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80mm Guide Scope for 80mm Main Scope

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#1 tommo4523

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Posted 22 November 2020 - 02:19 AM

Hi there,

 

I just bought an 80mm scope for my beginner astrophotography setup, and I am now looking at guide scopes, I have found a very affordable second hand 80mm celestron guide scope and a 30mm guide scope brand new, they are both very similar in price, I have a Orion Starshoot Autoguider and an EQ5 mount, my main scope weighs 3kg and has a Canon 500D attached, is the 80mm guide scope too heavy for my setup and will it be suitable for it or is the 30mm scope a better option.

 

On a seperate note, will a t-ring to M42 to 2 inch tube be fine for my APS-C sensor or is M48 required, I only ask as my local store only has M42 in stock and am wondering if it is worth the several month wait or spending more elsewhere.

 

Thanks in advance.

 

Tom,


Edited by tommo4523, 22 November 2020 - 02:20 AM.


#2 Stelios

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Posted 22 November 2020 - 02:34 AM

Don't burden your lightweight EQ5 mount with a second 80mm scope. It's entirely unnecessary. The 30mm guide scope should be plenty. 

 

The M42 should be fine for APS-C. 


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#3 tommo4523

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Posted 22 November 2020 - 05:16 AM

Thank you, ill do that



#4 TelescopeGreg

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Posted 22 November 2020 - 01:54 PM

If that Celestron 80mm "guide scope" is the one that's an f/7.5, stay away.  Far away.  It's poorly made, heavy, and the mounting rings are designed for a Losmandy rail, which if you buy the one from Celestron, will add another several pounds to the setup.  At f/7.5 it is also dim, requiring a higher-end guide camera such as the ASI175mm Mini.  I don't know what they intended it to be used for, but guiding a small refractor is definitely not it. 

 

I got one for my Celestron 8" f/5 Newtonian scope, back early on when I had to use a 2x Barlow to get my DSLR to focus.  I had read the books and dutifully done the math, and with a 2,000mm effective focal length and desiring a 3:1 pixel ratio for the guider, realized that I needed a guide scope with a focal length of at least 480mm.  That "guide scope", especially being on sale at the time, seemed to be the right answer.  Being stubborn, I made it work, but what a mess...

 

The real answer was to get a coma corrector for the scope (which I later discovered I needed anyway), and that the slight Barlow effect of the corrector would also solve the DSLR focus issue.  That brought the effective focal length for the OTA down to a much more manageable 1,150mm, which put a raft of other, shorter, lighter, and significantly higher quality guide scopes within range.  I now have a 130mm f/7 refractor (910mm focal length) and guide it with the ZWO 60mm f/4.6 scope.  Much better.

 

A  30mm guide scope should be ok for your refractor, though I would look for perhaps a slightly larger one for some future proofing.  The 30's tend to want to be mounted in the "finder shoe", which can be a source of trouble ("differential flexure").  Try to find one that has two mount points.  Keep it centrally mounted, as close to the imaging scope as you can, and in line with the counterweights so that balance is maintained in all directions.  That said, if you're careful, and it fits your budget, it's probably going to be fine.  We too often recommend the ideal, when taking one step back from perfect is totally good enough.



#5 AZ Maverick

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Posted 22 November 2020 - 02:29 PM

I agree with the others on using the 30mm guide scope.

While I used to use a 60mm guide scope on my 80mm APO, I primarily had that guide scope for use on my other longer focal length telescope.

I don't know the exact specs on your optics but generally speaking it looks like an average 80mm telescope with a focal length of say 450mm with your camera will be around 2" per pixel and a 30mm guide scope with a focal length of say 125mm with that guide camera will be around 8" to 9" per pixel - that still gets you in the ballpark of the often stated 3 to 5 times pixel ratio for a guide scope.

But I also agree with the above post about 'ideal' vs 'good enough' - still, I'd try for a 50mm guide scope if I could find one. smile.gif



#6 tommo4523

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Posted 22 November 2020 - 08:51 PM

I was hoping to get a 50mm but unfortunately there are none in stock near my location but I definitely am going with the 30mm now, I will see how that goes and use it until it becomes a bottleneck and upgrade then. Thanks for the help. 



#7 alphatripleplus

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Posted 23 November 2020 - 07:47 AM

Good decision to go with the 30mm guidescope, as it should be fine.




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