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To give a little background, I’ve been imaging now for about 3 years and have often used my C8/CGEMII with a 60mm guide scope with no real problems. I’ve often been warned about differential flexure and that an OAG was ideal for this focal length, which makes sense to me. I recently purchased an ASI 2600MM Pro to replace my 1600MM camera which of course led me to need to perfect my spacing due to corner stars being elongated. With the Celestron FR, I was never able to get it right and read that I likely wouldn’t be able to and that the Starizona Corrector was the best option. I decided to upgrade to the Starizona and was able to get pretty good stars at the 90.3mm + ~0.6mm (to account for 2mm thick Antlia filters) but the corners were still elongated. I assume that’s probably expected because of the C8 and not much can be done and I’m good with that. Then, I upgraded to a ZWO OAG-L and an ASI174mm mini to see if I could get a little better guiding. After using the same 90.9mm spacing with the OAG included, the stars are a mess across the entire field now, all elongated in the same direction indicating guiding issues I guess. My current thoughts are that I probably need to lower the prism a bit more as well as adjust the RA and DEC aggressiveness down lower which are currently set to 70% and 100%. I’m using 2s exposures for guiding at Bin 2, maybe I should use 1s instead? Just to test, I added 5mm to the imaging train and am now seeing much better stars in the center but still not good at all. All of this to really ask if anyone is using a similar combo and what your spacing ended up being? Here are some examples below. First being a 10s lum exposure at the 90.9mm spacing  Then, a 10s lum exposure (different target) with 95.3mm  And here’s a 120s Ha exposure still at the 95.3mm  Here’s my imaging train, which brings up another potential issue with tilt, the Baader click lock that I’m using to connect the starizona corrector, I don’t love that setup but not sure what else would work in this case. Also, I roughly checked collimation and as far as I can tell, that looks good but may need to be more rigorous in that testing.  My guiding error in AsiAir is typically showing about 0.60-0.70” in RA and 0.30-0.45 in Dec. I appreciate anyone’s help with this as I would love to get things going better for Galaxy season and hate to waste clear sky time on so much troubleshooting. |
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| My only advice in this and similar cases is to break down the problem in simple unique steps that don't present overlap of different issues. In your case I'd investigate the spacing issue, if exists, by pointing at the NCP (or SCP) without tracking and taking short exposures, 5s being sufficient. Now you can check if the spacing is really an issue and how well corrected are the stars. Once this is sorted I would tackle the issue with the guiding. Also make sure there is no flexure in the imaging train which is quite long. |
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andrea tasselli: Gotcha, thanks for the advice! I’ll try this next clear night and update the post. However, I may need to purchase some more spacers first… |
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Hi David, I have almost an identical set up. I wouldn't want to give you bad advice but it does look as though the elongation is in the same direction across the frame, top left to bottom right. It could well point to something being loose either in the imaging or guiding set up. It's happened to me when something is loose. Maybe worth just triple checking everything in daylight to make sure everything is tight. Sorry for giving basic advice but sometimes easy to miss the easy stuff. Many times I've given a Honer Simpson doh! but great really when you do because it's then an easy fix. I've sometimes tried short exposures unguided too. You could also try slewing in dec and then in ra for a short exposures to see if the direction repeats the elongation you are seeing. The Baader click lock works well for me. I usually fit the optical train with the scope pointed directly to the ground so that it holds the train as true as possible with gravity working with me. But once engaged, it holds tight. I do insert the train as far through the click lock as I can. Cs Steve |
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Steve Spring: Thanks @Steve Spring, I’ll double check everything tonight to make sure everything’s good and tight, very much appreciate the advice! Also, I like the idea of inserting the imaging train while the scope is facing down to ensure everything is lined up properly. Do you happen to remember what your spacing ended up being on your setup behind the corrector? |
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| Much the same as yours. I used the recommended spacing and added a spacer for filters. |
