[Aavso-photometry] NEw or used CCD's

[arne]

David Higgins wrote:
> I’ve never used scopes that small for any CCD work (other than autoguider)
> but in theory a 5” or 6” scope would do OK for stars up to about Mag 8-9
> (though B may be a bit of a struggle – or long exposure).
> 

> 
> From: BFerguson at ucok.edu [mailto:BFerguson at ucok.edu] 
> Sent: Sunday, 2 December 2007 3:08 AM
> To: David Higgins
> Cc: aavso-photometry at mira.aavso.org
> Subject: RE: [Aavso-photometry] NEw or used CCD's
> 
>  
> 
> Thank you David.
> 
>  
> 
> I do B and V on the AAVSO PEP list which, generally, is stars greater than
> 8th magnitude.  Since a CCD has so much greater sensitivity than a
> photoelectric photometer, I wondered if observations of the same caliber
> could be made using a CCD on a scope of only five or six inches aperture
> instead of a photometer on eight..
> 
>  
> 
> It's something I have never seen addressed and the discussion on older CCD
> chips brought it to mind.
> 
CCDs really change your limiting magnitude.  I've used the 4-inch apertures
on the TASS camera down to V=12 or so with good precision and 2-minute
exposures.  ASAS uses roughly 4-inch apertures, as does XO, TreS, HAT
and other exoplanet surveys.  So 5 or 6-inch telescopes will work fine
on millions of stars with a CCD system.

What generally limits you on the bright objects is finding suitable
comparison stars.  With a wide-field system like a camera lens, you
have poor angular resolution but lots of field, so you have your comp stars.
With a typical 6-inch telescope and 1/3-1/2" CCD, the field of view is
significantly smaller, and what you have to do is find those fields where
there are good comp stars nearby - not impossible by any means, but you
do need to pick and choose.  An example is EW Sct, a nice cepheid
with a nearby similar-brightness star; or the 10th magnitude cepheids
and eclipsing binaries in a cluster like NGC7790.

Again, as has been said already: any telescope and any CCD camera will
give decent photometry when calibrated.  If it is your first venture into
the CCD world, I'd go cheap and learn the ropes, and then decide where
you want to go on the next round (and use the original CCD for other
purposes like guiding, or give it away to the next generation of observer).
The older CCDs typically have longer download time or poorer blue response,
but you can work around that as long as they are supported by the most
recent releases of control software (not orphans).
Arne