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Long Focal Length Imaging

Sky-Watcher EQ6-R Pro Equipment Guiding Troubleshooting
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blastrophoto avatar
blastrophoto avatar
Trying to figure out if I'm going crazy or if I have an equipment issue. I just started trying to image using a C11 and Starizona .63x reducer. I just purchased a used EQ6-R Pro for it to sit on and I get round stars.. but guiding error is around .9 arc second when it needs to be down to .5 arc seconds for optimal resolution. 
OAG w/ 174mm stars are tight and round. Seeing wasn't great both nights I tried imaging. The thing that I'm being thrown off with.. I had another scope running at the same time at 952mm FL.. That scope was getting .6 arc second guiding on an EQ6-R Pro. So was it the seeing that was really the problem for the longer scope? Or could it perhaps be something else? Guide graph looked fine in Dec but the Ra was a little all over. Polar alignment was perfect. 

I guess my thoughts are at the higher focal lengths, the scope will be more sensitive to seeing and thus show more error.. but I'm hoping someone can confirm that. It's just frustrating seeing one scope get .6 arc sec guiding and actually need that on the other. I played with guide settings for a few hours before my eyelids grew heavy and I crashed and had it smoothed out to around .88. 

I love this hobby..
Gabriel Wiklund avatar
Gabriel Wiklund avatar
With a large aperture and long focal length the system is cabable of more accurately "measuring" the effects of seeing, so if the seeing is bad it will be more evident. If the seeing is very good however, then it's the object that will be "more accurately measured"

To give an example that is extreme but very illustrative, Wolfgang Promper was imaging with the ASA AZ 1500, which i believe has almost 9 meters of focal length, if i remember correctly he wrote something in the comments that seeing was not very good for the system at 1.5 arc seconds during one of the captures, and i think he bins when the seeing is not very good since there is no gain in resolution anyway.

If you on the other hand had 1.5 arc second seeing imaging with a Sky-Watcher 200PDS at 1 meter focal length, the image would be razor sharp!
But the size of the image would of course be much smaller, not as close in and not as well resolved details (stating the obvious).

Personally i like the challenges of long focal length imaging, because if good results are obtained they are appreciated much more!

Have fun!
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Robert Sahari avatar
Robert Sahari avatar
I've learned that the resoltion of your guidecam/scope combo shouldn't be less than half the seeing in arcsec, so that the guidestar never jumps more than half a diameter. At long focal length this get's trickier and trickier the worse the seeing gets.

With your scope, reducer guidecam combo you have 0.69" resolution, so your seeing should be 1.4" or better to get optimal results. If the seeing wasn't great, I assume it was 2" or worse, so we're talking atleast three times the guidecams resolution, which makes the guidestar jump more than a full diameter and guiding gets worse ofc as centering the guidestar becomes less reliable once it jumps more than half a diameter.
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blastrophoto avatar
blastrophoto avatar
Both of these responses were very insightful and helpful. Thank you for the knowledge! I will be highly anticipating a night of great seeing and hope that any issues are ironed out before then.
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