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 I have the same C8, Starizona Corrector IV, and ASI2600 camera.I setup my imaging train like this: SCT to M48 adapter: https://www.amazon.com/gp/product/B09JVMR455 Starizona SCT Corrector: https://starizona.com/products/starizona-sct-corrector-63x-reducer-coma-corrector ZWO M54 Male Thread to M42 T/T2 Female Thread Zero Optical Length Adapter # M54M-M42F-0: https://agenaastro.com/zwo-m54-male-thread-to-m42-female-thread-zero-profile-adapter.html Blue Fireball M54x0.75 Spacer Ring with 4mm Extension # S-M54-04: https://agenaastro.com/blue-fireball-m54-0-75-spacer-ring-4mm-extension.html Blue Fireball 360° CAA with M54x0.75: https://agenaastro.com/blue-fireball-360-camera-angle-adjuster-rotator-m54-thread-r-06.html Askar M54x0.75 Thread Spacer Ring / Backfocus Adjuster: https://agenaastro.com/askar-m54x0-75-thread-spacer-ring-backfocus-adjuster-with-variable-16mm-20mm-extension.html Askar Off-Axis Guider with 1.25" Holder and M42, M48 & M54 Threads # PAPI-AS-M54OAG: https://agenaastro.com/askar-off-axis-guider-with-1-25-holder-and-m42-m48-m54-threads.html Askar 5-in-1 M54 Filter Drawer # PAAR-AS-FD-M54-AS: https://agenaastro.com/askar-5-in-1-m54-filter-drawer.html]https://agenaastro.com/askar-5-in-1-m54-filter-drawer.html ZWO ASI2600MC Pro APS-C Cooled Color Astronomy Camera # ASI2600MC-P: https://agenaastro.com/zwo-asi2600mc-p-cmos-color-astronomy-imaging-cooled-camera.html Backfocus calculation is: M54/M42 adapter screwed into the M54 4mm spacer sticks out just a bit. My caliper measures: 4.8mm total extension. Blue Fireball CAA optical path length: 13.5mm Askar Backfocus Adjuster: 18mm +/- 2mm Askar OAG optical path length: 19.5mm Askar Filter Drawer: 18mm ASI2600MC: 17.5mm ----------------------------------------------\ Total: 91.3mm Now shorten the Backfocus adjuster by 1mm for no filter or by 0.3mm (or 0.35mm) for a 2mm filter. Use the backfocus adjuster to tune you corner star shapes. Elongated stars all pointing inward toward center = too close Elongated stars all pointing perpendicular to center = too far. I've found that 0.1mm - 0.15mm adjustments make a difference for me. Also, I've noticed a bit of elongation pointing inward in the corners on my APS-C chip that seems to be from chromatic aberration since the stars are blue on the closer edge and red on the further edge and grow in size with exposure length. Running BlurXterminator in Correct-Only makes them perfectly round. Lastly, I originally started with the Starizona Virtual View Visual Back but I struggled with tilt until I switched to an all threaded design. |
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John Stone: Thanks very much @John Stone, very good info here! I’m certainly going to look into eventually getting rid of the click lock adapter as I’m fairly certain it’s causing tilt in a different way each time I connect it making it impossible to fix. Very much appreciate you taking the time to list out your imaging train for me, this gives a lot to think about! |
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David, I have the same setup and started with the Baader Click Lock. I found 27 degrees of tilt was my problem. No matter how careful I was to get the imaging train "straight" I could not get rid of the tilt the Click Lock introduces. Another Astrobinner, Cesar, advised me to get an adapter that could be threaded (SCT to imaging train). It was good advice. Got the tilt down to 4 percent and the stars are round almost all the way to the edges. However, I never had any success with the Celestron OAG and the ASI174. I went back to the WO guidescope (200mm focal length) and the 174 camera. Guide figures are good and stars round. Too many times, especially imaging galaxies, I couldn't find guide stars. Current setup:  Guiding:  |
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Mike H: Thanks for the info @Mike H, I had assumed the threads inside the reducer were not quite long enough to support the weight of the imaging train but that may not be a bad way to go. I certainly like the idea of everything being threaded to reduce tilt. |
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I didn't know if they were either. Found out, they were  I got down to 4% which for me is nothing. Did a session with M51 last week. Cropped a little off the edge for stacking. Stars are sharp and round edge to edge. I'm amazed. Never dreamed I accomplished that with an SCT. Mike |
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Mike H: I think I’ve decided to ditch the OAG and go back to the guide scope as well. Just too much fuss for me and several targets I’ve tried had no guide stars available at all. I want to have fun with this hobby and not be fiddling with things in the middle of the night so whatever’s easier is best for me. Now I’m ready for another test run whenever I’m blessed with some clear sky time! Thanks again for your help! |
