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Does it make sanse to have two 8" telescopes?

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#1 txo

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Posted 24 March 2019 - 05:40 AM

Hi there,

 

I recently started with astrophotography with 8" 812mm newtonian telescope which is a fine cheap instrument. I paired it with Altair 183M camera that is slightly oversampling the image.

 

Now the problem. I quickly realised that there are som objects that I'd like to shoot that are quite small for the setup - e.g. 100 pixels on the camera. So I started looking around for either longer focal length telescopes or barlow. The main limitation I have now is that it must not weight more than say 10kg and have a bigger apperture than 8". During this search I did come across a interesting number called resolving capacity and found that mine actual telescope's 5.5 is quite a good one. From that I came to the conclusion that unless I would be willing to move to much bigger appertures there is no reason for having a different telescope this size since mine current equipment already reaches physical limit of a telescope of this class and I could not get any more detail from it.

 

Is this conclusion correct or am I missing something? 

 

thank you


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#2 Ian Robinson

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Posted 24 March 2019 - 08:06 AM

My take on this is :
if you have the $ , why not have two ?

You can look through one as a dob while the other one on the big expensive GEM is being used to image stuff.

I'd go to the extent of suggesting an f/4 system for imaging + f/5 or f/6 system for looking + a 2" 2x Powermate and a 2" 4x Powermat + off cause a coma corrector or maybe even if you have the $ an ASA 2" Newtonian Coma Corrector and 0.73x Reducer for Astrophotography.

Edited by Ian Robinson, 24 March 2019 - 08:13 AM.


#3 Eddgie

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Posted 24 March 2019 - 08:21 AM

I don't think you are missing anything at all.  If you can maximize the resolution for a given aperture by using a focal length extender, then adding a second scope may not do anything for you.

 

On a practical side, a faster system is generally better (in my opinion) because it is easy to make a faster system slower (extend focal length) than making a slower system faster (speed up the focal ratio).  Since the scope you have is faster than the scope you were thinking about getting, there may be very little to gain by getting it.

 

And just because your camera may not be working optimally with a given focal ratio/aperture does not mean that it can't still be used to image with.  For example, if the subject is too big to fit into the field of the optimized focal ratio/aperture when a Barlow is used, your choice is to only capture a part of it at less than optimal match, or capture all of it but loose some small amount of detail.   If you have the larger, slower scope that can't be further reduced, you are limited to only capturing part of the subject on the frame.  

 

I would say that there is little point to be had by going to a slower scope when you can Barlow your existing one to get the same result.


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#4 MalVeauX

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Posted 24 March 2019 - 08:59 AM

Hi there,

 

I recently started with astrophotography with 8" 812mm newtonian telescope which is a fine cheap instrument. I paired it with Altair 183M camera that is slightly oversampling the image.

 

Now the problem. I quickly realised that there are som objects that I'd like to shoot that are quite small for the setup - e.g. 100 pixels on the camera. So I started looking around for either longer focal length telescopes or barlow. The main limitation I have now is that it must not weight more than say 10kg and have a bigger apperture than 8". During this search I did come across a interesting number called resolving capacity and found that mine actual telescope's 5.5 is quite a good one. From that I came to the conclusion that unless I would be willing to move to much bigger appertures there is no reason for having a different telescope this size since mine current equipment already reaches physical limit of a telescope of this class and I could not get any more detail from it.

 

Is this conclusion correct or am I missing something? 

 

thank you

Heya,

 

You can raise your imaging scale. While aperture defines resolution, DSO is not always about resolution and often resolution is compromised. You can raise your imaging scale with a 2x extender to F8. I use a C8 Edge for this reason, so my scale can be F10 or F7 immediately. F7 and F8 are not much different really. You can still do DSO imaging, just with a little longer exposure time. But that larger scale gives you more options. So really, I don't see much of a reason going to yet another 8" scope that produces similar scales (a different design would be something like an 8" RASA or hyperstar C8, but these are lowering the image scale a lot more, not increasing it). And it sounds like you're trying to increase your image scale. You need focal length for that. But, also, you need more aperture with that longer focal length to benefit it under OK seeing, so ultimately, going 8" to 8" doesn't do this for you. You'd have to go bigger. Or,  you can push the scale up and be ok with not having ideal sampling (which is common with DSO!).

 

Very best,


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#5 Geo.

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Posted 24 March 2019 - 09:14 AM

Makes more sense than having 5!  Opps, make that 6.



#6 gnowellsct

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Posted 24 March 2019 - 09:15 AM

Judging from your solar work you know a thing or two about gear. I can't read your signature on my phone so I'll just say putting money into a high class mount should take precedence over the optic.

Another way to spend some funds usefully is to go to the North east Astro Imaging Conference. Hang out with imaging nuts. It's in two weeks so better hurry. A way to make friends and get into serious gear head discussions. You'll have a stronger sense of what direction you want to go in.

#7 carolinaskies

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Posted 24 March 2019 - 07:10 PM

If you camera is capable of binning have you tried that with a barlow?  

With a barlow you'll be decreasing your field (enlarging your target) and by binning you'll have less over-sampling, ie more effective use of the pixels in question.  This of course depends on the camera and it's capabilities whether this will help you.  

Unless you're planning on shooting very small planetary nebula I think binning with a good barlow may be the way to go.   On the other hand if your prospective targets are falling too near the resolution of your telescope to get good detail then moving up in aperture will be necessary. 



#8 drjustice

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Posted 27 March 2019 - 07:38 AM

I think the OP meant that he wished to stick together two telescopes one looking into the other.

Eg: Look through the small end of one telescope, out the big end, then in to the big end of the 2nd telescope, and out through the small end of the 2nd telescope. Presumably with some duct tape holding the contraption together.

:-)


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