significantly from previous sessions. I now know that even though you may think your in best focus, using the focuser and an inline helical focuser for fine focus, unless you have the back focus right, you’re not going to truly achieve focus. So now, I have to experiment to find out the precise back focus I need at native, reducer/corrector and when using my barlows.

[@Aero19612]

Thanks for watching! It's frustrating to deal with this fine tuning but it does pay off. Good luck!

[@dirkschwarze7478]

I am using a back focus of 77mm with my Celestron 0.63x reducer, which surprisingly is approx 105mm from the base of the thread where the reducer is screwed on. Platesolving determined my focal length to be 1483mm. So maybe Celestrons recommendation uses the same reference point with and without reducer. I am using no OAG with the reducer, because the image circle is too small to fit my APS-C Camera and the 8x8mm prism.

[@dirkschwarze7478]

BTW.: with 77mm, the stars a really good....

[@michael.a.covington]

Extremely useful study! Thank you! I wonder how tightly Celestron holds the manufacturing tolerances on the compressor, and also how much SCTs vary from manufacturing lot to manufacturing lot.

[@michael.a.covington]

A few technical remarks. (1) As you know, we should be looking back at the Celestron camera adapter and SLR camera bodies with T-rings, which are 55mm flange to film. Celestron photographic accessories were originally designed for film SLRs. The 0.63 reducer was designed to have the same camera adapter behind it (same back focus) as the telescope used natively. My recollection is that for the C8, this is 105mm, but not very critical when used natively (another way to say that is you're not going to get rid of the field curvature). (2) You're using a C9.25. My impression was that the 0.63 reducer was designed for the C8, though they now advertise it as compatible with all their SCTs. Each of them, natively, has some field curvature, which is partly dependent on back focal distance. But they are not the same. It's not surprising that the C9.25 works best with a different back focal distance. (3) Your big point is that the best back focus should be determined experimentally and can be surprising!

[@Aero19612]

I think most people are finding that the Celestron-recommended back focus for their 0.63 reducer of 105 mm is not ideal and is closer to the 85 mm. I'd like to see them take this old product out of circulation and introduce a properly designed replacement. Of course, they do this for their EdgeHD line. Oh well.

[@alejandrohenriquez7316]

I went to Agendastro and saw several adapters but I don't know which ones I should buy

[@luboinchina3013]

Celestron recommends to focus on stars that are between the mIdQqdle and the edge of the field of view in order to get the best star shapes. I used to have standard sct, but the coma was awful. Sold it, bought HD and never looked back. Celestron corrector doesn't correct for coma. Sell it and get the Starizona corrector/reducer. Also how about the Hyperstar? I also zwo OAG and cannot complain at all.

[@Aero19612]

The EdgeHD line is definitely the way to go. Wish I had known that when I started in this hobby. The C9.25 was my first scope... Oh well. Thanks for watching!

[@MrDirtyRod]

Reminds me of my struggle to get my Edge8 backfocus right. Love those BlueFireball adapters and spacers. Just curious why you are not counting the camera backfocus. IIRC, Celestron is measuring to the actual sensor. ZWO cameras usually have the sensor set back from the front by 6.5mm or 17mm so, when you add those in, you are much closer to the factory numbers. My C8 needs 106.6 for near perfect stars which includes the camera backfocus.

[@Aero19612]

I am including the camera sensor setback of 6.5 mm. See table at 3:45 in the video. It's the first entry in the table.

[@MrDirtyRod]

@@Aero19612 My bad. Not sure how I missed it. Sorry.

[@Aero19612]

No worries. I'm the guy who forgot to turn off the automated sprinkler system one night and watered my scope, so forgetting 6.5 mm would not be the worst thing I've done in this hobby.

[@devilsheadcoffee4149]

Have you ever used the starizona .63 focal reducer/flattener? I have a very similar telescope, oag and camera configuration, and would hope their distance of 90.3 mm for the back focal distance would be more accurate than the celestron reducer. Thanks for the detailed video, it is much appreciated!

[@Aero19612]

No, I don’t have the Starizona reducer/corrector, but I’ve heard very good things about it. Just not sure I’m interested in a another $400 to $500 investment in this scope. The perfect is the enemy of the Good…. I may just settle for “good” in this case. Thanks for watching.

