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Ratio between guidescope and main telescope resolution

Astrophotography Beginner Equipment Imaging Meade Accessories
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#1 dellavolpe

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Posted 22 September 2021 - 03:29 PM

Hello everyone; first of all I would like that I am a beginner in both astronomy and astrophotography, so possibly some of my questions may appear even meaningless or obvious.
I watched a video today on choosing a Guidescope that suits the focal length of the main telescope. In this video, the author uses the astronomy tools website to determine the resolution (in square inches per pixel) of both the main telescope and Guidescope and calculates a ratio between them, mentioning that the result should "should not exceed 5x". Based on this video and my search for a Guidescope to buy I ask the following questions:
0) I failed to mention my equipment: 10" Meade LX200GPS Telescope (FL = 2500mm, ap. = 254mm, f/10); ZWO ASI462MC planetary camera; ASI294MC-PRO camera; ZWO ASI120MC-S guide camera, f/6.3 focal reducers ef/3.3. Meade resolution with the ASI294: 0.38"x0.38" per pixel; 1.14"x1.14" per pixel (f/3.3 reducer); 0.6"x0.6" per pixel (f/6.3 reducer).
Question 1: What is the considered "ideal" ratio (if any) between the resolution of the main telescope and the Guidescope?
Question 2: Due to FL, would it be correct to use OAG?
Question 3: I understand the limitations of the 10" LX200 for DSO, but I really like it for planetary photos (and I bought it recently, I don't want to change it for now): I thought about the option of buying an Apochromatic Guidescope (today there are options: Evoguide 50DX, Orion, WO, etc.): it serves as a guide and I can also use it for DSO (also aware that small apochromats are not "ideal") But this possibility depends on the ratio between the resolutions (guidescope/Telescope ). Has anyone had this experience?
Thank you very much, sorry for the long post.



#2 t-ara-fan

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Posted 22 September 2021 - 05:09 PM

  the resolution (in square inches per pixel) of both the main telescope and Guidescope and calculates a ratio between them, mentioning that the result should "should not exceed 5x". Based on this video and my search for a Guidescope to buy I ask the following questions:
 

 

with the ASI294: 0.38"x0.38" per pixel; 1.14"x1.14" per pixel (f/3.3 reducer); 0.6"x0.6" per pixel (f/6.3 reducer).
 

The double quote means arc-seconds, not inches. And FYI the single quote means arc-minutes.

 

A ratio of 5 or less is a good rule of thumb.  If the system can guide and keep the centroid of a star in the guide scope steady to 0.2 pixels of the guide scope sensor, then that is steady to 1 pixel with the main camera.

 

An OAG is a good choice for those LONG focal lengths.

 

I can't comment on how good the fork mount is for DSOs. Definitely a good scope for sun, moon, and planets.


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#3 dellavolpe

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Posted 22 September 2021 - 05:51 PM

Sorry for the mistake, I didn't know the quotes. I'm really a beginner chair.gif

The double quote means arc-seconds, not inches. And FYI the single quote means arc-minutes.

 

A ratio of 5 or less is a good rule of thumb.  If the system can guide and keep the centroid of a star in the guide scope steady to 0.2 pixels of the guide scope sensor, then that is steady to 1 pixel with the main camera.

 

An OAG is a good choice for those LONG focal lengths.

 

I can't comment on how good the fork mount is for DSOs. Definitely a good scope for sun, moon, and planets.



#4 TelescopeGreg

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Posted 23 September 2021 - 01:12 AM

5:1 is a comfortable upper bound, but most recommend an Off Axis Guider when guiding a scope with a moveable mirror, since the mirror can flop around a bit.  If your scope can really be brought down to f/3.3 (838mm focal length), you might have a shot at doing some deep sky imaging.  That's still a little long for many targets, but withing reach of many others.  I'm at 910mm, and many are a bit tight, even with an APS-C size sensor.

 

Besides the guider, I'm afraid you may need to either put the mount on a "wedge", or get a different mount altogether.  The problem is that the long exposures needed for deep sky imaging need an Equatorial type mount, and a good one at that with a scope that large.  You have a Alt/Az mount, so the Earth will be spinning around wherever the mount has aimed.  Those trails cannot be fixed in post processing.  You might try it as-is, and see if you can get some of the brighter targets with an increased camera gain and short shutter speed, but saving up for a shorter, lighter scope and good equatorial mount will probably be a good idea too.  Keep the big scope for what it's excellent at - viewing and imaging the Moon and planets. 


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#5 t-ara-fan

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Posted 23 September 2021 - 04:58 PM

Sorry for the mistake, I didn't know the quotes. I'm really a beginner chair.gif

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