Schmidt Cassegrain Telescope Imaging

Good idea, I'll start. 

 I am a newbie to SCT imaging. I have set up a C11 edge HD for imaging at native focal length 2800 mm in f10 

Controversially, I am using a zwo am5 strain wave mount.  I think many people would say this is undermounted for such a long focal length. However, I have achieved 0.5 Arc, second RMS and FWHM of two arcseconds with this arrangement in good seeing.  Also, I am not ready to drop $5k+ on a "pemium" mount, so this is how it will be. 

 One difficulty I came across in creating this rig was adding Auto focusing.  It sounds trivial, but there are factors that complicate things: mirror flop, and backspacing.

 1. Mirror flop.  Zwo makes an adapter that allows direct attachment of their eaf the native focuser knob. This seems like an easy solution.  The problem is that mirror flop causes image shift and failure of the autofocus routine. I've lost at least one night of unattended image capture data due to failure of this setup to find focus. Reading the forums, this is a known problem. The easiest solution is to use a rack and pinion or crayford SCT focuser between the field flattener and sensor train.  However, this is complicated by backspacing issues.

2.  Backspacing.  The edgehd series have a narrow range of backspacing for best flat field performance. On the C11 this is 146.05 mm +/- 0.5 mm.  Celestron makes a 91 mm adapter so that a sensor train with standard 55 mm back focus reaches the 146 mm exactly. The adapter can be replaced with an inline focuser to allow for auto focusing. Several premium focusers are available that work for this purpose, Moonlight makes one for example, but these can be quite expensive. The more reasonably priced GSO SCT focusers are too long. I did find an affordable short SCT focuser on Alibaba that works well, and I am very happy with the build quality. I can share details if anyone is interested.  With the inline focuser in place, the native telescope focuser then needs to be set, and the mirrors locked, at the point where the focal plane is at the proper back spacing of 146 mm. At that point, fine adjustment of focusing can be done through the inline focuser only by the eaf.

With the auto focusing problem solved, I think I am set up to give Galaxy season a go. Will be posting results here if we ever get some clear nights.

@Jeremy Wright @Dean Ostergaard when I got my C11 Edge Hd it came with a Starlight Feather Touch Micro Focuser and a Moonlite CHL focuser for the imaging train. I purchased a bracket from MWD Astronomy to attach to the moonlite for my EAF focuser. If you are not aware of this guy he makes various custom made clutches and  brackets. I would recommend checking his stuff out if you haven't come across him yet. Starlights focusers seem very nice quality like the moonlite.

With the moonlite focuser  I have no mirror flop. It does introduces other issues, what would astronomy be without problem solving . I think I have a astigmatism and or potentially tilt. When looking at the corners of the image I have oval stars in a corner or two not all. Now I take forever to process my data. I'm working on an image from 2 years ago well before I got this scope. so I haven't dug deep into the SCT data to see what is going on. 

For back focus I bought a caliper. the moonlite focusers tube extends roughly 20mm so when calculating the back focus I wanted 146mm land at the tubes 10mm mark. I use an OAG which landed me at 7mm.  I have to use the caliper to get me close enough to 146mm to get my autofocus to work. Anyway I bought a Husky caliper from Home Depot which works fine. I found other uses for the caliper that I didn't realize I may need. handy not required.

Something I have struggled with and don't think I have gotten perfect is collimation. I have tried a half dozen times maybe more. I think the images are sharp but i will only know when I get to processing the data. When collimating the stars (at various magnitudes) all seem super muddy. I read many collimation forum recommendations. I got rid of everything in the imaging train and used the Celestron spacer and camera only (modified canon 6D). I know seeing will play a role in this which as per Astropheric It was at average or mostly below average seeing.

This winter for nebula season I just focused on learning how to image with an SCT, learning The SkyX and my Myt Mount all at the same time so didn't have high expectations of getting usable images collected. I may have come away with one at best. 

I started with cleaning the corrector plate without removing it, just the outside. I got 99% alcohol and distilled water and it left a film. So I move on to other chemicals like formaldehyde and something else. All too scary for me and I did not want to ruin my scope before I used it. I'm not totally happy with the result but I think it's acceptable for now. If anyone has a lot of experience with this I wouldn't mind getting together. I will bring the chemicals and pec pads!

Brett Joslin:
@Jeremy Wright @Dean Ostergaard when I got my C11 Edge Hd it came with a Starlight Feather Touch Micro Focuser and a Moonlite CHL focuser for the imaging train. I purchased a bracket from MWD Astronomy to attach to the moonlite for my EAF focuser. If you are not aware of this guy he makes various custom made clutches and  brackets. I would recommend checking his stuff out if you haven't come across him yet. Starlights focusers seem very nice quality like the moonlite.

