[Aavso-photometry] RE: Need Advice, CCD photometry

[Tom Krajci]

>From: Dan <know_ledge at yahoo.com>

>  Tom K.:

>  I'd like to thank you for your suggestions on testing linearity, this has
inspired all sorts
>of other tests that one can think of and I'm VERY excited to try them
all....

Sadly, very few folks test linearity of their CCD.  I'm glad you're doing
it.  I'm interested to hear of the results.

>  I have three additional questions for you ... =)  :
>  - You mentioned you take 1:1 darks to images, what is your take on the
biases?

Actually I like shooting the longest darks I can, and scaling them in
AIP4WIN.  The 1:1 ratio I use is for flat darks to calibrate flats.

As for bias frames...since most cameras these days have decent temp.
regulation and repeatability from night to night...I take hundreds of bias
frames at my operating temp. on a cloudy night...and k-sigma combine (or
median combine) them for a master bias.  I can use that master frame for
months.

Same for darks and flat darks.  I can take oodles of them on a cloudy night
and make masters that last for months.  That means you only have to worry
about flats.  I use a light box for flats and take flats during twilight
just before I start imaging.  That way I don't waste any dark time to take
calibration frames.

>  - I'd assume it's better to scale down dark images rather than scale up?

Yes, if at all possible, take darks longer than your light/data frames and
scale down.  1:1 ratio of exposure times is OK, but scaling up is to be
avoided...unless you have no choice, and your master dark frame is a stack
of many exposures.

>  - I didn't quite follow the advice on aperature circles.   However, I
just noticed an e-mail
>from you hidden amongst my inbox so I'll rethink the issue and get back to
you.

That email should have some graphics showing time-series plots of C2 - C1 at
different aperture settings.  Too small an aperture (for non PSF fitting
software like AIP4WIN) will give you sampling artifacts.

Tom Krajci
Cloudcroft, New Mexico
http://overton2.tamu.edu/aset/krajci/

PS.  I CC'd this to the photometry list...maybe we can draw you over there.
;-)



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