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Can you convince me that bigger is not better for AP?

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#126 Rouzbeh

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Posted 18 September 2021 - 02:15 PM

Here's a table I made for myself when I was trying to decide to upgrade to a bigger telescope:

 

Using Lamberts performance indicators:

 

https://lambermont.d...telescopes.html

 

Given the image is only made of pixels, I don't see why we shouldn't consider Pixel Entendue and Pixel Signal.

Out of all the combinations, the CDK14 + reducer has the highest numbers

 

 


Edited by Rouzbeh, 18 September 2021 - 02:30 PM.

 

#127 dan_1984

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Posted 18 September 2021 - 02:22 PM

Very interesting, indeed. 

 

And we look forward to further graphs incorporating image scale.  

 

Thanks!

Hopefully the last version.grin.gif

Scopes:

  • 8 inch CFF APO FL=1280 f/6.5  Black Line with Black Dot
  • 10 inch TEC APO FL=2200 f/8.8 Red Line with Red Dot
  • 12 inch LZOS APO FL=2280 f/12 Green Line with Green Dot
  • 14 inch LZOS APO FL=4270 f/12 Blue Line with Blue Dot

Target:

  • Something very faint, Mag = 25

Desired SNR:

  • 10

SkyMag:

  • 20.88

CCDs-Moravian CCDs:

         For first image:

  • G316200, 8 inch scope, image scale 0.94"/pixel
  • G49000, 10 inch scope, image scale 1.13"/pixel
  • G49000, 12 inch scope, image scale 1.09"/pixel
  • hypotetical 24 pixel camera for the 14 scope, image scale 1.16"/pixel

        For second image:

  • G316200, 8 inch scope, image scale 0.94"/pixel
  • G416000, 10 inch scope, image scale 1.14"/pixel
  • G416000, 12 inch scope, image scale 0.94"/pixel
  • G49000, 14 inch scope, image scale 1.07"/pixel

 

Filter:

  • V-Band

 

Results:

Results 1st image:

  • CFF200   Total imaging time for SNR 10: 101 hours
  • TEC250   Total imaging time for SNR 10:  42.76 hours
  • LZOS304 Total imaging time for SNR 10:  30.61 hours
  • LZOS354 Total imaging time for SNR 10:  19.52 hours

Results 2nd image (keeping the same focal ratio of 6.5 for all scopes):

  • CFF200  Total imaging time for SNR 10: 101 hours
  • TEC250   Total imaging time for SNR 10: 41.68 hours
  • LZOS304 Total imaging time for SNR 10: 41.68 hours
  • LZOS354 Total imaging time for SNR 10: 23.20 hours.

 

 

So, for total imaging time ALL are important: Aperture, Focal Ratio, Pixel Size. The individual contribution of each factor to total imaging time is less important, because I think the total time  is what ultimately matters

Attached Thumbnails

  • FL1.png
  • FL2.png

Edited by dan_1984, 18 September 2021 - 02:24 PM.

 

#128 Rouzbeh

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Posted 18 September 2021 - 02:25 PM

TOA150 

99% APO transmission

 

Delta Rho

96% reflectivity mirrors

55% central obstruction

 

Attached Thumbnails

  • Screenshot 2021-09-18 12.22.52.png

 

#129 Rouzbeh

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Posted 18 September 2021 - 02:36 PM

Sorry, I'm a little slow, but better in ALL aspects? To me, it's horses for courses: the right combination of scope & camera per object. A f/2 600mm aperture scope will not capture as wide a field as an 80mm aperture f/2 scope, so for many objects, the 600mm aperture scope is NOT better for me. Right scope/camera for the right object (and pixel-size of camera is part of this, too). That's why the initial question is a bit vague in that I KNOW BH isn't going to want to use a long FL scope for a wide field object.

 

Paul

If you're priority is FOV at the cost of everything else then yes by all means the 80mm scope is the best. My Samsung cell phone camera at f/1.8 has even more FOV!


 

#130 dan_1984

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Posted 18 September 2021 - 02:43 PM

Here's a table I made for myself when I was trying to decide to upgrade to a bigger telescope:

 

Using Lamberts performance indicators:

 

https://lambermont.d...telescopes.html

 

Given the image is only made of pixels, I don't see why we shouldn't consider Pixel Entendue and Pixel Signal.

