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Backfocus RedCat51 w/ Starizona Filter Drawer and ASI183

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#1 Sven_Bortle5

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Posted 19 September 2020 - 03:33 AM

Hi there,

 

I had a Starizona Filter Drawer lying around and just saw they sell an adapter for the RedCat51. So, normally the backfocus from the backplate of a WO-scope (or DSLR-adapter) to the image plane is 54.80 mm.

 

This led me to the following setup:

 

Backfocus RedCat51 ASI183 V2
 
I added 0.7 mm of backfocus to adjust to 2mm glass of filters in the optical train. Additionally, I am able to move the Starizona Filter Slider closer to the ASI-camera.
 
What would you suggest in regard to the position of the drawer? Might the position closer to the cam reduce the risk of coma?
 
Regards and thanks,
Sven

 


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#2 Coconuts

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Posted 19 September 2020 - 05:44 AM

Here you go:

  1. Your claim of 54.8 mm back focus is incorrect; see attached.  It's 59.7 mm from the seating surface for the M48 threads, but your diagram shows that section removed, so your back focus is probably the 77.7 mm dimension.
  2. When you focus the RedCat 51, all of its optics moves together as one rigid body.  Given the enormous travel of the front focusing threads, the traditional notion of back focus barely applies.  You only need to nail it to get the focus index mark to align with the infinity symbol, but that is a feature easily dispensed with.
  3. Set the focus at the infinity mark, and measure how far the rear optical element is inset from the last mechanical surface of the RedCat 51. 
  4. Confirm the nominal back focus for your RedCat 51 configuration (again, that looks to me to be 77.7 mm). 
  5. Total up how much space your external gear and camera sensor depth needs.
  6. "Focus" the RedCat 51 to put the inset of that rearmost optical element such that it plus your external gear equals the infinity mark rearmost optical inset plus the nominal back focus.
  7. The location of the filters doesn't matter.  Your offset of the sensor backwards of 0.7 mm is close enough (0.68 mm is closer, assuming that your filters use a BK-7 substrate).

RedCat 51 back focus.JPG

 

All the best,

 

Kevin


Edited by Coconuts, 19 September 2020 - 05:55 AM.


#3 Sven_Bortle5

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Posted 19 September 2020 - 08:35 AM

Hi,

 

I know the original drawing of the RC51 backfocus, but started differently due to these images at the Starizona website.

 

We don't know the exact optical length of the Starizona adapter and have to add the filter slider.

 

Startizona says you just need to connect a Canon/Nikon T2-adapter in order to have the "required 55mm backfocus". WO has those T2-adapters of 10.7mm (Canon) or 8.3mm (Nikon), which receive a backfocus of 54.8mm with the flange distance of these DSLRs. This is exactly what WO shows in the drawings of most other telescopes.

 

But I didn't get what you said in point 7 (due to my english). I though one reason for coma could be reflections between the last glass element and the sensor of the cam. Reducing this distance should reduce the risk of such reflections.... am I wrong?

 

Thanks,

Sven 


Edited by Sven_Bortle5, 19 September 2020 - 08:39 AM.


#4 Coconuts

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Posted 19 September 2020 - 09:59 AM

Reflections aren't coma, and good filters shouldn't produce them to begin with.  If your filters have reflection issues, where you place them probably only affects where they show up in the image.  Further away might actually be better.

 

But to the back focus question, the RedCat 51, through its long focus thread travel, can support a wide range of gear length totals.  It is very flexible in that regard.  You merely need to get the distance from your sensor to the rear optical element correct.

 

All the best,

 

Kevin



#5 Sven_Bortle5

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Posted 21 October 2020 - 07:28 AM

Hi again,

 

I needed to update this thread and my image after some testing and adjusting. I wondered how my calculation using the Starizona adapter and the backfocus of 59.7 mm - as given by William Optics - could work.

 

The answer is simple: The pezval construction of the WO RedCat51 does not require a precise backfocus, because it's focuser works more like a camera lens and moves the optical train. You just need to be in a range of backfocus, which allows you to move the focuser enough forwards and backwards.

 

I found a backfocus of app. 54.8 mm to be an optimal length.

 

Backfocus RedCat51 ASI183 V3 (corrected)

 

Using the 59.7 mm which are stated by WO, my focus point is exactly where the focuser is completely moved in. Adding filters moves the focus point out of reach. 

 

Best regards,

Sven 




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