Pairing a ZWO ASI294MC with a Celestron SCT - Back Focus and OAG Setup

[@hbmike47]

Hi James, Long term follower here. Remember discussing dew shields? Some Big problems here that you've probably long since figured out in the past 9 months but I'll bring up here since it's directly on topic to this video. Let's start with the most obvious. You saw immediately the vignetting issue as soon as you looked at the OAG guide camera and were surprised, wondering why you don't have good field illumination. Don't blame the OAG hardware! The image circle is restricted by the field stop created by internal free optical path of the 3.25 to SCT threaded adapter on the rear of the OTA. That's about 40mm(rounded for the sake of the math here). With a focal reducer, the image circle is reduced by the ratio of the reducer, so you're down to about 25 mm exiting the FR. Overlay the image circles with and without FR on the diagram at 18:13 in the video and its clear that what you're seeing is severe vignetting caused by the field stop and FR. Even without the FR, you will still get significant vignetting as you are riding the edge of the image circle with the current spacing and prism positioning. The 294's 4/3" sensor has a diagonal of 23.5mm. (ZWO modified usage of the Sony sensor) compared to the 1600's 21.9mm I won't get into baffle measurements, FOV and optics calculations in this comment but even with a 40mm clear optical path at the 3.25 to 2 adapter plate you'll get edge dimming at 139mm backfocus. I'm sure you'll run the calcs and look at FOV and understand what's going on better than most. One thing to note, the 9.25 primary mirror is F/2.5 native, not F2.0 like the C11 and 14. So that's the 1st pair (with and without FR) of "big problems". What has hopefully not driven you completely nuts is that there's NO (good) SOLUTION with this set of parts. The best compromise I can offer with these parts is to forget about the advertised 2350 F/L and the FR. Connect the OAG with the SCT adapter directly to the 3.25 to SCT rear plate threads. Yes, you're measuring optical distance so the SCT threads that thread into the SCT to OAG adapter need to be subtracted (-7.5mm if there is a gap, 10mm otherwise). I'm not sure where you got that table with 139 mm from, but I strongly suggest you remove the extra SCT spacer before the OAG and aim for 105 mm total. Every standard Celestron and Meade SCT with was built around 105 mm backfocus. Proof? Both brands have always supplied standard SCT to T2 Camara adapters that add exactly 50mm spacing. Add to that 55 mm which is the standard still kept to this day for cameras with their brand specific T adapters. I'm not sure which OAG you are using. The ZWO OAGs and cameras with the standard spacers supplied are based on that 55mm total camera side distance design which of course has to match the prism to guide sensor focal distance. Naturally some prism depth adjustment is needed to set it just outside the image sensor FOV and that means messing with the guide camera position. My suggestion is a helical focuser on the guide camera if you can adapt the OAG, otherwise a focus locking ring that's more secure than that single thumb screw seen so often 😢. But for other OAGs you may need spacers. I totally agree with your statement about keeping that part of the imaging train as a unit. Once the focusing and prism depth position is worked out its not something you want to change. I did see you used guide sensor size not FOV in your calcs for stellarium, but this is trivial as it affects only the boundaries displayed in the program which is close enough. Something though is quite wrong with what you've got set up in Stellarium. It didn't recognize the limits of FOV of the image circle of the OTA/FR combo and has your guider way further out from center than I'd expect. Anyway, I'll leave you to sort that out. It's all got to fit in the image circle, and clearly it's not accounting for the field stop of that FR/SCT interface and the FOV restriction caused by the back distance from it. So that's the best I can offer. Keep backfocus to 105 mm or there abouts to keep in the image circle. Just don't even try the older SCT FR/FF. Not unless you center crop and go with a separate guide scope. Also, forget about EDGE scopes. They don't support an image circle large enough for this OAG + Camera combination either. Is this maybe the source of some of your dissatisfaction with the 1600mm and nb results? What's a Good solution with other parts? Replace the 3.25 to SCT rear OTA plate with a solution from Astro Physics. They make a 3.25 to 2.5" (67 or 68mm) threaded adapter plate. Field stop is now the primary OTA baffle edges. This works for 9.25 and larger scopes. Sorry, doesn't work for the 8" scopes. A-Ps 2.5" telecompressor is supposed to do a very good job. There are of course several options. Never heard of any issues with their solution. It will fix the issue the right way. You need a big image circle to get a good SNR for your guider. You'll benefit from the flatter field for your imaging camera too. Your way ahead not being stuck with the Edge version optics. There's no good solution for those scopes and the A-P solution is cheaper than the edge FR that wouldn't work well for you anyway. Sorry I haven't kept up on YT. Love seeing your vids. Be well, stay safe, clear skies this coming year. Michael B

