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Old 15-03-2019, 03:08 PM
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Stonius (Markus)
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Removing focal reducer yields *more ADU?

Hey there,

I was just going through some test data I shot with my RC10.

To get the native focal length of the scope I took a frame without the focal reducer in place (Telecompressor CCDT67).

When I looked at the statistics, the shorter focal length image (the one with the field flattener) captured less ADU than the one with; 787 ADU Vs 923. All variables were otherwise the same.

Seems counterintuitive that a slower system should be, erm, faster?

Weird.

I'll test it again when I get the opportunity to verify - this was based on two individual 1 sec exposures.

It could be anomalous short exposure response of the 1600.

Markus
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Old 15-03-2019, 03:45 PM
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Slawomir (Suavi)
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Quote:
Originally Posted by Stonius View Post
Hey there,

I was just going through some test data I shot with my RC10.

To get the native focal length of the scope I took a frame without the focal reducer in place (Telecompressor CCDT67).

When I looked at the statistics, the shorter focal length image (the one with the field flattener) captured less ADU than the one with; 787 ADU Vs 923. All variables were otherwise the same.

Seems counterintuitive that a slower system should be, erm, faster?

Weird.

I'll test it again when I get the opportunity to verify - this was based on two individual 1 sec exposures.

It could be anomalous short exposure response of the 1600.

Markus
Could it be that the bias / moon phase / target was different in these two scenarios?
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Old 15-03-2019, 05:03 PM
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Stonius (Markus)
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Thanks Suavi,

the culprit was, well, my own stupidity.

I was sure all variables were the same, but when I examined the FITS headers, I realised that somehow I'd managed to take one exposure at 1 second and the other at 10 seconds, hence the difference.
Duh.
I can only assume that I either adjusted the exposure or entered it wrong. Not impossible as I was more interested in getting fields to solve for focal length calculations than comparative ADU.

Ahem, as you were.

Markus
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