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Walking on the Moon

Guide scope upgrade


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It's a tradeoff between extra weight on the mount against the advantages of brighter stars and a longer focal length. If you've already got enough focal length on your existing guidescope compared to your main scope to give accurate tracking, and you've never had problems getting a bright enough guide star, then it's probably not worth it. I think you're probably looking at 200mm and 600mm (without reducer) respectively which should work ok for all normal camera pixel size like the ASI120. After all , you only need 1 star to guide on and that's not usually an issue with a separate guidescope. 

If the pixel size on your main camera were very small compared to your main camera then you could make a case for greater guiding accuracy but looking at your kit list I don't think this is the case here..

What's your expected total weight? And will it require an extra counterweight because the scope/ guidescope is too far from the axis?

The focuser on the ST80 might want some work to make it stable enough. A lot of the 50mm finders dispense with the r+p focuser using the threads on the objective to set focus which is more stable in my experience. 

I like big guidescopes if they can double up as finders with a flip mirror assuming the focuser has enough in-travel, but that's a different argument! An old ETX60/70/80 OTA is good and they're very light. 

Not sure if that helps any...

RL

 

Edited by rl
clarification
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I regularly guide with stars with a SNR less than 20 without too many issues, with a 2 second exposure in PHD.

The increased aperture will definately  increase your SNR  in the ratio of 64/25  (areas) which is good, assuming light pollution is not the limiting factor. In addition , if the ST80 at f/5 is slower than your current 50mm guidescope (f/4?) the light pollution will be reduced improving the SNR.

On the other hand, doubling the focal length means the guide setup will see only 1/4 of the sky area with the same guide camera which decreases your chances of having a bright guide star in the field in the first place.....

Swings and roundabouts I'm afraid!

Edited by rl
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If you've already got a filter then it's worth a try.  One practical test is worth a lot of posts here, but I would not expect much benefit.  It depends heavily on the nature of your light pollution. A star shows a continuous spectrum rather than a line emission spectrum. The filter is going to cut down the star's light along with the light pollution. If your light pollution is old-fashioned sodium light with a line spectrum there might well be an improvement but most modern streetlights are unsportingly white in their output, which will leave the optical SNR little changed but the reduced photon count will only serve to emphasize the read noise on the camera. 

 

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i've got a mix of street lights near me but none are causing too much issue. they seem pretty well designed and focus down where they should, even the older sodium one. i did notice a cleaner image with the DSLR but even without the filter 10min subs were still darkish at ISO 800

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It sounds like your skies are a lot better than mine...is light pollution really a problem for guiding? Can't you just up the guide cam exposure time? Try 2 or 5 sec..should still be ok if your polar alignment is up to scratch.

 

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Start from scratch. What is your imaging scale in arcseconds per pixel? (Divide 206.265 by your focal length in mm and multiply this by your pixel size in microns.) Now set up your guide scope and camera in PHD (you may already have done so) so PHD knows your FL and pixel size and gives you an RMS in arcseconds. If your RMS is no more than half your pixel scale in arcseconds then you don't have a problem and needn't waste time or money in trying to solve it. If your RMS is indeed greater than half your pixel scale in arcseconds then you are probably losing resolution to guiding error and might want to work on that.

The notion that you should guide at twice the precision at which you image is not set in stone but it's a value respected by serious imagers. I settle for it rather then embroil myself in arcane discussions.

Olly

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Taking up Olly's suggestion on the maths...

Main scope resolution at 600mm f/l = 1 arcsec per pixel for ASI290 with 2.9 micron pixels

Guide scope resolution at 200mm f/l = 3.8 arcsec per pixel for ASI120 with 3.75 micronpixels

Autoguiders will generally control to 1/10 pixel without too much bother, say 0.4 pixels so if it's all working ok you should be well in. If not, are there other issues like backlash or wind, polar alignment?

What does PHD2 report as the guiding error? I have a very similar setup (ASI120MM 60mm guidescope) and generally get better than 0.5 arcsec RMS on a wind-free night. 

Bear in mind that the Rayleigh limit for an 80mm in green light is only about 1.5 arcsec. If your RMS error were 1 arcsec it would'nt be a disaster. 

Edited by rl
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