[@Aero19612]

Hi Mike. Welcome back and checking. You got in just under the 2023/2024 wire! Lots of good comments from you so I’ll try to touch on most/all of them: 1. I have a love/shrug relationship,with the focal reducer on the SCT. I really prefer to use native focal length for galaxies. The ~0.6FR does provide a nice alternative FoV for some targets, but the Celestron FR isn’t that great and I’m not that interested in dumping more money into the SCT. So I’m in the live and let live mode with that configuration. 2. The 139 mm back focus table is from Celestron (it’s somewhere on their website). I know what you mean about the 105 mm and the T-adapter. When I first bought the scope, I also bought the T-adapter for my DSLR. I used 105 mm with the focal reducer and without. I believe the 139 mm is appropriate for the native focal length and I agree that worrying about not getting 2350 mm is not an issue. Is the image quality much better/different at 139 mm vs 105 mm? Probably not. 3. Totally agree that guide sensor clipping is caused by light blockage from the scope. I have since lowered the prism by a ~2 mm. I’ve been at peace with the SCT. I like it and have gotten good-ish images from it. Nothing beats the focal length it provides…or the many challenges it presents. I feel no real gravitational pull to get an EdgeHD, so I’ll settle on a configuration and roll with that for a while. Have a safe and happy New Year, my friend! Jay

[@sergeyyatskevitch3617]

Thank you! Great analysis and a wealth io info I was looking for. Cheers!

[@ntldr2005]

Guiding is even more challenging with the Edge HD 8, OAG, and .7x reducer. The 8" reducer has a much smaller image circle (26.7mm) and obstructs the OAG prism by as much as 40% based on my estimate. Had I known this beforehand I may have paid the extra for the 9.25.

[@Aero19612]

Wow. I didn’t realize that. In that case,you definitely want the long dimension of the guide sensor to be perpendicular to the axis of the scope.

[@hbmike47]

The 9.25 is better, but not the HD Edge version. None of the Edge FRs have a large enough image circle to use with a FR, OAG and APS-C sensor. They barely support APS-C, so there's no room for the prism to get illuminated unless it's tucked up hard along the long edge of the imaging sensor FOV. That means you can't independently rotate the imager from the guider which can make it tough to find guide stars and frame your object. With the XLTs non-edge version you can change out the rear 3.25 to SCT adapter and go with an A-P or a couple of other solutions that yield a larger image circle. With the Edge you're stuck with limited options at huge expense.

[@Ronbo765]

I look forward to your videos. Absolutely a goldmine of insight. It's not all peaches and cream!

[@Aero19612]

Certainly not peaches and cream! Thanks for watching, Ron!

[@mrkutube]

Excellent video! It is very useful personally for me for the back focus, as I am going to build a similar equipment set up (also I am a beginner in practical astrophotography, for that reason only your video is a gem to me). Thanks James.

[@Aero19612]

Glad you found it helpful. Thanks for watching!

[@textandtelescope8199]

Gonna follow this closely. I have a C8 SCT which I have not used for a while and have been debating selling or just having it handy for observation for brother's grandkids. I will use my 2600mc as I sold my 294s and I don't have an off axis. Now, waiting for our "stars at night" to show up! Good stuff brother.

