Celestron SCT Back Focus Study at Native and Reduced Focal Lengths 

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

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.

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

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! :-)

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

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.

Excellent work!

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.

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!

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.

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...

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!

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.

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

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.

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.

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

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.

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

Can I apply this configuration to my C5 nexstar

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?

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.

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!

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.

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.

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?

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.

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

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