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SCT OAG Vignetting/Reflections Help

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#26 Scott Mitchell

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Posted 05 April 2020 - 06:23 PM

Scot,

Before spending money on a new focal reducer or OAG, I would consider going to a guide scope. I quit using my OAG when I went from film to digital.The time I save focusing and acquiring a suitable guide star is significant and better spent improving polar alignment.You already have a guide camera, so all you need is a guide scope which can be had for less than $100. By liminitng the length of your lights an taking more of them you can minimize the vignetting from the focal reducer. The concerns of differential flexure, much discussed i've never foumd to be a problem.

 

 

attachicon.gifM13.jpg

 

This is an image obtained with Celestron Ultima 8 and Orion SSAG. Lights were 240 seconds at iso 800 (using Celestron focal reducer,) You can judge for yourself the quality of the guiding and Vignetting.

Thanks for the suggestion ELDavis. I do use a guide scope successfully with my refractor based setup. I wanted to avoid that with the SCT for 2 reasons: avoiding the issues related to mirror flop, not overloading my mount due to the extra weight. I'm positive I would have to upgrade my mount if I went that route. So, hopefully I can get this working. If not then I'll kinda be back at square one to evaluate options for longer focal length imaging.



#27 Scott Mitchell

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Posted 05 April 2020 - 06:42 PM

OK, the definitive way to determine if you need a different focal reducer (like an Optec) is to measure the image circle on your current FR.
 
Install the FR and nothing else on the scope.  Aim the scope at a bright outdoor scene with the rear of the scope heavily shaded.  Easiest way to do this is aim it outdoors through a window at a bright scene.  Turn off the room lights and pull the drapes around the scope to keep light from coming in.  If that's not possible, you might have to use a blanket over the rear of the scope instead of a darkened room.
 
Now get a piece of white paper or cardboard and hold it up behind the focal reducer.  You should see a circular patch of light.  Move it back and forth until the scene is in focus.   Put a couple marks on the paper with a pencil on opposite sides of the circle, then measure that to get the diameter.
 
If it is 28mm or more, then another focal reducer won't help with your stalk positioning.
 
-Dan

Dan, I went through this exercise, and while it is hard to be super precise with this, it appears that I'm between a 26-28mm image circle with the current reducer. 
 
The CCDT67 looks like it might present a larger image circle which could be helpful:

  • Optimized for 0.67x compression with a fully illuminated circle of 29 mm when the back spacing distance is 85 mm from the rear flange to the camera sensor is used
  • Expect a 0.75x compression with a fully illuminated circle of 34 mm when the back spacing distance is 50 mm from the rear flange to the camera sensor

 

I think my next steps are to do more experimentation at f/10 to see what I can learn. I question how well I'll be able to guide at that focal length, but a real world experiment is in order. 

 

 

I looked at the flat and light you supplied, and it is certainly not a perfect match. 

 

Something worth considering, is how have you taken your flats?  It's possible that your flats are just not optimal.  It might be worth taking flats again and/or trying a new method for acquiring to see if you can correct for the shading. 

Chris, I'll also do some more experimentation with my flats. The reference image appears to correct almost perfectly. The interesting question to me at the moment is what is different between the various lights that might cause an issue. I actually wonder if there are reflections that are shifting as the night progresses.

 

Regarding how I take the flats, I use an LCD set to a white image then cover the scope with 2 layers of white t-shirt material. I'll have to do some more experimenting on that front, as what I've succeeded with with the refractors might not scale up successfully here. However, ultimately I think there's some variation in the illumination of the lights that is causing trouble as well.



#28 Midnight Dan

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Posted 05 April 2020 - 06:58 PM

>> Expect a 0.75x compression with a fully illuminated circle of 34 mm when the back spacing distance is 50 mm from the rear flange to the camera sensor

 

Based on the size of your OAG in the side-view photo, it doesn't look like you can get the camera sensor that close to the focal reducer.  But you'd have to measure to be sure.  But yeah, if you can get that extra 6mm of image circle, it would help with your stalk location.