[@Aero19612]

Had one clear night last night and did some testing for a few hours before the clouds realized I had set things up. It's a struggle for sure. Thanks for watching!

[@robg735]

Excellent video James!! I have the 8se, OAG, Celestron FR, filter ring along with 533 mc …I haven’t been able to get focus yet …but could you 1) advise how a 2inch diagonal affects the back focus distances ….2) with 2x Barlow for example ….does it affect the back focus at all ? I’m really interested in planets eg Saturn at the moment …so 3) should the Barlow go before the diagonal eg attach directly to Celestron ? Thanks again for the very informative video…definitely subscribed!

[@Aero19612]

Thanks for the sub! The diagonal doesn't affect back focus, but it does take up a lot of distance that may cause you to violate the back focus of the optical system. For the diagonal path length, just add each leg of the light path to the mirror and then from the mirror "up" to the camera (shoulder to mirror horizontal + mirror to shoulder vertical). I have a 2x Barlow but haven't used it in a long time. For finding focus, maybe point to a bright/large Moon so you can see the out of focus light. Maybe start by removing the diagonal and just hold the Barlow along the light path until you see the imaging coming into focus. I do remember having to play with it quite a bit to find focus with the Barlow. I assume you have a camera in the Barlow? I would leave out the diagonal. Hope this helps some.

[@leeratliff7588]

This video is super-helpful because I have an almost identical rig (I have an 8SE instead of the 9.25”) and I’ve run into many of the same issues. I bought the Celestron OAG having done the math and knowing the back focus would be tight, but OK. Then I received it and realized the SCT adapter is way too thick and I can’t get down to the 105mm ideal distance. Now I’m considering two options. One, find a much thinner SCT-M48 adapter. Or two, Upgrade to the Starizona FR. The Starizona FR has shorter back focal distance (90.3mm), but it has integrated M42 threads so it can mate to the Celestron OAG without needing the thick SCT adapter. I’m leaning toward option #2 because I haven’t been happy with the optical quality of the Celestron FR. It’s poorly documented and the incorrect focal ratio highlights the fact that it was designed decades ago for film cameras. The Starizona FR has 5 elements and, by all accounts, far superior optical quality. The downside, of course, is the price. It’s $400. Also, it doesn’t thread onto the SCT, so it needs a 2” visual back. Cheap visual backs that use a single tightening screw are insecure and can introduce off-axis tilt, so most people recommend using the Baader ClickLock visual back. That adds another $100 or so. So about $500 total, but still far cheaper than upgrading to an Edge. Keep us posted on your results. They are directly applicable to my own setup.

[@Aero19612]

Hi Lee. What camera/equipment are you trying to attach behind the focal reducer? There’s not much, if any ,fat in my stack-up, but I do get the 105 mm. I’ve pondered the same philosophical “what if” scenarios as you. The question I haven’t answered is how much of my image quality is dictated by seeing/guiding vs the focal reducer quality? I’ll address my star shapes in the corners in my next video. Not great. But also not $400 bad either, especially considering typical galaxy targets live in the middle 1/3 to 1/4 of the sensor so I can just crop out the “bad” stars. As for the EdgeHD, I am tempted, but is my typical seeing going to erase the optical benefits? For now, the perfect is the enemy of the good and I’ll roll with this setup this galaxy season and “see what happens”. I use the ClickLock on my ED102. I also have the SCT ClickLock. It’s a great product. Fantastic holding power. Thanks for watching!

[@leeratliff7588]

@@Aero19612 Actually, you're right. I double-checked my math and it works now that I know I should measure the back focus from the top of the FR threads instead of the bottom. I'm using the ASI294MC Pro for imaging and the ASI174mini for guiding, just as you are. You should re-check your math on your pixel scale. I think you used 3.8um for the pixel size, but the 294 is 4.63um. That might affect some of your conclusions (not about back focus, of course).