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John Stone, I found your post to be very helpful. In fact, I didn't know a backfocus adjuster existed. I ordered four parts off your list. Thank you for the specific links as I brought them up and right into my cart. $250 later I had what I needed to get the back focus distance of 90.3mm but also a camera rotator, a way to make small changes to the backfocus distance and the two adapters to attach them to my imaging train. It's perfect! I also have a super short 9mm visual back made by Baader so I'll have room for in and out focus travel. I haven't had it out yet but I'm sure this is the ticket. Thank you so much  Can't wait to try out the new Starizona Corrector. Mike  |
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Hello David and everyone, First thing: Ditch the Baader click-lock and substitute for a an SCT to M48 thinner adapter. This one for example in amazon is perfect for the job: https://www.amazon.com/gp/product/B09JVMR455 Screw it into the M48 threads of the corrector. This will put your optical train straight to your sensor. If you want to go with the OAG to do a better guiding, my sugestion is the optical train and before the OAG, to put a manual rotator, this will help you a lot in finding a good guide star, but in put it as much as you can right after the corrector in order to you have a perfect backfocus also in the guiding camera. I have the same OAG as you, so in my case this was the solution, I adjust all during the day in order to check if I could achive a focus. I did. Then if you have any doubts about the weight in the treads, honest and I say this from my heart because I did the same thing: ditch the filter wheel and put a filter drawer. Is more light weight, much cleaner regarding dust in a long term, and to be honest I doubt most people who do monochrome imaging, can do more than two filter per night. Think in acquire a better signal per channel, and do only one filter at least each night. You'll have better results and discipline all along the way. This way I think is a solid solution. You will avoid tilted images, a better backfocus, you can guide if you want with the OAG and you'll have a lighter imaging train. I sugest if you buy a rotator, avoid a cheeper one or the TS-Optics 360º Rotator T2 that allows you to take part of the imaging train temporary out. I have one of those, and he's not a paper weight. This model specific, he's well build and thin, but the way he's contructed put in my imaging train a lot of tilt and in a disform way, because of the lateral screws pushing and creating pressure in the threads of the central piece. Bottom line, just avoid that one and similars. Go for a more robust one. Keep us informed of your progress. Regards, Cesar P.S. - This is the one that I was talking about. My new paper weight: https://www.teleskop-express.de/en/adaptors-10/adapter-quick-changing-rotation-133/ts-optics-t2-thread-360-rotation-and-quick-changer-5-5mm-short-5547 |
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Mike H: Mike, Glad to help. I've got a couple questions about your setup. 1.) Exactly what Baader SCT adapter are you using? If it's this one (https://www.baader-planetarium.com/en/baader-sc-ultra-short-t-adaptor-for-c-581114.html) then I'm really confused because it provides a male T-2 thread but the Starizona SCT IV has M48 filter threads. How do you connect them together? Why not buy the $8 Amazon adapter I pointed out? 2.) Exactly what are the devices between your camera and the Askar backfocus adjuster? It looks like a filter wheel (what brand/model?) and maybe a tilt plate? then a spacer ring then your camera (which also has a tilt plate?). Is that a ZWO camera you're using? I've never seen an ZWO ASI camera with a red tilt plate. Is your camera a IMX571 sensor or something else? Starizona says their SCT Reducer/Corrector IV will cover a APC sized sensor, but I find that tilt and backfocus are very sensitive with the stars on the edge of my IMX571 ASI2600MC Pro camera. Super happy you got everything sorted. I've been waiting for a clear night for weeks as well. Let's hope they break soon so we can continue to try out the Starizona reducer! |
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John, 1. Baader SCT to T thread: https://alpineastro.com/products/baader-sc-hd-ultra-short-t-adaptor-7mm-optical-length I know you mean well but the Amazon piece you recommend is junk. It's poorly made and won't thread anymore than a quarter turn on the SCT threads. There are several bad reviews on that piece. However, stupid me bought it anyway and of course, it didn't work. I would not recommend it anymore my friend. 2. Tilt plate was only temporary. I found the adapter (spacer) I needed this morning. Revised train below. Filter wheel is a ZWO 5 position 2" model.    |
