Re: APPM Image Scale Question and Suggestions

Ray Gralak

Hi Marty,

2x2 binning of 0.26 arc-secs/pixel should be 0.52 arc-secs/pixel, not 1.0 arc-secs/pixel.

However, since this is a color camera, if each group of 4-pixels is considered 1 pixel, then you should enter 0.52 arc-secs/pixel in APPM for the X and Y image scale values.


-----Original Message-----
From: [] On Behalf Of mjb87 via
Sent: Saturday, April 17, 2021 9:12 AM
Subject: [ap-gto] APPM Image Scale Question and Suggestions

Hi everyone.

I have used APPM successfully with my Mach2 and 130mm Starfire GTX. I am now trying to build a model for my
1100 with a CFF 300 on it. I have some specific questions about image scale. I'm using USNO A2.0.

The camera is a ASI1600MC-Cool with a 0.67 focal reducer on a 300mm f/15 Cassegrain. I calculate unbinned
image scale at just under 0.26, which is of course pretty small. I binned 2x2. My initial run was not successful --
many failures, often reported as due to potential image scale issues. Later, using the image-link-test solve
capabilities, I was able to get a couple of the earlier images to solve, but the estimated image scale in those
calculations was reported just over 1.0. When I re-ran the image-link-test solving with that 1.0 entered for image
scale I got about half the images to solve easily.

Question #1: Is this inconsistency because of the 2x2 binning? Does the plate-solve results in the Image-Link-Test
display estimated unbinned or binned image scale? The math works: 4 x 0.26 is about what was reported.

Question #2: Just to confirm, then, that when I enter X-scale and Y-scale factors in the platesolve setup, I should
enter the unbinned values, right (0.26)? The software will then adjust, I assume, for the binning.

Question #3: What options should I play with to reduce the potential for failed solutions due to image scale issues.
How high can I set (realistically) the Image Scale Tolerance? Is 50% too high? 100%? I also assume it would be
helpful to set Catalog Expansion to a higher value, maybe to the max at 0.8. I'n not trying to get fast solutions, just
more solutions.

Question #4: Any other suggestions for running APPM with such a small image scale? I was worried about getting
enough stars so I increased the exposure time on the camera (from 5 to 15) and reduced the sigma above mean
(from 4 to 3). Anything else?



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