[@Aero19612]

Yeah, that measuring from the back end of the threads is confusing. In that table, I used the advertised 0.63x for the reducer/corrector. In fact, it's more like 0.57x for my C9.25, so the pixel scale is 0.72 arc-sec with C9.25+reducer. At 0.72 arc-sec/pixel, that translates into 1.44 arc-sec for minimum FWHM (2x0.72 from Nyquist). So, if I get a higher FWHM from the images, then the pixel scale is OK. For example, last night seemed pretty stable and the best FWHM I got was 2.5 arc-sec (> 1.44). The seeing/scope is the weak link rather than the pixel size from a resolution perspective.

[@AstroQuest1]

This is such a good and informative video James. Well done! I have been using the ASI294MC with my Edge800 and 0.7 Celestron FR specially for galaxy season. The ASI294 sensor pairs up really well with the Edge for the reasons you already discussed. Your backfocus research was great. I had to play around with my backfocus and may explain why I had to add more distance (~4mm) to get decent stars. I may still have a little tilt but that is another story. The only thing I don't use is an OAG, I have been using the my Orion ST80 with an ASI120mm. Of course in theory the OAG is better since you are using the scope itself but the separate 400mm FL guidescope has a couple of advantages in that I never have to worry about finding stars even on objects in sparse regions. Also, I have it mounted on a second narrow dovetail with ADM adaptors which I can slide the full length of the scope so I can use to balance the rig. Cheers Kurt

[@Aero19612]

Hey Kurt. Finally got a few hours of no clouds and no Moon, if you can believe that. Spent most of my time dialing in the altered imaging train. That's one advantage you have with a guide scope - it's just plug and play. Meanwhile, I'm outside trying to dial in focus with the OAG. Plus, your EdgeHD is more geared to astrophotography. I've often wondered what I'm sacrificing with my non-Edge SCT.

[@jasontichy244]

I have the exact same lighting issue with my new ASI174 and the Celestron OAG. I have trouble keeping stars in PhD2 as they will be a mixture of oversaturated in the bright spot and barely noticeable in the dark region. Nobody has touched on this in any video before you did. Thank you for at least confirming that I am not crazy. I'd love to see a video of focusing the guide camera in the Celestron OAG and then getting guide settings done correctly. I have the C11 and CEM70EC, even with the encoders I have not achieved guiding sufficient enough for long duration images of galaxy.

[@Aero19612]

I had that very problem the other night. PHD2 adjusts the gain automatically. I had multi star guiding selected so I’m wondering if PHD2 ramped up the gain to see the stars in the dark zone, thereby blowing the “good” star in the bright area. Will try single-star guiding next time to see if I can just rely on the 1 good guide star. It’s disappointing that we can’t make use of a uniformly illuminated ASI174 sensor. Seems like an OAG design issue. I’ll have to repeat this with my ZWO OAG. The CEM70 is a nice mount, but the C11 is a beast. Very demanding. Maybe I didn’t confirm you aren’t crazy, after all, haha. BTW, I show a very sped up process of adjusting the focus of the OAG star in my latest video. I first adjust the in/out position of the guide camera, then lock the set screws. Then I use the helical focuser to dial in the smallest star size I can. I will be discussing a process for dialing in OAG parameters for Stellarium in the next video and will mention focusing then. Good luck with that C11, Jason!

