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Size of scope to attach guiding CCD?

astrophotography beginner equipment imaging
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#1 wsullivan

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Posted 19 October 2020 - 12:43 AM

I want to have a guiding chip on a separate telescope. I am hoping I can do this with something the size of a finder scope since it would be difficult to put anything larger on the scope. Through what size scope do I need to be able to guide adequately? Thank you.



#2 TelescopeGreg

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Posted 19 October 2020 - 01:02 AM

Lots of choices; some math will help guide your decision (er, sorry...).

 

The software that guides the scope (often PHD2) does a pretty good job at knowing where a guide star is, down to sub-pixel accuracy.  So, the guider doesn't need to be as "powerful" as the imaging scope it's controlling.  If you look at the pixel scale, the portion of the sky that each pixel covers, the practical guideline is that the imaging scope should be no "worse" (larger scale) than the imaging scope by about 5:1.

 

Pixel scale is 206 * pixel size / scope focal length.  Pixel scale in microns, and focal length in millimeters, gives scale in arc-seconds.  My own setup has a 280mm guide scope with a camera that has 5.86 micron pixels, and an imaging scope of 910 mm focal length and camera with 3.76 micron pixels.

 

Guider:  206*5.86/280 = 4.31 arc-seconds

Imager: 206*3.76/910 = 0.85 arc-seconds

 

Ratio:  4.31/0.85 = 5.07  Good enough!

 

So, depending on your imaging scope and camera, do some browsing for a monochrome guide camera (mono is important) and guide scope that fall into the above range.  Go as light weight as possible, and consider your mounting options.  The guide scope needs to be very rigidly attached so that the guider moves exactly as the imaging scope.  Do not put it on the finder shoe, for example, as that's not stiff enough.  Bars of metal should be viewed as stiff rubber, nothing more.  Microns count in this hobby, and microns are very very very small.


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#3 wsullivan

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Posted 19 October 2020 - 01:30 AM

Thank you for your very detailed answer. My imaging scope is 530 mm and each pixel is 9 mm. giving me about 3.5 arc-seconds. So by your calculation I could go as high as 17 arc-seconds on a guide scope. My problem is that I have not figured out a way to attach a suitable guiding scope to the imaging scope. Also, wouldn't the guide scope have to point pretty close to that which is being photographed? There is no guarantee of that if it is rigidly mounted and cannot be redirected. How critical is it that the guide star be close to the photographed area? Any advise as to how you attach another scope to a Takahashi 530mm would be appreciated. 

Thanks again.

Walter



#4 Stelios

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Posted 19 October 2020 - 02:32 AM

A simple Orion 50mm guidescope and ASI120MM-mini camera is all you need. It has 162mm F/L and 3.75um pixels giving 4.77"/px. 

 

As for mounting it, take a look at the ADM Accessories site. The best way of mounting it is with a top dovetail bar and mounting rings. How easy this is depends on what rings exist now on your scope (model # or picture would help). 

 

You could also simplify your life by getting an OAG instead. At 530mm the ASI120MM-mini should suffice to find stars, although the ASI290MM-mini would be better and allow for use on longer F/L scopes. 



#5 wsullivan

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Posted 19 October 2020 - 03:55 AM

Thanks again for your thoughtful advice.

 

Walter




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