Quote:
Originally Posted by Logieberra
Peter, I would use a 2x barlow if I was you. From memory, an image scale of around 1 is ideal.
Some useful info on PEC image scale from Ray Gralak, the Pempro guy:
What Image Scale is Best to use?
I am going to start out this section with something that I think we all learned early on with CCD Imaging and focusing. You cant focus at a 2x2 bin and then image at 1x1 binning. You can, although, focus at 1x1 binning and image at 2x2 binning. If you are going to image at 2x2 binning only, then you can focus at 2x2 binning. The rule is you must focus at the highest binning mode you plan on using.
I have found that PEMPro follows the same rule. Think of it as focusing the periodic error of your mount. If you only image at a 3.5 arc seconds per pixel for wide field work, then you can use that image scale to program your mount with PEMPro. But don't expect to throw on your 12 inch Meade at .55 arc second per pixel on your mount and still expect the same level of performance.
Use the smallest image scale you will use with your mount to program your mount. The better then sampling rate (i.e. smaller the image scale) the better PEMPro can analyze and correct your mount. Remember, ideal sampling is the seeing in your local area divided by 3.3. If your local seeing conditions averages 2.0 arc seconds, then you should aim for a image scale of ~.60 arc seconds per pixel. Anything more than that wont achieve much better performance.
Now, if you only image at 1.75 arc seconds per pixel, then use that to program your mount. I have programmed my mount with PEMPro at 3.5 arc seconds per pixel and it worked great with my wide field set-up. But when I measured my PE with a C9.25 SCT, I found the error to be to large for that image scale. So I re-programmed my mount with the C9.25 and decreased my PE error by 4X! Now the next time I do wide field imaging, my mount will be 'super-tuned' for the job, and that's just what we want.
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Good post.
Very informative.
Focusing at 1x1 binning though can be a pain on many cameras that have slow downloads. I suppose you get it exact at higher binning and then switch to 1x1 for a small adjustment.
Greg.