Affiliation
American Association of Variable Star Observers (AAVSO)
Wed, 08/27/2014 - 23:28
I recently made calibration runs to establsh eV and eI on two diferent telescopes in preparation for V-I photometry. The runs were made on the same night, same red/blue star pair, same photometer and filters, same location, with a 10" SCT and a 24" classical cassegrain. The net star counts appear to show that the Cass optics are comparatively less efficient towards the blue end of the spectrum. To wit:
(SCT V counts/I counts) / (Cass V counts/I counts) ~= 1.26
Hence, the Cass count ratio is smaller than the SCT count ratio, implying an excess of I counts or shortage of V counts in the Cass ratio, as compared to the SCT ratio.
What would be the likely cause of this effect? Inferior or aging coatings on the Cass? Dust on the optics?
Tom
How interesting! Actually, what this shows is that the Ic signal in the SCT is low. A straight Cassegrain system should have the most uniform response, since it contains all reflective optics. Once you introduce refractive elements, all bets are off.
What might be more instructive is the ratio Vcounts_SCT / Vcounts_Cass, to see if it matches the ratio of light collecting area.
Arne
Hmmm... The collecting area ratio of 24" vs 10" is 5.76.
In V band, I get 6.83x more counts on the Cass. In Ic band, 8.66. The V and Ic magnitudes are very close for one of the stars, and when I got very similar readings for that star in those bands on the SCT I took that as a consistent sign. However, that reasoning assumed a similar detector response curve for V and Ic, which is not the case. The SSP3 is much more sensitive in the red than the green, so the Ic counts for that star should have been higher.
I agree that the SCT is weak in Ic, but I'm surprised that the corrector would affect Ic more than V.
Tom