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C8 with f/6.3 reducer backfocus help with zwo 533

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#26 ERHAD

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Posted 22 April 2021 - 08:11 AM

ERHAD,

 

... having read thread after thread after thread on f/6.3 SCT focal reducers (Celestrons and Meades and Antares, oh my) recently while researching in preparation to get one for my C8, I have come to only 2 conclusions:

 

1) The topic is a giant morass of confusing, conflicting information

 

 

Amen!


Edited by ERHAD, 22 April 2021 - 08:11 AM.


#27 ERHAD

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Posted 22 April 2021 - 08:13 AM

In fact, ERHAD, it sounds like your scope/reducer combo is even more out of spec, as you have a C925 not a C8. I believe, according to Celestron, the correct back focus for the 925 is actually 117mm from the base of the camera side reducer SCT threads. That is a full 32mm longer than what you are using!

That is probably true. Let me just state, however, how extraordinarily happy I am with my out of spec setup...lol.gif smile.gif


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#28 AhBok

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Posted 22 April 2021 - 05:18 PM

The only significant number here is the distance from the focal reducer to the chip. For Celestron FRs, that is 105mm. Variability is only in the +_5mm range for optimal performance. Meade and other reducers such as the AstroTech have different backfocus requirements.

The backfocus stated for the C8 is dependent on the position of the primary mirror. The spec was originally set at the focal point using the stock SCT 1.25” visual back and the stock prism diagonal. This was the point where the focal ratio was 10 and the focal length 2032mm. As soon as you mount the FR on the rear port, the position of the primary changes in order to reach focus and the focal length is changed by .63x. But what is x? It is no longer 2032 because the position of the primary has changed. This is why platesolves focal length varies from what some think it should be. The SCT focal length changes within a range depending on the position of the primary.

Any attempt at forcing a specific focal length by varying the backfocus distance of the FR de-optimizes the system. Just set the length to 105mm and accept the final platesolved focal length for what it is.

Just the ravings from an old-timer! My feelings are not hurt if you do not believe me. (Lol!)
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#29 ERHAD

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Posted 23 April 2021 - 03:00 AM

The only significant number here is the distance from the focal reducer to the chip. For Celestron FRs, that is 105mm. Variability is only in the +_5mm range for optimal performance. Meade and other reducers such as the AstroTech have different backfocus requirements.

The backfocus stated for the C8 is dependent on the position of the primary mirror. The spec was originally set at the focal point using the stock SCT 1.25” visual back and the stock prism diagonal. This was the point where the focal ratio was 10 and the focal length 2032mm. As soon as you mount the FR on the rear port, the position of the primary changes in order to reach focus and the focal length is changed by .63x. But what is x? It is no longer 2032 because the position of the primary has changed. This is why platesolves focal length varies from what some think it should be. The SCT focal length changes within a range depending on the position of the primary.

Any attempt at forcing a specific focal length by varying the backfocus distance of the FR de-optimizes the system. Just set the length to 105mm and accept the final platesolved focal length for what it is.

Just the ravings from an old-timer! My feelings are not hurt if you do not believe me. (Lol!)

While what you say is probably very true, there are other considerations at play.

 

First, is focal length/reduction: by following the 105mm BF rule, my setup is reduced a bit above f/4. Framing is a consideration, one might not want such a "wide " field; one images with a SC for the long focal length after all. This is my galaxy/planetary nebula setup...

 

Secondly, there is the issue of off axis performance, and I can attest that the aberrations are considerably worse at 105mm back focus than at 85mm. Using an of axis guider is made difficult and the stars are completely deformed at 105, much improved at 85. 

 

Again, all the above is true FOR MY SETUP and yours might vary wildly. My whole point since the beginning has been that simply recommending 105mm back focus to everybody "blindly" might not be the best piece of advice since it is clear that there are tolerances at play here and that it could induce problems for certain setups that might be difficult to understand/identify for a newbie who is following the "common knowledge"  as gospel.

 

Not trying to be controversial here...Thanks for reading.

 

Erik.


Edited by ERHAD, 23 April 2021 - 03:02 AM.

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#30 WadeH237

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Posted 23 April 2021 - 07:06 AM

Secondly, there is the issue of off axis performance, and I can attest that the aberrations are considerably worse at 105mm back focus than at 85mm. Using an of axis guider is made difficult and the stars are completely deformed at 105, much improved at 85. 

I fully accept your results, but I'm not sure that I agree that the explanation is variability in tolerances.

 

We're talking about an optical component that's fully in the light path.  Any issues with the optics would completely spoil the resulting field.  I find it difficult to swallow that different batches of units would be able to produce an acceptable field of view, yet be off by 20% in the spacing requirements.  That would mean that two examples that differ by this much, have different figures on the lens.  That's not impossible, but it would be pretty rare on an item that is produced by machines that crank out the same thing over and over again.

 

Something is definitely going on with the different results that people get.  It would be interesting to compare the complete imaging trains of two systems that demonstrate the extremes of spacing.  Since it's a purely academic question, though, there would need to be two people willing to give up a night or two to testing, and could tightly control variables, test different spacings and evaluate the results.


Edited by WadeH237, 23 April 2021 - 07:07 AM.