[@andrebremer7772]

I previously used the same setup for my image train with comparable results (same OTA, OAG, but with 533MC). It always bothered me how much the guide camera stuck out in order to achieve focus. Subsequently, I took a different approach trading distances scope and camera side which ended up working better for me. First off, camera-side I replaced the M48 OAG adapter, with a 2mm M48m/M42m adapter ring and the included OAG M42-F adapter. That reduced the distance camera side for both guide and imaging camera by about 5.5mm. I added that distance back by using an SCT to M48 adapter scope side and various M48 spacers feeding right into the OAG M48-F adapter (instead of using the SCT OAG adapter). The result is a more stable fit for the guide camera (due to shortened distance), and a fixed setup from the OAG to the cameras regardless of reducer use. The necessary back focus distances can then be easily achieved scope side with M48 spacers alone

[@Aero19612]

Hey Andre'. Yes, that's worth considering, especially since my images with this focal reducer suggest I need to reduce the back focus. Like you, the logical way to reduce the back focus (not your goal, my goal) is to swap out the 12.5 mm OAG adapter with the 4.5 mm internal thread adapter and use an external-to-external M48 adapter (about 2 mm to 2.5 mm) to connect to the filter drawer. That leaves me about 4 mm to make up on the scope side if I want to move inward by 1 mm. Thanks for watching!

[@andrebremer7772]

@@Aero19612 Thanks for your response! I was able to test last night and achieved almost identical results to yours (104mm resulting in 1350mm focal length). Using the slimmer OAG adapter with internal threads got me down to about 58mm between the sensor and the OAG center line. That was enough to focus the guide camera without needing to attach additional spacers. In my case, I used the M42 OAG adapter camera side, since I will need the female M48 adapter on the telescope side to reduce distance there as well. Tonight, I am hoping to test a shorter 99mm solution to see how close it can get me to 1450. Although after reading everything, I am still not sure if we're just optimizing for correct focal length or actual image quality. I haven't seen noticeable changes in star shapes at varying backfocus lengths (yet).

[@davidmisisco]

Great video, James. I always enjoy your in depth analysis. Question: how did you measure the off-axis position of the guide camera? You stated the obvious inability to use a pair of calipers.

[@Aero19612]

Hi David. Go back to the video at 17:30, I explain the series of measurements and show the math there. Good luck!

[@davidmisisco]

@@Aero19612 thanks. It must have gone over my head. I’ll review it again.

[@Aero19612]

@@davidmisisco My diagram may be too simple. I open the OAG body and measure the inside diameter and then measure the distance from the inside edge to the bottom of the prism. And then the math starts...

[@davidmisisco]

@@Aero19612 ah, so there were some physical measurements. Got it now. Thanks.

[@sreeshab4093]

Thank you James. Great video. I have an Celestron Edge 8. And also 0.7x celestron reducer. Do you think the latest zwo 2600mc DUO will work well with this - both for imaging and guiding?

[@Aero19612]

You should be good to go! The 2600 is a good camera: larger sensor and higher image bit depth. The Duo is an interesting concept. Some targets have few guide stars, so you may have to rotate the camera to get that guide star in the field of view of the Duo guide sensor which will affect your framing. Probably won’t be an issue since targets with few guide stars are typically galaxies that won’t challenge the framing. Sounds awesome! Would love to hear your feedback on how the camera is working for you. Thanks for watching!

[@sreeshab4093]

@@Aero19612 Thank you James, appreciate your inputs and time. This really helps. Am in India so it will take time for orders to reach. That said, I will surely respond again after I do my tests. BTW, the astro season for us in Nov-Feb.

[@old_photons]

Great work as always, James. I have the C8 SCT (and their FR) and at the start of this galaxy season I'm questioning if upgrading the existing OTA with a better FR/corrector or if an EdgeHD or something else would be better performance and cost justified. The Starizona SCT corrector reportedly shames the Celestron version and was my first consideration. You may note its specifications shows different size SCTs will get different reduction ratios. My intuition has me thinking this makes more sense that it being fixed - disappointing Celestron can't publish clear information like this. I've contemplated guide cam rotation and appreciate your coverage of this topic so I need to relook this again. I've been chalking up the sensor shadowing to vignetting, as in the pre-dusk hours, I've noted the rounded edge of the aperture is apparent. Right or wrong, that has been my thinking. Always a trade space, but I've opted to keep the prism oriented to the long edge of the main sensor and as close as possible - of course this forces rotation of the whole setup vs full swing of the guide cam/prism around to get suitable guide stars - at least until the desired image composition is compromised.