 

I agree that using the system at f/10 and seeing how it works out is a good next step.  I do it with my Edge8 and it is indeed challenging.  Imaging DSOs at 2000mm makes you very susceptible to seeing issues and guiding is definitely harder.  You need a good, sensitive guide camera to be able to find guide stars.  A larger guide camera sensor can be a big help here too, but not with the ZWO OAG because it has a relatively small pickoff prism and won't fully illuminate larger sensors.

 

-Dan



#29 Scott Mitchell

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Posted 11 April 2020 - 01:02 PM

Just closing the loop on this. I got another clear night last night (finally). So, I was to grab new lights and then spent some good time working out flats this morning. I think I'm on a productive path now. The big miss on my part was removing the dew shield. This time I kept it on and was very careful to try to mitigate any potential light leaks around the shield, given that I was now taking the flats in the day time. These flats are correcting the lights much more accurately now. I still have the full integration running so the jury is still out on exactly how well they match, but I know I'm much better off now. Here's an illustration of what I'm seeing (just screen shots out of APP):

 

Single light:

oyK39esb_CtAx_joqLN4VR9M7kD2GDXNwP-KoSG1

 

Master flat:

eJuN91tzwW6KW_zTKhBWhIp0CMpO7Ddr6zLquM10

 

Integration of the two:

O-_uyWAeoSn6vlCP76K64TQvXNEmnrfm7HuGJURj

 

So, thank you all for working this through with me. I'm much more hopeful about what I'll be able to do with this setup now. I will be getting a more appropriate focal reducer, and I'll see how far I can get with the ASI120MM. 

 

Thanks!



#30 Scott Mitchell

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Posted 23 April 2020 - 01:00 AM

Yet another update... I picked up the Antares f/6.3 Focal Reducer and removed 1 spacer to reduce the spacing as much as physically possible with this setup. I can't quite reach their optimal 85mm spacing (I'm probably closer to 90 from my rough measurements). However, the image circle is enough larger now that I can withdraw the OAG stalk enough to stop impinging on the light path to the DSLR. I'm still only illuminating about half of the guider's imaging chip which isn't ideal, but at least I'm used to that. 

 

The field isn't entirely flat with this setup. Stars only the extreme left and right edges of the images are stretched vertically (ie. not pointing to the center of the field) which seems to jive with the incorrect spacing. However, I wasn't sure how important spacing would even be given that the reducer claims not to be a corrector. The other slightly confusing thing is that plate solving shows that I am only down to f/6.6 which seems to imply that I my spacing is less than the optimal length? For the record though, I'm not overly concerned about either issue.

 

Image calibration is *much* improved now. My flats still aren't perfect, but I have an LED panel arriving later this week that will hopefully give me another incremental improvement. Some of my issue may also just be severe light pollution gradients that I haven't managed to fully correct. I'm not used to doing broadband imaging from home, and the light pollution is really bad.

 

I'll post up a new image later today that shows the results of this move. 

 

Thanks again for everyone's help!



#31 Scott Mitchell

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Posted 23 April 2020 - 05:56 AM

Here's an example of a flat-corrected single sub out of the new setup and the master flat itself to show the level of vignetting (and lack of OAG stalk). Both of these have had a stretch applied by APP.
 
Corrected Sub: https://photos.app.g...gVdr3UcCcp8xtv6
zfPOaN0g8kUgWgp7d-dh6vGnzwFyALYx8GBlexJ3
 
Master Flat: https://photos.app.g...8awA6RVpwXLRau8
2d-v8X1RLHgFnJ7Uq94iiLmdsuZ8zbqxN1hMKZ5u


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#32 EXT64

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Posted 23 April 2020 - 07:26 AM

I too found the reducer can make a big difference.  I went to a Starizona corrector for my C9.25, and to my surprise (without changing the depth of my OAG stalk) it no longer has a visible impact on my images or flats.  Only a subtle vignetting outside of the 27mm advertised illumination circle.  And that kept the OAG well illuminated too.


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