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Possible dumb question of the day: short refractor vs Mak

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#1 unimatrix0

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Posted 22 July 2021 - 01:28 PM

I tried to find some informative threads and even youtube videos on the type of telescopes people use for these planetary photography. 
Here is what I found: 
8"-10" Reflector

6-8" SC/Mak

I understand the planets require a high magnification and large aperture. That's fine . 
I mean go ahead on youtube, look for videos regarding this topic and I see people capturing a video of Jupiter , that single frames of his video looks 10x better than I could ever stack together.  I just don't see too many videos regarding their gear they use unless it's a $1500 Meade S.Cass.

But what's the shortest focal range that still produces good images?  

Do I need to be really in the 2000mm focal range to produce anything that shows the bands of Jupiter? 

 

I mean, i tried 560mm and the results were poor, even with Autostakkert + Registrax corrections. I just went ahead and deleted it, because it looked completely unfixable actually single frames looked better than the stacked one even though I only selected to stack 10%


Edited by unimatrix0, 22 July 2021 - 01:31 PM.


#2 WarmWeatherGuy

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Posted 22 July 2021 - 01:48 PM

When you use the Rule of Thumb to get the optimal focal ratio then the size of Jupiter, in pixels, will be the size of your scope in mm. For example, an 8" (200mm) will give you a Jupiter around 200 pixels wide. So you should be looking at the diameter rather than the focal length. Once you choose a diameter then you adjust your focal length using a Barlow to get close to optimal.

 

Optimal focal ratio = 5 * sensor pixel size in microns.

 

In practice 5 to 7 works fine. For example if your chip has 4 micron pixels then you want to be between f/20 and f/28.



#3 WarmWeatherGuy

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Posted 22 July 2021 - 01:57 PM

This thread has pictures of Jupiter along with mention of what gear was used.

 

Small bore challenge: Jupiter w/ 6" or less



#4 RedLionNJ

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Posted 22 July 2021 - 02:05 PM

I tried to find some informative threads and even youtube videos on the type of telescopes people use for these planetary photography. 
Here is what I found: 
8"-10" Reflector

6-8" SC/Mak

If this is what you're finding, you're either looking in the wrong place (perfectly possible, real hi-res imagers may not use YouTube) or your search criteria are a bit off.

 

Ideally, a large Newtonian or classical Cassegrain should be used.

For many people, a large SCT is more practical (easy to obtain and maintain) than a large pure reflector.



#5 petert913

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Posted 22 July 2021 - 02:06 PM

You need both !  lol.gif



#6 BQ Octantis

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Posted 23 July 2021 - 03:01 AM

I've shot Jupiter and Saturn at f = 8mm, 15mm, 50mm, 200mm, 250mm, 300mm, 750mm, 2700mm, 5700mm, 9000mm, and 10,400mm, and a handful of others in between.

 

You can start to see bands on Jupiter and that Saturn has rings at f=300mm D=54mm (f/D = 5.6):

 

post-273658-0-76425000-1532822649.jpg  post-273658-0-79472900-1532822660.jpg

Canon 70-300 IS II, Canon 600D/T3i, single RAW frame

 

But honestly, for good results you need both aperture and focal length.

 

I shot this at f=9000mm D=127mm (f/D = 71):

 

post-273658-0-13752400-1564396346.jpg

Celestron C5/750mm SCT, Fujiyama 9mm ortho, Canon 600D/T3i LiveView stack

 

And I shot this at D=180mm f=4600mm (Saturn, f/D = 26) and f=8500mm (Jupiter, f/D = 47):

 

post-273658-0-54825200-1598081103.jpg

Skywatcher Mak 180, Fujiyama 18mm ortho (Saturn), 12.5mm ortho (Jupiter), Canon 600D/T3i LiveView stack

 

But the typical big guns around here are D=12-16 in Newt or SCT, with f/D = 5 × pixel size in microns.

 

So you have plenty of options. It's all just a matter of money. Or perhaps what you're already starting with…

 

BQ


Edited by BQ Octantis, 23 July 2021 - 03:02 AM.


#7 luxo II

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Posted Yesterday, 08:01 AM

The other candidate scope is a big maksutov like mine, but that’s serious $$$$$. I’m just starting to try imaging FWIW despite the consistently poor weather we are having combined with a COVID lockdown.

Edited by luxo II, Yesterday, 08:04 AM.



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