[@Aero19612]

Hey David. I agree, the little aperture in the OAG stalk is the likely cause of the light contours in the guide cam. I should think the central bright spot would be in the center of the sensor. I tried to see if I could get it there, but no go. I’ve entertained thoughts along the Starizona/EdgeHD lines myself…with no resolution. For my seeing, I hesitate to throw a lot of money at such a long focal length. A better focal reducer or EdgeHD won’t solve that issue. If I were getting FWHM near 1 to 1.5 arc-sec maybe I would consider it more seriously. As it is, I’m getting a little over 2 arc-sec on the best of nights. Many people complain about the Celestron focal reducer and praise the Starizona version. For me, I use this scope for galaxies which are small in the field of view, especially with a focal reducer in place. Seems like cropping “bad” stars is a cost effective option compared to the Starizona focal reducer. But look who’s talking about conserving dollars; I just shelled out dough for a dedicated camera for my SCT. Let me know if you go the EdgeHD and/or Starizona route and what your informed opinion is. I might be right behind you. Thanks for watching!

[@old_photons]

​@@Aero19612 So I had multiple improvement ideas coalescing at the same time. I added an ASG Photon Cage to my camera and the Starizona. In retrospect I have yet to make the best possible images with both correctors to give the best results to compare. I am convinced that Starizona flat field is superior to Celestron's but I probably haven't taken enough trials to ensure "perfection" with the back focus on it where I spent considerable time with that as well as the tilt adjustment. Perhaps too much candy for a nickel for either the ASG or the Starizona if cropping the image anyway - I fully concede your point. But I'm hopeful as I progress with the ASG it will also help with my FLT91 setup as the draw tube was giving some movement/tilt before I moved back to the C8 for galaxy season. But I also got a replacement focuser/drawtube through Agena Astro/WO, which I've yet to reshoot with. Last point on the Starizona is that it connects with the Virtual View and not a threaded connection. VV isn't just screws but a clamping mechanism so I'm content and aids rotation (but Celestron OAG already had that) - it just makes me nervous to keep a tight hold as I adjust to avoid a drop. Long story but it is over :) Four clear nights in a row last week but back indoors for a while again.

[@Aero19612]

Sorry for the late reply, David. Wow! You went crazy on that issue. I had to look up what a "ASG Photon Cage" was. Very clever/serious piece of hardware. And I have not doubt that the Starizona focal reducer/corrector is much better than the Celestron FR/corrector I'm using. As you fight with tilt and backfocus, remember: "The perfect is the enemy of the good." It's amazing that WO and AgenaAstro were able to jump in and address your concerns. I buy most of my gear through AgenaAstro and they've helped me out in the past too. The VirtualView sounds like the Baader ClickLock. It's a very secure connection. I use one on my ED102 and also have one for the SCT but I'm not using it at the moment. Keep me informed on your progress. Are you using the "Aberration Inspector" in NINA's Hocus Focus to dial in tilt adjustments?

[@old_photons]

@@Aero19612 I think I would prefer the surety of the click in the ClickLock whereas VV has me double / triple checking screws - "I tightened that good, right?" :) Yes, I'm using Aberration Inspector. On the perfect / good enough front - great advice! BTW, have you seen Russ Croman's MTF Analyzer (free tool) something similar to what I've working on in Excel (with the addition of configuration tools to track my backfocus, equipment weight, etc). I found it useful and it led me to some setting choices for when to drizzle not (Bin or not or general scope/sensor pairing). Oh, and no progress by the way - nothing but clouds to image

[@sjpp71]

I'm also in this process with my C6 and a 294MM. So far, at 105mm back focus, stars look not good at all (like a star trek's federation badge, lol). My main concern is star shape, I can live with less focal length as long as stars look decent enough. Good luck (and yeah, very relatable: what did I get myself into?)

