[Aavso-photometry] Used CCDs

[Gary Billings]

Chris,

Almost any camera can be used for photometry -- I've never heard anyone 
say camera X absolutely cannot be used for photometry.  However, the 
more one "steps back" from an ideal camera, the more difficult it will 
be to produce good data.   That might mean more trouble "in the field", 
more elaborate calibrations and study of accuracy / errors, etc.  While 
we all like a challenge, I find there is more than enough challenge 
getting good data, that withstands really careful scrutiny, even with 
an "ideal" camera.

A nonABG ST-7/7E, for example, is a good camera because:
- linear response
- self guiding.  Aperture photometry doesn't require pinpoint stars, 
but spreading photons/electrons across more pixels hampers your ability 
to work on fainter stars...  demands that you use a bigger measurement 
aperture and hence more sky noise...  and if you want to do all-night 
time series, e.g. for determining the time of minimum of a binary, or 
time of max of an RR Lyr, then the self-guiding will keep the chip on 
the same field all night
- a usable FOV (on your C-9.25, f/10, the main chip sees 6.7 x 10 
arc-min)
- 16 bit A/D
- tolerable download speed, even with the original parallel version

ST-237A:
- your FOV will be 7.1 x 5.4 arc-minutes
- does it come in a nonABG version?
- is a frame transfer chip, allowing 0.01 s exposures.  The quickest 
exposure for an ST-7 with its rotating shutter is 0.12 sec, and for 
reliable flatfielding, you should stick to much longer than that.  
However, when fast exposures are required, e.g. for "beat the seeing" 
planetary exposures, a webcam is probably preferred with it's 30/s 
frame rate, rather than with the many-seconds download deadtime of even 
a subframed ST-237A
- note that you would not want an ST-237 (non-A version) with its 12 
bit A/D.  The 237A is 16 bit.

DSLRs:
- are not temperature regulated.  Thus in principle you need to shoot 
darks through the night as temperature changes.  In practice, I don't 
know
- in any one colour, are quite underampled, i.e. there is space on the 
focal plane between the pixels you'll actually be able to use if 
shooting through a photometric filter.  This wastes photons (shame!) 
and depending on the place scale / seeing, can give inaccurate / noisy 
results (as the centre of the PSF -- the place where most of the 
photons from one star lands on the focal plane -- drifts around with 
respect to the locations you are actually able to sample)
- may or may not give you actual raw pixel arrays from the camera.  
Christian Buil's site informs us that Nikon DSLRs apply a digital 
smoothing filter before outputting even the supposedly raw image.  JPEG 
images are definitely unacceptable, by the way, because their lossy 
compression is not designed to preserve photometric precision of points 
in the image
- use with small / short focal length refractor or telephoto might be 
suitable for brighter stars, but flat fielding may be much more 
difficult.  The hugely useful and successful ASAS survey used/uses a 
135 mm telephoto -- but with extremely careful calibrations etc.
- see Hoot's paper from this URL:  
http://www.socastrosci.org/2007%20pdf/07pdf.htm
- I haven't come across any other papers about using DSLRs for 
photometry, but they may be out there...  I asked Arne Henden about 
DSLR photometry at the AAVSO meeting last month:  he suggested they 
probably deliver accuracy comparable to visual estimates (0.05 to 0.1 
mag, yes yes, a few visual observers do much better that that...).  
With astronomical cameras, I'd say 0.03 mag is not too hard to achieve, 
with 0.01 the goal...  One could talk about THAT for many pages, but I 
don't have time!

HX516 etc:
- I don't know anything about these cameras

Your Schuler filter sounds like the right one.

Mobile setup:  do consider an observatory (any kind of shelter can 
leave your equipment set up in).  You'll do a lot more photometry.  Any 
place in Victoria has sufficiently dark skies that you can do lots of 
useful photometry.  I've been working from near the centre of Calgary 
for many years -- a much worse sky.  However, a self-guiding camera, or 
even a separate guiding camera, will not require recalibration at each 
setup, unless you change the guiding rate of the mount, so long as you 
put the camera back in within, say, 5 degrees of your normal 
orientation.

g.



On 29 Nov, 2007, at 1:35 PM, cspratt wrote:

> Thanks for the replies on my previous question about new or used CCDs
> for photometry. I was looking at a used  SBIG 237A CCD which is light 
> in
> weight but doesn't have a guiding chip (do I need one?) . My old
> computer has a parallel port so that isn't a problem. I have an AP600E
> GOTO mount which can handle up to a 9.25" SCT. But this is a portable
> setup and takes time to calibrate the guiding chip, thus I wouldn't use
> the guiding chip in any case. The are aslo some older Starlight Xpress
> CCDs on the used market at a reasonable price. I'm looking at the HX516
> or MX512 as an alternative to the SBIG 237A. Both of these use a
> parallel port interface and are small and lightweight. Again, I don't
> know if either of these are suitable for photometry.
>
> I could also mount a Canon DSLR on the scope and use a telephoto lens 
> or
> small refractor instead of a regular CCD through the main scope, Is 
> this
> the way to go?
>
> I also have a Schuler green 1.25" filter marked "1.25"V bvri VISIBLE".
> Is this the correct V one for photometry?
>
> Thanks again,
>
> Chris. Spratt
> Victoria, BC
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