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| @Mike H Thanks for your info. I'm guessing you're using another M42 - M48 adapter on that Baader SCT to T thread to connect it to the M48 filter threads in the Starizona SCT Corrector IV? |
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Here’s a different option that I use. I posted this picture on different thread regarding achieving focus with the Starizona reducer on a C8 so you may have already seen it. I’m getting reasonable results using a Televue SCT adapter, the Starizona reducer, Starizona SCT 35.3mm corrector adapter, ZWO tilt plate, ZWO LOAG, ZWO filter wheel, ASI2600mm. This gives the image train 90.3mm backfocus. I don’t add 1mm to accommodate the filter thickness because, if I recall correctly, Starizona states nominal +/- 2mm. In my case (and perhaps being overly cautious), I hesitated to use the threads of the reducer to support the image train instead of a visual back, they don’t appear adequate to me and Starizona only mentions them in the instructions for filters. Another comment, on some old C8s, (mine is ~1976 or so), the T2 threads are cut slightly different on the rear cell and in some cases won’t work with modern (non Celestron brand) T2 threads. I had to modify mine.  |
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| @Marc Dickinson interesting. Don’t you have tilt problems from that thumbscrew slightly moving/tilting the starizona corrector when you clamp it down? |
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Looks like everything I would've said has been covered, so just to add a +1 to them:
Make sure all things in the optical train are incredibly strong and can handle the weight. Also, I found a guidescope on an SCT unusable, giving terribly trailed stars even when the guiding was tight. If you decide to go back, the trick to make an OAG a simpler and pain-free experience is to program your information into stellarium (there's a section specific to off-axis guiding prisms) on a cloudy night, test the distances with platesolving when a known object is in the pick-off prism (like a planet or very bright star), adjust the values, and now you know exactly which guide star you're aiming for. when you move the imaging train to the target in stellarium. I could never imagine trying to randomly catch a bright enough guidestar with the OAG; that would be a nightmare. My guidestar is planned as carefully as the target ahead of time, so when I slew to the planned coordinates, the guidestar pops right into phd2 as expected. |
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John Stone: The Televue visual back has a brass compression ring and two tightening screws, but yes, there’s probably a little tilt, it’s not too bad, probably no worse than the corner distortion. It also makes image rotation easy.  |
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Thanks for posting the picture of your setup Marc. My orange tube is an '81. The threads are standard SCT fare. The threads on my Starizona are more than adequate for the train attached. An analysis shows 4% tilt. Certainly good enough for me Mike |
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This configuration does not work. The camera will not come to focus. The correct and adjustable back focus is 90.3 give or take. Do I have the wrong idea trying to get the Starizona corrector as close to the scope as I can? The directions from Starizona is to get it as close to the scope optics as possible for the correct reducing effect. I've just about had it with the Starizona unit. Many have had good success with it but I have not. It could be the orange tube scope it's connected to. Maybe the focus travel is not as great on mine as it is on more modern C8s? I've had much greater success with the Celestron Corrector/Reducer. |
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Mike H: Mike, as you probably recall, mine was mirror travel...I outlined my fix from your earlier thread,https://www.astrobin.com/forum/c/astrophotography/deep-sky-processing-techniques/galaxy-is-not-sharp/ it now works great, but if you try what I did, take care not to run your mirror off the end of the baffle, especially with an EAF. Also, I could reach focus fine with a Celestron reducer, it was only the Starizona I needed more travel. |
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Thanks for your input Marc. I'll have to see if my scope has the same focuser that can have the screw and washer stop removed. You said you got five more turns. I'm not sure how many I would need to reach focus. You say yours has another safety stop to keep the mirror intact. I don't know whether mine does or not. Can I tell by just removing the focus knob? That Pacman nebula you took was very good. I'd like to achieve similar results. Mike |