Out of all the combinations, the CDK14 + reducer has the highest numbers

Cool stuff.

The link doesn't work. How do you compute Pixel Etendue, Pixel Signal and Object Signal?

If the formulas are too long, can you attach the excel file maybe?


 

#131 Peter in Reno

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Posted 18 September 2021 - 02:59 PM

Again:

 

14" Delta rhof/3   vs   150mm APO   f/7.5

 

Both the telescopes will have the exact  same FOV the same Focal length same pixel scale.

 

The big reflector will just be several times faster.

 

I'm a bit amazed you are still debating this.

I never said the big reflector exposure time is the same as the smaller refractor. The exposure time will be 6.25 (7.5^2 / 3^2) times faster thanks to f/3 focal ratio, not because of aperture. 

 

For the sake of the argument, if you hook up both scopes to a DSLR body, the camera's light meter will measure the exposure time based on focal ratio, not aperture size. 

 

Peter


 

#132 Rouzbeh

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Posted 18 September 2021 - 03:08 PM

Cool stuff.

The link doesn't work. How do you compute Pixel Etendue, Pixel Signal and Object Signal?

If the formulas are too long, can you attach the excel file maybe?

Here is a more detailed page:

 

https://github.com/d...are-telescopes/

 

This should work.


 

#133 Rouzbeh

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Posted 18 September 2021 - 03:19 PM

I never said the big reflector exposure time is the same as the smaller refractor. The exposure time will be 6.25 (7.5^2 / 3^2) times faster thanks to f/3 focal ratio, not because of aperture. 

 

For the sake of the argument, if you hook up both scopes to a DSLR body, the camera's light meter will measure the exposure time based on focal ratio, not aperture size. 

 

Peter

Small scopes do have their uses, that's why they exist. Size, weight, cost, simplicity, etc...

And yes for widefield imaging a small scope is enough.

 

 

Perhaps I should have said larger aperture is better for:

 

  • More light capture
  • At the same focal length, faster speed if needed
  • Higher image scale if needed (ie finer details)
  • Much better for small objects

 

Nothing is perfect but I'd say 8 times out of 10 I'd want more aperture.  


 

#134 psandelle

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Posted 18 September 2021 - 03:24 PM

If you're priority is FOV at the cost of everything else then yes by all means the 80mm scope is the best. My Samsung cell phone camera at f/1.8 has even more FOV!

Sorry, but that's being disingenuous. They don't make extremely high performance (and expensive) wide-field scopes for nothing. There is a huge group of imagers who use them (and professionals as well) and dismissing them by saying they're the same thing as a Samsung cell phone is simply showing a bias. From the TEC 300VT-7Degree, to the Tak FSQ and Epsilon lines, to OS' RH line, AP RH scope, etc., etc. If you don't match camera and scope to object, larger aperture is NOT better and no amount of dismissing that face is going to make it correct. It depends on the object.

 

Paul


 

#135 Rouzbeh

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Posted 18 September 2021 - 03:25 PM

Here's a recent bigger aperture (CK12.5 -  318mm  -  f/5.3) example.

 

Ha 3nm from Bortle 6

300s x 3 = 15 minutes

Scale is 0.92 arcseconds/pixel 

I've made it match my 2.5" seeing by  using a 0.66x reducer and bin2 of the IMX571      3.76u pixels.

 

Ive traded some of the extra native resolution for speed and very happy with the results.

The image scale is still "high res" at 0.9"/pix

 

 

The full 4 hours of integration at 0.57"/pix

https://astrob.in/full/hz5u8e/0/

Attached Thumbnails

  • Ha 300s x3.jpeg

Edited by Rouzbeh, 18 September 2021 - 03:28 PM.

 

#136 psandelle

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Posted 18 September 2021 - 03:28 PM

Oh, almost forgot what might be the best reason for BH going bigger: less chance his wife to pick it up and throw at his head. I'm SURE that's why he went for the Mach 1. grin.gif

 

Paul


 

#137 TareqPhoto

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Posted 18 September 2021 - 03:34 PM

Ok, i got the answer, i must buy several scopes and cameras so i am covered for all kind of debates and conversations about which is which, if i want aperture i will have, if i want focal ratio i will have, if i want pixel size i will have, it is no sense to come and ask which is better for open possibilities and many factors there, so at the end it is about results, many amhy people are really not into those so smart **** mathematics calculations and analysis, i even bet that some with amazing great images even don't know about focal ratio much or pixel size or sampling.