[@Aero19612]

I know what you mean about the Star Trek (NASA logo, really. Haha) star shapes, Sergio. I have plenty of them. I plan to go over that issue along with some other observations about the L-Pro and Antlia Triband RGB filter in my next video. I’m not as picky as most are with respect to the star shapes though. Taking pictures of galaxies with the C9.25+focal reducer leaves a lot of space around the galaxy, so I figure I’ll just crop those bad boys out during post processing… Thanks for watching!

[@RobertSmith-pf6lt]

Great info it helps me with my 8se. My question is how did you figure out the spacing for the guide camera?

[@Aero19612]

Thanks for watching, Robert! The distance from the center of the OAG prism back to the imaging sensor has to be the same as the distance from the center of the OAG prism to the guide camera sensor so both cameras will be in focus. There is a small difference because the guide camera will have non-filtered light. Anyway, for a long focal length scope, you’ll probably have to make final adjustments at night with a star in the field of view of the guide cam and then adjust the position of the guide camera within the OAG holder until you get good focus. With a shorter focal length scope, you can do this during the day with the main camera focused on a distant tree/roof and the adjust the position of the guide cam for optimum focus.

[@christopheguigot841]

trés bon tuto...bravo James.....c'est cool merci a toi...france.

[@DubbleD69]

Hi James - Thank you for this video. I have a celestron 6se, with the Celestron focal reducer and the same camera and filter drawer as you do. I do NOT have the OAG. For the life of me I'm unable to figure out what I need to attach the filter drawer to the focal reducer. I was assured by the place I purchased it from that it would fit but alas, it's too small to screw on. Are you able to provide me with any guidance? Thanks in advance. - Dave

[@Aero19612]

@@DubbleD69 Hi Dave. Your focal reducer (if it’s the same as mine) has an SCT thread that is larger than the M48 thread on the filter drawer. Go to Agena Astro and look up this adapter: “Blue Fireball SCT Female Thread to M48 (2" Filter) Male Thread Adapter # C-04”. It should provide the connection you need with minimal impact on back focus.

[@DubbleD69]

@@Aero19612 So I'm not going crazy! lol. Thank you so much for this!

[@Aero19612]

@@DubbleD69 Haha. Well, you ARE crazy. You chose astrophotography as a hobby. Your bad.

[@TheNakism]

James you may want to check out freestar8n's OAG calculator where he models the offset FOV in the guide cam

[@Aero19612]

Very interesting. Thanks for the heads-up Keith. I'll have to study it more, but looks like it will explain a few things I was wondering about.

[@anata5127]

If no corrector, is it backfocus for SCT? I am not sure. If you don’t keep backfocus according to these numbers, then image will not be blurred. It will just have different f.

[@Aero19612]

I’m pretty sure Celestron designed the non-EdgeHD SCTs with only the visual observer in mind and the EdgeHD series with astrophotographers in mind. The corrector on an SCT is a very mild lens. I suspect the only real effect of back focus is to get the optimum focal length with minimal distortion. I have used different back focus distances and can always focus. There may be more distortion in the corners when I’m not using the “correct” back focus.

[@anata5127]

@@Aero19612 Interesting. I could never get proper pictures with non-Edge SCTs. No matter focal distance, no matter seeing, no matter guiding. Non-Edge was great to learn. Now, time to move on.

[@KJRitch]

I've got the C8. I've entered the same numbers for the OAG you use for the ASI174MM Mini (which I own) and the same numbers for the ASI294MC Pro, which I'm considering purchasing. In Stellarium the ASI294MC rectangle corners exceed the inner circle. It doesn't fit inside the inner circle like in your video. I've double checked my figures for ASI294MC Pro; 19.1mm x 13.0mm, 4144x2822 resolution; OAG 13.20, 7.10, 11.30. How does Stellarium calculate the inside circle diameter. Thanks for any advice.

