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Guide scope selection..


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All, I've been mulling over guides scopes, so this is more of a discussion..

The guiding principle for me, and one I'd like to verify with you, is that the guides scope should match, the arc-second resolution of the imaging scope and camera (ignoring airy disc star bloat).

I understand that the atmospheric abberation will change quicker than the mount can react, thus AO helps within reason.

With a long exposure time the error will cause loss of clarity due to the photons being scatted in a distribution pattern against the correct target pixel.

I'm thinking if I get an f4 SW ST80 (400mm) as a guide scope, the arcsecond/pixel resolution for the guiding will be higher than the main imaging scope (675mm fl and perhaps later with a longer fl) with the result that there will be more loss of detail.

I know that just adding an ST80 would work and all the pictures would be good, but I'm interested in peoples thoughts.

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Crikey! I didn't know you needed to think that deeply about what type of guidescope to use. I just bunged an ST80 on the top of my 250mm Newt and off I went.

Must have missed something, or my OCD rating is lower than I thought, but it works fine for me (most of the time)!

Edited by lukebl
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Yup, I understand that they work off flux, based on a far higher precision. Although this is susceptable to airy discs bloating the star depending on the wave length of the star and the aperture size (not assuming the abberation is linear in it's position/effect for each wavelength).

I was wondering if it's worth going for a longer fl/larger aperture guide scope to screw as much accuracy out of the NEQ6 or a higher resolution mount as possible in terms of PEC tracking.

I suppose that the accuracy of a seperate guide scope is a moot point as the mount can't move that accurately (although it would help increase the accuracy of the feed back and PEC, even using sub-pixel flux centre tracking). An AO would obviously using the main scope's optics and thus would be operating resolution would be defined by the guide cam only.

luke - trouble is I look at a telescope, the target, resulting image and I see mathmatics! :D:p

Edited by NickK
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The guiding principle for me, and one I'd like to verify with you, is that the guides scope should match, the arc-second resolution of the imaging scope and camera (ignoring airy disc star bloat).

I understand back in the bad old days, that was the perceived wisdom but as Peter says, these days we have software that works on sub-pixel calculations so you can be much more flexible with the f/l of the guidescope. I think you're complicating the process by overanalysis when a simple ST80 or even finderscope would do the job just fine.

The only choice in my view is would you go for a cheap scope like the ST80 and use that only as a guide scope or go for a small APO like a Megrez 72 so it could double up a wide-feld imaging scope as well. My usual scope for guiding is my Zenithstar 66 but when I want that wider field it does imaging duty while the ED80 guides.

Tony..

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I use a 500mm fl f8.0 telephoto lens as my guidescope with a meade DSI IIc as the guidecam never failed to find a guidestar in the fov or needed to adjust it i use this whilst imaging with the Megrez 72 and CPC800 upto f20 and once at f30 using PHD as the guiding software. I have bought a ST80 for use with the Synguider on the HEQ-5

Billy on the San Fran

.....

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