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#31 Paul AZ

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Posted 20 July 2021 - 05:31 PM

I have my image train set up but need to test it. With the Celestron OAG and FR I am at 104.55mm

 

Here is my set up from rear of C9.25:

  1. Focal Reducer
  2. Add back focus:
    • Female SCT to T2 male adapter = 8.4mm
    • OAG female T2 = 4.5mm
    • OAG body = 29mm
    • Male T2 OAG adapter = 12.5mm
    • ZWO filter drawer 2 inch = 21mm
    • OAG medium T2 spacer = 11.55mm
    • Meade DSI IV = 17.5mm

Total backfocus is 104.55mm ... close enough.

Attached Thumbnails

  • PSX_20210720_131615.jpg

Edited by Paul AZ, 20 July 2021 - 07:12 PM.


#32 Paul AZ

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Posted 20 July 2021 - 05:52 PM

I have my image train set up but meed to test it. With the Celestron OAG and FR I am at 104.55mm 😁 Here is my set up from rear of C9.25 Focal Reducer Add back focus: Female SCT to T2 male adapter = 8.4mm OAG female T2 = 4.5mm OAG body = 29mm Male T2 OAG adapter = 12.5mm ZWO filter drawer 2 inch = 21mm OAG medium T2 spacer = 11.55mm Meade DSI IV = 17.5mm Total is 104.55mm ... close enough. 👍

 

Attached Thumbnails

  • PSX_20210720_131556.jpg


#33 charlesgeiger

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Posted 20 July 2021 - 07:25 PM

I have read here that the original Celestron f/6.3 units were made in Japan.  They were stated to be at 105 mm back focus.  Production units changed from Japan to China and the early Chinese units had from about 80 mm back focus to 100+.  I believe the same could be said for the Meade's.  Also, the Antares units usually had shorter back focus lengths.  There was a thread here showing the actual units attached showing the varying back focal lengths by measuring the image of the sun and measuring it.  Anyhow, if I remember correctly, this would lend credence to the idea that the focal reducer's varied in manufacturing and assembly after the Japan units were completed and the manufacturing began in China.



#34 Paul AZ

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Posted 20 July 2021 - 08:26 PM

I have read here that the original Celestron f/6.3 units were made in Japan.  They were stated to be at 105 mm back focus.  Production units changed from Japan to China and the early Chinese units had from about 80 mm back focus to 100+.  I believe the same could be said for the Meade's.  Also, the Antares units usually had shorter back focus lengths.  There was a thread here showing the actual units attached showing the varying back focal lengths by measuring the image of the sun and measuring it.  Anyhow, if I remember correctly, this would lend credence to the idea that the focal reducer's varied in manufacturing and assembly after the Japan units were completed and the manufacturing began in China.

I use the same focal reducer for my visual set up on a CPC800 with a 2 inch diagonal and have had no issues even though the backfocus with the large 2 inch diagonal is much greater than 105mm. Even with the Pentax 30mm XW eyepiece it was fine.



#35 Paul AZ

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Posted 20 July 2021 - 08:57 PM

In fact, ERHAD, it sounds like your scope/reducer combo is even more out of spec, as you have a C925 not a C8. I believe, according to Celestron, the correct back focus for the 925 is actually 117mm from the base of the camera side reducer SCT threads. That is a full 32mm longer than what you are using!

I’ll need to find the article but what I recall reading is that regardless of the placement of the focal reducer, within the bounds of the scopes focus path, the focal reducer optimum backfocus is still 105mm. There is a reason that the FR adjusted Moonlite focuser enables a fixed distance between the focal reducer and eyepiece/ camera sensor. Same with the Meade Zero Image focuser, with the focal reducer screwed onto the SCT adapter on the back end of the focuser. waytogo.gif



#36 Paul AZ

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Posted 20 July 2021 - 09:05 PM

I have read here that the original Celestron f/6.3 units were made in Japan.  They were stated to be at 105 mm back focus.  Production units changed from Japan to China and the early Chinese units had from about 80 mm back focus to 100+.  I believe the same could be said for the Meade's.  Also, the Antares units usually had shorter back focus lengths.  There was a thread here showing the actual units attached showing the varying back focal lengths by measuring the image of the sun and measuring it.  Anyhow, if I remember correctly, this would lend credence to the idea that the focal reducer's varied in manufacturing and assembly after the Japan units were completed and the manufacturing began in China.

I need to comment on this a bit more. Just because the manufacturing moved from Japan to China would not result in a difference in shipped product quality. What could cause such a shift in shipped quality would be a shift in Celestron quality requirements. Your IPhones are pretty much made in China and quality is fine. Lots of ODM’s operations are in China and it all boils down to quality control requirements from the company putting their name on the product. As the DPM requirements tighten so does the cost but it is not impossible.  



#37 ldipenti

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Posted 17 September 2021 - 06:39 AM

Following closely this thread as a new SCT owner.

 

I have a Celestron C5 Spotter XLT that I intend to use for astrophotography with my Nikon D500 DSLR.

 

I have purchased the Celestron f/6.3 reducer, T adaptor and T-ring for Nikon and I am having issues with aberrations at the edges of the frame:

 

https://siasky.net/f...FrPfxKpRXzuFnyQ (for some reason the website doesn’t allow me to attach it)

 

I have been told that these aberrations are consistent with the sensor being too far from the reducer, and on this thread I have read lots of comments about shortening the distance. Can you explain how? Should I get a shorter T adaptor? I don’t think Celestron offer those, where to buy them?

 

Sorry if my questions sound dumb but it’s my first experience with SCT, I come from using refractors.

 

thanks!




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