With the moonlite focuser  I have no mirror flop. It does introduces other issues, what would astronomy be without problem solving . I think I have a astigmatism and or potentially tilt. When looking at the corners of the image I have oval stars in a corner or two not all. Now I take forever to process my data. I'm working on an image from 2 years ago well before I got this scope. so I haven't dug deep into the SCT data to see what is going on. 

For back focus I bought a caliper. the moonlite focusers tube extends roughly 20mm so when calculating the back focus I wanted 146mm land at the tubes 10mm mark. I use an OAG which landed me at 7mm.  I have to use the caliper to get me close enough to 146mm to get my autofocus to work. Anyway I bought a Husky caliper from Home Depot which works fine. I found other uses for the caliper that I didn't realize I may need. handy not required.

Something I have struggled with and don't think I have gotten perfect is collimation. I have tried a half dozen times maybe more. I think the images are sharp but i will only know when I get to processing the data. When collimating the stars (at various magnitudes) all seem super muddy. I read many collimation forum recommendations. I got rid of everything in the imaging train and used the Celestron spacer and camera only (modified canon 6D). I know seeing will play a role in this which as per Astropheric It was at average or mostly below average seeing.

This winter for nebula season I just focused on learning how to image with an SCT, learning The SkyX and my Myt Mount all at the same time so didn't have high expectations of getting usable images collected. I may have come away with one at best. 

I started with cleaning the corrector plate without removing it, just the outside. I got 99% alcohol and distilled water and it left a film. So I move on to other chemicals like formaldehyde and something else. All too scary for me and I did not want to ruin my scope before I used it. I'm not totally happy with the result but I think it's acceptable for now. If anyone has a lot of experience with this I wouldn't mind getting together. I will bring the chemicals and pec pads!

I think you get some nice gear with that set you picked up. Those are some pricey focuser accessories, but I'm sure they are top-notch.  

 Re collimation, I've spent tons of time perfecting Newtonian collimation, but new to SCT.  My understanding is there's no good way around waiting for a night of high quality seeing to do an in focus star test.   You can try an artificial star but you need a long separation with a C11, more than I have easily available.

Regarding cleaning, I try not to touch the large optics unless they're very dirty since the common wisdom is that doesn't degrade the images too much.  I did have to muster the courage to finally clean filters and the camera sensor housing because of obnoxious dust motes on every image.  Flats are great, but I can say now from experience that a clean sensor train is way better.

 We have to prod you to start processing some of that old data and see what you've got!

Brett Joslin:
I’m working in an image now and should have something soon haha. Trust me I would like to move on.

If you ask me, images are never done. You can always use more integration time, another run at culling and stacking, more tweaks to the workflow, and better adjustments to the histogram, curves, and layers.

I am trying to remember if I have ever posted anything since my earliest images that I didn't consider a work-in-progress.

BTW, I am sitting here waiting for a drive recovery software to finish scanning two dead external drives that contained almost all of my raw data from 2024.  I just bought a NAS to prevent that happening again.

Protect your data!

So question about collimating a SCT. Seeing aside, my understanding is you go to both sides of focus starting at a larger donut, call this rough collimation. Then you reduce your donut just beyond focus, zoom in on the donut this being fine collimation. Again working both sides of focus. The other piece is exposure being careful not to over saturate your stars and using a magnitude 4 star.

I look at images on the forums, people have these nice well defined circular lines which I have yet to see. I always chalked it up to bad seeing.

Am I on the right track?

Maybe I am just trying to do too many things at the same time. 

I had the C8 SCT out several nights back and I was unable to achieve focus using an EAF neither "manually" by using the move in and move out buttons in NINA, nor via the Autofocus function.

I could get close with HFR in the high 4s and low 5s but the next small step would take me past what should have been best focus and HFR would increase again. What was more frustrating was that the stars never stopped being donuts, albeit very small and sharp ones. When well out of focus the collimation seemed pretty good to me looking symmetrical and even all around.

The odd thing is that I had successfully imaged with this exact same configuration a few months ago. The calculated back focus, or more accurately back spacing, is 125mm which is 2mm short of Celestron's recommendation of 127mm for a C8 SCT.

Given that there are no corrective optics in the standard SCT I didn't think 2mm would matter much. Maybe last time I had not tightened the extension rings and T-adapter as much resulting in a slightly different backspacing?

I was about to order some M42 spacers for this but thought I'd toss it out here to see what others think before throwing money away on something that might not work.

Is HFR in NINA given in arc seconds or pixels?  If pixels then I think four to five isn't bad for an 8" SCT, since you are sampling at something like 0.5 Arc seconds per pixel.   I can't explain the donuts though.