[@KJRitch]

In the Tool bar that allows you to cycle through different sensors for the ASI294MC Pro the dimensions are 51.29' 34.91'. In your video your dimensions are 45.12' 30.71'. Why are yours smaller? My rectangle representing the ASI174MM is the same as your example in the video. Thanks

[@Aero19612]

The numbers you're quoting are the field of view in arc-minutes. Mine will be smaller since I have C9.25 with a longer focal length as for how does Stellarium determine where to draw the circles...it depends: Which version of Stellarium are you using? If before V23, then enter the distance from the scope axis to the BOTTOM of the prism or chip (whichever is greater) in Prism/CCD Distance if v23 or after, then enter the distance from the scope axis to the CENTER of the prism in Prism/CCD Distance

[@KJRitch]

I have Stellarium 23.3. I have it on my Mac and a Windows 11 partition. In Windows its the same issue, I used your dimensions to set up the ASI294 with the OAG since I have the same guide camera in Oculars but the rectangle that represents the sensor still exceeds the inner circle boundaries. I measured the opening of the M42 male component of the OAG (the camera side) and is roughly 37mm wide which is more than wide enough for the ASI294 sensor of 19.1mm. Even an APS-C sensor would fit, 23.6mm. Are you running the same version of Stellarium? Thanks.

[@KJRitch]

I your video the pop up help for the Prism/CCD distance states "This is the distance of the imaging center to the bottom of the prism". In the 23.3 version the help pop up states "This is the distance of the imaging center to the center of the prism" I wonder if the if they changed the calculations. I guess the inner circle doesn't mean anything. Your calculations match the new help prompt text. I'm looking to purchase a OSC. I'm looking at the ASI533MC, ASI294MC and ASi071MC. I have the Celestron OAG (haven't used it yet) and a C8 SCT. I have the ASI174MM Mini. The 071 has an APS-C sensor of 23.6mm x 15.6mm. The diagonal of the 071 sensor is 28.4mm and half 14.2. The distance from the top of the 071 sensor to the bottom of the prism is only 2.6mm. Looks like you can only view guide star from along the four sides and not on the corners. Do you think using an APS-C sensor might be problematic? I like the ASI294 except for amp glow but some are complaining about calibration problems with this camera. Thanks

[@Aero19612]

@@KJRitch Yes, they changed the way the CCD/Prism Distance is calculated - see my first response to your initial comment. An APS/C sensor can work. You may need to raise the OAG prism up. You may still get a shadow, but Flats will correct for it. Darks calibrate out the ASI294 amp glow quite well and I never have any issues with calibration, so I don't know what the other users are seeing.

[@RL7282]

Love this video I have the same setup and constantly struggle with these numbers. Thank you for this. One very basic/ stupid question.....when shooting how does back focus play a role? Can simply adjusting rather focusing knob not help this? Is it only to enter in stellarium from a calibration perspective OR does this cause blurred stars when shooting? I.e. if back focus is not taken into account

[@Aero19612]

Thanks for watching, Raghav! Good question. You can turn the focus knob and get the center of the field of view in focus, even with the wrong back focus. However, the stars get increasingly elongated away from the center. If the stars are stretched radially, then you need to increase the back focus. If the stretch direction appears to be circumferential, then you need to reduce the back focus distance.

[@RL7282]

@James Lamb wow! Thank you for that speedy response will work on this and update

[@screwyouyoutube5493]

Hi James, I could use some information, what gain can you use with your 174mm guide camera? I cannot get mine to see any stars unless I use a gain of 400 and 3 second exposures. Is that normal?

[@Aero19612]

I let PHD2 control the gain automatically, so I don’t know what gain it’s using. I see plenty of stars at well below 1 sec exposure. You might try the camera in daylight with ASICap or SharpCap and Mel sure it’s working.