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CPC 800, 6.3 reducer and ZWO ASI294MC. Most efficient optical train?

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#1 chummy

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Posted 10 October 2021 - 02:30 PM

I am frankly a bit confused how to get to this 105mm optical train from the reducer to the camera.  Can anyone help? I also have a small assortment of 1.25" filters.  Should I stay at 2" or can I incorporate the filters along the way without sacrificing anything? So what goes between the cam and reducer?



#2 dcweaver

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Posted 10 October 2021 - 02:48 PM

I read somewhere that Celestron designed their standard SCT T-adapter to give 105 mm when combined with a T-ring/camera that gives 55 mm backfocus (i.e DSLR).  For a Zwo camera, their site shows several options for how to get 55 mm with their cameras and supplied spacers.

 

https://www.celestro...66afc9fbb&_ss=r



#3 Lastinline

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Posted 10 October 2021 - 03:24 PM

If you have a t2 to 1.25” thread reducer (that comes included with some flavors of the 294mc), you can use that to place the filter directly in front of the camera window.

#4 mfratto

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Posted 10 October 2021 - 03:24 PM

The working distance of the F6.3 reducer is 105mm. The camera has a distance of 10.5mm from the sensor to the top of the mounting ring, so you need to make up the difference of 94.5mm. The T-Adpater is 55mm, so you need to make up 39.5 mm. Find adapter rings that will get you there.

 

You can also just call up the astronomy stores, tell them what you have and let them find the parts. None of the on-line stores make finding spacers easy.


Edited by mfratto, 10 October 2021 - 03:28 PM.


#5 dcweaver

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Posted 10 October 2021 - 09:02 PM

Good info in all the replies.  Here are a few articles on focal reducers that may help.

   

https://agenaastro.c...cers-guide.html

Good general information

   

https://skyandtelesc...focal-reducers/

Confirms the 105 mm design spacing - Thanks Charles (Chuckwagon) of Utah

   

https://www.cloudyni...k#entry10909906

Some work I did on the effect of spacing with a GSO 0.5x reducer

   

https://astronomy-im...cus-length.html

Several ways to get 55 mm backfocus that adds to the 50 mm spacing of Celestron's standard T-adapter to give 105 mm

   

That's a lot of information, but the take away is that unless you have an EdgeHD series, there isn't a single "correct" backfocus spacing.  You can tune the reduction with longer or shorter distances than the design value of 105 mm.  Longer spacings will provide more reduction, but they introduce more aberrations.  To clean up the view if there are problems at 105 mm, make the spacing smaller.



#6 Noah4x4

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Posted 11 October 2021 - 12:49 AM

I don't have a CPC, but I suspect you can probably stop worrying about achieving the exact 105mm desirable optical train. This is NOT critical on SCT telescopes where focus is achieved by moving the primary mirror.

I spent months fussing with this challenge and hence struggling with cameras that wouldn't offer sufficient rear end clearance on my 8" Evolution when affixed in a straight 105mm optical train. Eventually, I tried a Celestron 2" dielectric diagonal behind the 6.3x focal reducer that considerably extends the optical train. Guess what, perfect focus was still achievable with the added benefit that due to the extension, I now enjoy circa f/5.1 rather than f/6.3, so my scope is a little "faster".

Achieving 105mm is critical with most other types of focuser, but on a typical SCT the range of travel of a primary mirror will suffice. I have read that a range of 75mm to 140mm will work fine with a typical SCT with moving mirror, but not tried the extremes. I know folk that use either 1.25" or 2" dielectric diagonals that won't deliver 105mm and they work fine with an SCT. I hence doubt if a CPC800 differs.

Edited by Noah4x4, 11 October 2021 - 12:51 AM.

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#7 alphatripleplus

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Posted 11 October 2021 - 07:57 AM

For  EAA,  I don't worry about operating at the quoted working distance of 105mm. I would suggest a little experimentation with different working distances and the camera and take a look at how sensitive aberrations towards the edge of the FOV are to changes in working distance. For smaller sensor cameras (compared to the 294), you can vary that distance quite a bit before edge aberrations are noticeable - they may be more noticeable with a large sensor like the 294.  With the 294, you'll probably find vignetting is also an issue.



#8 Mark Lovik

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Posted 12 October 2021 - 03:31 PM

For  EAA,  I don't worry about operating at the quoted working distance of 105mm. I would suggest a little experimentation with different working distances and the camera and take a look at how sensitive aberrations towards the edge of the FOV are to changes in working distance. For smaller sensor cameras (compared to the 294), you can vary that distance quite a bit before edge aberrations are noticeable - they may be more noticeable with a large sensor like the 294.  With the 294, you'll probably find vignetting is also an issue.

Not to hijack (much) on this thread -- alphatripleplus -- I know you currently stack reducers on your SCT for galaxy imaging. I am configuring my old Meade 8" SCT for EAA and purchased a single 6.3 reducer for it.  My camera's are much smaller than the x294. 

  • Currently I can hardware bin and get reasonable results with the single reducer - but that feels lame. 
  • Getting the core system working - can plate solve most of the time, now working on a mask and indicator knob for accurate manual focusing.  Can easily collimate the SCT when needed.
  • Have had success with some globulars and the fireworks galaxy, but I am interested in pushing what is possible for the scope and camera combination.
  • I am using color and mono versions of the xx178 image sensors - about a 9mm diagonal and 2.4um pixels.  I am looking to get down to the range of F3.5 to maybe F4.5.  Currently I image on a 72mm APO at F4.8

 

Looking for any guidance (I know there are a number of SCT EAA users out there that have fiddled with stacked reducers) on how to best start this process.  Just sent the order out for the second reducer -- hope to have it in a week or so.  I have a modest set of M42 extension tubes that have not been claimed for other purposes -- may need to enhance this in the future.

