[Aavso-photometry] Calibration Frames

[Bob Crumrine]

Hi Jeff,

Then you understand my frustration with the cloudiness around here this 
time of year!

What's the best way to really tell if a set of darks has 'expired' and 
needs to be retaken?  I remember early on seeing my images get speckled 
after a while (points in the image that weren't really stars), retaking 
the darks and then the backgrounds were dark again.

Bob

Jeff Hopkins wrote:
> Hi Bob,
>
> You can take bias frames with any camera. Just set the exposure to the 
> lowest setting and take a bunch. I twill only take a second or two.
>
> While darks are important, when the outside temperature is in the 40's 
> F or colder, the dark frames become less significant. From where you 
> are located (I'm from upstate NY) I know the night time temperatures 
> this time of year hover around freezing. Taking one set of darks early 
> (after the equipment has stabilized, should last through the evening. 
> Since the darks contain a  bias frame, unless your are trying to make 
> master darks (not really necessary) just take a set of darks before 
> each observing session and do an automatic dark subtract on the 
> images. I do that with my DSI Pro and it works well.
>
> Jeff
>
> At 11:54 -0700 01/06/2008, Bob Crumrine wrote:
>> Hi Gary,
>>
>> I'm just getting into this, have a Meade DSI Pro (V filtered but
>> uncooled), and a portable setup, so darks & flats generally eat into
>> observing time.  I don't think I can take bias frames with that camera.
>>
>> I keep a thermometer near the telescope and take darks whenever the temp
>> changes by more than a degree or two, but certainly every 30 min or so.
>> Each time, I take five darks at the exposure I plan to use next and this
>> usually doesn't take much time.  Is five OK?  BTW, I wait at least 15
>> minutes for the camera to cool down after turning it on before I do
>> anything, and keep the camera in the garage, taking it outside as soon
>> as it gets dark on clear nights.
>>
>> I build a light box and right now take my flats partially into the
>> observing session when I'm sure everything is working right (cables,
>> focus, camera orientation, etc.).  Since the flats (and flat darks) take
>> time from observing, I have been taking just a few flats, say 5-7 and a
>> similar number of flat darks.  Sounds like I should be taking more flat
>> frames but what's a minimum number in this situation?
>>
>> Bob Crumrine (CRR)
>> Rochester, NY
>