 

#138 Rouzbeh

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Posted 18 September 2021 - 03:36 PM

Sorry, but that's being disingenuous. They don't make extremely high performance (and expensive) wide-field scopes for nothing. There is a huge group of imagers who use them (and professionals as well) and dismissing them by saying they're the same thing as a Samsung cell phone is simply showing a bias. From the TEC 300VT-7Degree, to the Tak FSQ and Epsilon lines, to OS' RH line, AP RH scope, etc., etc. If you don't match camera and scope to object, larger aperture is NOT better and no amount of dismissing that face is going to make it correct. It depends on the object.

 

Paul

On the contrary, I completely agree with those fast scopes.

 

They are in fact VERY large aperture scopes for those focal lengths. If I were to do widefield I would love those very scopes myself.

 

9 times of of 10 those focal lengths are achieved with 3 to 4 inch apos with much smaller apertures at the typical f6 / f7 ratio.

 

I was rereferring to those examples where people try to compare a typical APO to a fast large scope.

 

So for almost all situations you want more aperture as you demonstrated with these telescopes:

 

 

 

 

"TEC 300VT-7Degree, to the Tak FSQ and Epsilon lines, to OS' RH line, AP RH scope"

 

 

 

Obviously aperture with the correct f/ratio. A 16" classical cassegrain at f/20 is almost useless for deep sky imaging.

 

There are other things like pixel scale, image circle, etc to work out too.


Edited by Rouzbeh, 18 September 2021 - 03:45 PM.

 

#139 Rouzbeh

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Posted 18 September 2021 - 03:40 PM

In fact I just placed an order for a larger 14" CDK with the exact same focal length of the smaller CDK12.5.

 

With the dedicated focal reducer at  f/4.7 the f/ratio is only 1700mm  thats a lot less than an 8" edge SCT.


 

#140 dan_1984

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Posted 18 September 2021 - 03:47 PM

Here's another way of looking at it.

Each factor in isolation. Image scale appears to be the biggest contributor, followed by aperture, followed by f ratio.

So the OP needs a bigger APO, lower F ratio, and another camera to match the scopegrin.gif

Attached Thumbnails

  • pic1.png
  • pic2.png
  • pic3.png

Edited by dan_1984, 18 September 2021 - 03:59 PM.

 

#141 dan_1984

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Posted 18 September 2021 - 03:51 PM

Here is a more detailed page:

 

https://github.com/d...are-telescopes/

 

This should work.

Thanks, already written in python. You saved me from writing the code from scratch flowerred.gif  yaaay


 

#142 Rouzbeh

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Posted 18 September 2021 - 03:55 PM

Here's another way of looking at it.

Each factor in isolation. Image scale appears to be the biggest contributor, followed by aperture, followed by f ratio.

Very interesting stuff here.


 

#143 555aaa

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Posted 18 September 2021 - 04:04 PM

Bally-man, you should get something in the 9 to 11 inch aperture and 2000 to 2500mm focal length. I think these are still safely luggable and it opens up some new targets that are challenging for the smaller refractor. You will learn some new stuff and it should be fun. At 10”, either at f/2 ( as in my RASA) or f/10 ( as in my SCT) I can detect stars down to 20th magnitude in a five minute sub. That’s hard to do with a small refractor.
 

#144 dan_1984

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Posted 18 September 2021 - 04:05 PM

Very interesting stuff here.

Yes, I am surprised image scale has such contribution. Maybe it has something to do with bigger pixels and the signal/pixel ratio or going to a little higher image scale is like lowering the f ratio from 6.5 to 5, then it makes sense.. 


Edited by dan_1984, 18 September 2021 - 04:12 PM.

 

#145 Peter in Reno

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Posted 18 September 2021 - 04:12 PM

Both pixel size and focal ratio share contributions. 

 

Peter


 

#146 555aaa

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Posted 18 September 2021 - 04:32 PM

You need a different scope for galaxy season than nebula season.
 

#147 imtl

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Posted 18 September 2021 - 04:35 PM

Thank you all for your contributions. Ballyhoo got his answer(s....). This topic is over-chewed and is done.


 


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