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Is a DIY astrograph just to use a smaller central obstruction worth it?

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#1 arun289

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Posted 27 February 2021 - 10:37 AM

Hi,

 

I recently got a ASI533MC camera which is on the way. It has a rather small sensor with a 16mm diagonal. I am not able to make a build vs buy decision on a compact 6" F4 astrograph for this camera. I was wondering if I build one myself, I can use a smaller central obstruction as the scope would be dedicated for use with this camera. I am new to this, so correct me if I am wrong anywhere. Since the required fully illuminated image circle is a tiny 16mm, I hope to get away with a small secondary ~32mm instead of the tradiional 63mm which comes with off the shelf scopes to carter to larger camera sensors. The central obstruction is 20% for the 32mm compared to 42% for the 63mm. Am I overthinking this? Does the central obstruction matter much? I understand it reduces contrast and nothing else much. Does the difference in contrast worth the effort? Just for background, I am used to DIY and have descent capability there.

 

Regards,

Arun

 



#2 MitchAlsup

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Posted 27 February 2021 - 11:59 AM

Am I overthinking this? yes

 

Does the central obstruction matter much? only on planets

 

I understand it reduces contrast and nothing else much. it illuminates the outer field

 

Does the difference in contrast worth the effort? contrast in astrophoto is a slider in the post processing application.

 

https://www.bbastrod...om/diagonal.htm

 

I plugged in 6", f/4 = 24" focal length, diagonal to focal plane = 3"+3"+57mm, and the calculator says the smallest secondary you can "get away with (assuming you are using a Paracorr) is 2.14.

 

The first 3" is that you don't want the bottom of the Paracorr to enter the light column,

The second 3" is the length of the Paracorr

The 57mm is the backfocus from the top of the Paracorr to the focal plane.


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#3 arun289

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Posted 28 February 2021 - 05:08 AM

Thanks @mitchAlsup for the information. I think I will just buy a 6"F/4 then.

One question on the contrast though. Contrast increase in software will not really increase the real dynamic range right?

 

Regards,

Arun



#4 MitchAlsup

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Posted 28 February 2021 - 11:54 AM

I would not use the word "real". At best the word might be "apparent".

 

You will get an image off the camera with a real dynamic range:: that is it contains with a white point and a black point; The white point is the most saturated pixel, the black point the least saturated.

 

In post processing, after stacking lots of images, flat fielding,... you manipulate the black point down to about 0 output brightness, and adjust the white point up to 256 output brightness (jpg) and this spreads the information over the largest output dynamic range. An adjustment in gamma and you have a pleasing image.



#5 jgraham

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Posted 28 February 2021 - 12:31 PM

For imaging the size of the secondary has a teeny tiny effect that is likely swamped by other factors like seeing. I have a 6” f/3.6 Schmidt Newtonian (a Meade Comet Tracker) with a 3.1” secondary and it works great!

 

The one thing that I would highly recommend for a 6” f/4 imaging Newtonian is a Baader Mk III MPCC. The Mk III does a fantastic job cleaning up the focal plane.

 

Food for thought.


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#6 clivemilne

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Posted 01 March 2021 - 03:01 AM

The overall merit function for astrographs places a VERY low importance on relative size of central obstruction.

 

Some of the finest astrographs available at any cost have 50% to 60% central obstructions.




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