 

Initial plan

1. Dual 6.3 reducers stacked together.

2. Standard Celestron - SCT-T adapter  (50 ish mm back focus?)

3. Add M42 spacers to fiddle with back focus -- maybe start with the Camera directly on the Celestron SCT-T adapter, then incrementally add extensions to the back focus.  

4. Image a few different objects, and use plate solving to calculate the effective focal length and field of view.

 

No interest in adding filters initially, just trying to get a baseline EAA system running first.

I have a sneaking suspicion that flats will be fairly important for this system (expecting some vignetting here).

.


Edited by Mark Lovik, 12 October 2021 - 03:45 PM.


#9 steveincolo

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Posted 12 October 2021 - 03:39 PM

Not to hijack (much) on this thread -- alphatripleplus -- I know you currently stack reducers on your SCT for galaxy imaging. I am configuring my old Meade 8" SCT for EAA and purchased a single 6.3 reducer for it.  My camera's are much smaller than the x294. 

  • Currently I can hardware bin and get reasonable results with the single reducer - but that feels lame. 
  • Getting the core system working - can plate solve most of the time, now working on a mask and indicator knob for accurate manual focusing.  Can easily collimate the SCT when needed.
  • Have had success with some globulars and the fireworks galaxy, but I am interested in pushing what is possible for the scope and camera combination.
  • I am using color and mono versions of the xx178 image sensors - about a 9mm diagonal and 2.4um pixels.  I am looking to get down to the range of F3.5 to maybe F4.5.  Currently I image on a 72mm APO at F4.8

 

Looking for any guidance (I know there are a number of SCT EAA users out there that have fiddled with stacked reducers) on how to best start this process.  Just sent the order out for the second reducer -- hope to have it in a week or so.  I have a modest set of M42 extension tubes that have not been claimed for other purposes -- may need to enhance this in the future.

 

Initial plan

1. Dual 6.3 reducers stacked together.

2. Standard Celestron - SCT-T adapter  (50 ish mm back focus?)

3. Add M42 spacers to fiddle with back focus -- maybe start with the Camera directly on the Celestron SCT-T adapter, then incrementally add extensions to the back focus.  

4. Image a few different objects, and use plate solving to calculate the effective focal length and field of view.

 

No interest in adding filters initially, just trying to get a baseline EAA system running first.

I have a sneaking suspicion that flats will be fairly important for this system (expecting some vignetting here).

.

There are at least three old threads on this.  Errol and Roel did the ground breaking.  I just started a couple nights ago with stacked 0.63x reducers, C8, and 178MM.  Back focus of 87mm worked for me. 
 

https://www.cloudyni...stacking-at-c8/

https://www.cloudyni...focal-reducers/

https://www.cloudyni...ith-a-lodestar/


Edited by steveincolo, 12 October 2021 - 03:57 PM.

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#10 Mark Lovik

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Posted 12 October 2021 - 04:02 PM

There are at least three old threads on this.  Errol and Roel did the ground breaking.  I just started a couple nights ago with stacked 0.63x reducers, C8, and 178MM.  Back focus of 87mm worked for me. 
 

https://www.cloudyni...stacking-at-c8/

https://www.cloudyni...focal-reducers/

https://www.cloudyni...ith-a-lodestar/

Thanks -- this is what I am looking for!


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#11 alphatripleplus

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Posted 13 October 2021 - 08:10 AM

Yep, those topics that Steve mentions do discuss stacking f/6.3 reducers with a relatively small sensor on SCTs. The problem here is that the OP has a relatively large 294MC sensor, and I am almost certain the stacked f/6.3 reducer approach will yield unacceptable aberrations across the FOV of such a large sensor.


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#12 Noah4x4

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Posted 14 October 2021 - 01:26 AM

Yep, those topics that Steve mentions do discuss stacking f/6.3 reducers with a relatively small sensor on SCTs. The problem here is that the OP has a relatively large 294MC sensor, and I am almost certain the stacked f/6.3 reducer approach will yield unacceptable aberrations across the FOV of such a large sensor.


Concur with this. I gave up on the twin stacked reducers experiment, albeit using a 1.25" diagonal clipped away some vignetting. But what then is the point of using a 4/3" sensor?

I get far more satisfactory results with my ASI294 by keeping a 2" optical train, hence 6.3f/r > 2" Celestron Dielectric Diagonal > Camera (with 2" nosepiece from my Atik Horizon). The optical train vastly exceeds 105mm, but that offers circa f/5.2. With an SCT you adjust focus by moving the primary mirror. You can therefore relax and stop fretting about chasing a precise 105mm. This resolves a myriad of other challenges, like rear end clearance on especially long cameras like the Atik Horizon (also 4/3").

I also use Hyperstar at f/2. That is the easiest form of imaging as it is lightening 'fast', but many DSOs are then too small. Using a 6.3x focal reducer but extending optical train to adjust to circa f/5.2 is a great compromise. I discovered this after giving up waiting for the x0.4 NightOwl which is also unsuitable for large sensor cameras.

Edited by Noah4x4, 14 October 2021 - 01:31 AM.

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