[Aavso-photometry] In defense of the ST-9 & CCD choiceConsiderations

[Tomas L. Gomez]

This interesting discussion about pixel size came at a very appropriate
time for me: I was about to buy an ST9 camera for my new
LX200R 8-inch, but thanks to this mails I am now pondering about
going for the ST8!. This will be my "travelling telescope", meaning
that I will take it to dark places in the countryside, so I guess that
the seeing will be good (before I buy, I will check this).

I have a couple of questions:

1) What do you think about defocusing a bit in order to increase
the size of the FWHM?

2) As far as I understand, both ST8-XME and ST9-XE cameras have a
CCD in which each pixel has two detectors (a "normal" gate and another
blue sensitive gate). If the usual rule is to have an FWHM of 2-3 pixels,
but, to avoid that this pixel structure spoils the photometry,
I guess one should increase this rule to, maybe, 3-4 pixels, isn't it?

3) Is it OK to take a flat with bin 1x1, do a "software bin" to 2x2, and
use this flat for pictures taken at bin 2x2?. What about darks?

As far as I can see, the only disadvantage of this method is that a
"software bin"
will have more noise that a bin done on the camera, but taking more flats
this will overcome this problem.

Thanks!

Tomas L. Gomez (GOT)


On 6/12/07, Wolfgang Renz <w_renz at onlinehome.de> wrote:
> Hi Michael, Hi Arne
>
> Then please enlight us and let us take part in your experiences.
>
> BTW:
> The mentioned Apogee page says
>     "A good rule of thumb to avoid undersampling is to divide your
>      seeing in half and choose a pixel size that provides that amount
>      of sky coverage. For example, if your seeing conditions are
>      generally 4 arcseconds, you should achieve a sky coverage of
>      2 arcseconds per pixel. If your seeing conditions are often 1
>      arcsecond, you'll want a pixel size that yields 0.5 arcseconds
>      per pixel."
>
> So they actually don't recommen a fixed 2"/pixel, but spreading the
> light (FWHM?) over (at least) 2 pixel (in diameter) to avoide under-
> sampling.
>
> Clear skies
>  Wolfgang
>
> --
> Wolfgang Renz, Karlsruhe, Germany
> Rz.BAV = WRe.vsnet = RWG.AAVSO
>
>
>
> ----- Original Message -----
> From: "Michael Newberry" <mnewberry at mirametrics.com>
> To: <mlfleenor at charter.net>; <aavso-photometry at aavso.org>
> Sent: Tuesday, June 12, 2007 6:03 PM
> Subject: Re: [Aavso-photometry] In defense of the ST-9 & CCD choiceConsiderations
>
>
> > If CCDU makes such statements, then I would kindly suggest that Apogee's
> > website is not giving the best advice. To my knowledge, no one at Apogee is
> > an astronomer. Arne and I (and others) have "been there and done that" for a
> > lot of years. Take Arne's advice!
> >
> > Michael Newberry
>
>
>
> ----- Original Message -----
> From: "arne" <arne at aavso.org>
> To: "AAVSO photometry" <aavso-photometry at aavso.org>
> Sent: Tuesday, June 12, 2007 3:54 PM
> Subject: Re: [Aavso-photometry] In defense of the ST-9 & CCD choiceConsiderations
>
>
> > Mike, have you seriously found a recent posting or reference that
> > gives a guideline of 2arcsec/pix?
> > Arne
>
> > ----- Original Message -----
> > From: <mlfleenor at charter.net>
> > To: "AAVSO photometry" <aavso-photometry at aavso.org>; "arne" <arne at aavso.org>
> > Sent: Tuesday, June 12, 2007 8:45 AM
> > Subject: Re: [Aavso-photometry] In defense of the ST-9 & CCD choice
> > Considerations
> >
> >> > Mike, have you seriously found a recent posting or reference that
> >> > gives a guideline of 2arcsec/pix?
> >> > Arne
> >>
> >> Re: 2 arc-second/pix sky coverage  check out CCD Univeristy @ apogee's
> >> website.
> >> http://www.ccd.com/ccd113.html
> >>
> >> In re-reading Tim’s post I see that he said 2-3  pixel sky coverage not
> >> 2. I was wanting to illustrate that as long as you are not running
> >> under-sampled that this criterion was not something that has to be
> >> explicitly followed.
> >>
> >> I use my SCT with a compressor and often run binned 2x2 yielding 1.24
> >> arc-sec/pix resolution or ~ 2.4 pixel sky coverage- however many times I
> >> am running 1x1 @ 0.62 arc-sec/pix  resulting in 5-6 pixel sky coverage
> >> with my typical seeing. Clearly outside of the 2-3 pixel criterion yet my
> >> results are excellent. ( I often achieve 1mmag precision with R-band 5
> >> minute averages with extra-solar planet candidates during out of transit
> >> baselines. )
> >>
> >> My frustration with the ST-9 was due in part to some of the images having
> >> blocky stars- I was under-sampling! I also was running @ 2260mm focal
> >> length. I could’ve run the SCT @ f/10 but here again the fov would be much
> >> too small for me.
> >>
> >> I do take a slight sensitivity hit but also need the resolution to resolve
> >> close stars.
> >> While the ST-9 may better meet the  magic sampling requirement there are
> >> other considerations.
> >>
> >> I bet there are a lot more folks doing photometry with SCT’s using ST8’s
> >> instead of ST-9’s .
> >>
> >> Mike
>
>
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