[@anucha98]

Can I apply this configuration to my C5 nexstar

[@guntergeorgi1347]

Hi Jay, great video again. I have a similar problem with my Edge and can not reach the suggested backfocus distance. But do not have very enlongated stars in the corners. Is the suggested backfocus for an Edge more strikt as for non Edge SCT's?

[@Aero19612]

Hi Gunter. Yes, the back focus is more critical for the EdgeHD OTAs because the Edge has an optical lens in the center tube that the non-Edge OTAs don't have. Sounds like you're getting pretty good images though. Better than what I'm getting!

[@Ronbo765]

Random thoughts: How can you rule out tilt in the mechano/optical train, collimation error, sensor tilt and coma? Also, changing the backfocus may introduce aperture loss based on what I’ve read on Cloudynights. Binoviewer people have to live with it. Have you used ASTAP or NINA’s aberration analysis tools? Whats next on your agenda?

[@Aero19612]

Haha. I'm sure all of those issues are present to some degree in the image. ASTAP shows some tilt. Collimation would affect the whole image. At some point when changing back focus you'll run out of improvement and be left with the remaining issues. I do see an improvement when going from 95 mm to 85 mm, so I'm assuming that's back focus. But the remaining star shapes are not great so coma and tilt are likely contributors. Next on the agenda? Hmm. I've got some additional observations on Celestron's dew heater ring and its effect on stars -- maybe we can start to find a solution for that. I've also got to share my results with the Antlia H-beta Sii filter. I've also added a few 3D printed parts to my refractor setup that I'll go over. Always something.

[@alejandrohenriquez7316]

Hello! Excellent video. Who can help me? I have a Meade LX200 10" and I have the Celestron 0.63X reducer. Could any of you give me the adapters, Drawer Filter and OAG that work best for me to achieve focus. Thank you, I await your responses.

[@Aero19612]

Hi Alejandro, I don’t anything about the Mead equipment. If you are using the same focal reducer as I have, I decided that a back focus around 85 mm works (check with my video to be sure). As far as what filter drawer, etc that works “best”, I’m not sure. You might that the Large OAG from ZWO doesn’t take up as much back focus as the Celestron OAG. Just add up the numbers, like in the video, to see what adapter lengths you need to make up the difference. Good luck!

[@alejandrohenriquez7316]

@@Aero19612 Thanks!

[@ObservaDome]

👍James, Thanks For Your Work👍 🔭Explaining A Complex Subject🔭 🪐I Will B Setting Up A C-11 XLT🪐 🌜Your Insights Should B Helpful🌛

[@Aero19612]

Thanks for watching. The C11 is a beast. Good luck!

[@thierrymartin8378]

This work shows the fact that more you are far from the ideal focal length, worse the picture with the reducer. The most important is the focal length of the reducer. Small is better. Up to 300mm focal length your are fine. Above the object of the focuser will be too far away from the ideal Focus length . And you can't get nice picture from ugly object

[@Aero19612]

@@thierrymartin8378 Haha. I think this work shows that Celestron’s focal reducer is not (was not) designed for later SCT designs that are used for astrophotography.

[@TheNakism]

James where did you get the "Celestron" value of 105mm from? I have never seen that stated by them in any official document or manual. But maybe I missed it

[@qonos68]

Neither did I, and this is really a shame for Celestron not having included this information in their product's datasheet. There are literally tons of posts on Cloudy Nights on this issue. Maybe 105 comes from the sum of the lenghts of Celestron's SCT to T2 adapter (50mm) + T2 to camera adapter (10mm) + the standard distance from the camera flange to the sensor (45mm), which gives 105. Incidentally Svbony produces a 30mm length SCT to T2 adapter, which gives a total length of 85mm. I'm going to try it.

[@Aero19612]

I don't think I've ever seen that number from Celestron. In my earlier video on this topic (kzbin.info/www/bejne/bGnLgp6ggth9o7csi=uizKbjfOJO39Lcvw) I infer the 105 mm back focus from Celestron's T-Adapter/T-Ring used with DSLRs. When you put those parts together, you get 105 mm. Have no idea why Celestron won't clearly state the number.

[@TheNakism]

Agree they should state this. But at the same time I think one should be cautious saying Celestrons number is wrong since that attributes the 105mm to them and perhaps incorrecly, It could be the case of someone inferring way back when and everyone ran with it as official. Heck even some vendors state this on the product page, maybe they've got this from Celestron but who knows

[@Ronbo765]

I’ve seen it in their EdgeHD whitepaper

[@TheNakism]

That's for the Edge reducer, not this one