[vsnet-chat 881] Re: Magnitudes, sequences, etc

["William S G Walker" <astroman@voyager.co.nz>]

Brian, Bruce, Berto, Taichi and all,

Excuse the length of this post but it is a serious offer to the problems of
southern visual observers. The results won't come overnight but they will
come. But first I need some feedback. Is there much of a problem for
southern sequences? Perhaps someone could really clarify this. Is it only
CVs or does it apply to Miras as well? We would like to help but don't want
to do a lot of work if there is no need. 

Several groups in NZ have been getting BVRI filters from Mike Bessell to
use with the ST6. The price is rather better than those quoted in US. There
is a minor problem in fitting them but this has been resolved and I'll
mention it back to him. Looking at the response of the chip there is a
possibility of a low quality +/-<10% U-B measure and we're working on this.
It would be useful in confirming novae and throwing out reddened O and B
stars from sequences. These latter cause visual estimation problems at low
altitudes.

Since it's more than a decade since anyone down here has done any sequence
work we need some assistance. Checking my E Region material I find that it
only goes down to about V = 11 which puts a strain on extrapolation down to
V = 15 or more. Are there other sequences which go fainter? No doubt this
has already been mentioned but I haven't been following these discussions
consistently. Presumably the calibration techniques are the same as for UBV
photometry?

I've just been studying an image of V803 Centauri. This is 11.7' x 8.8' and
in 30 seconds unfiltered shows a lot of stars. My comparison is ~13.2 in V
and CCDOPS will recognise images down to comp + 5.1 in whatever system this
is (~R  equivalent I gather). The ST6B with its fan assisted cooler seems
much better than the ST6 and its colour response is better suited than the
ST7 to BVRI photometry. Putting in the filters will change all of this
around but I suppose in a good cause the exposure times could be extended
to 120 or more seconds. All of which offers about V = 16 plus BRI colours
in about 8 minutes a field. The problem is that the small image size
probably requires some overlap of images - say at least 4 to cover a
reasonable area. So with location and other things it's about an hour per
field for sequences. How reliable are the brighter magnitudes? It would be
good if these were already available.

We've been using Munidos for general reductions of unfiltered images. This
is a nice piece of software and would be able to handle sets of four images
without problems. The internal accuracy of fields would be good after a
couple of sets of images but the observers would have to sort out any
variables. Tieing the field in to the standard system would be a bit slower
- probably requiring images of this and an E Region or two on several good
nights. I have a lot of these and no light pollution except for the
distractions of TV! There are several other people in our section who could
also help.

We also have an interest in Mira stars and will be monitoring these using
BVRI. One frame per field with all sky photometry. This can be tied in with
the CV fields I expect. We're not really interested in the lower parts of
the Mira light curves but the measures will be there and we'll send the
frames down to some of our non-observing people for sorting out values of
the fainter stars. 

The accuracy of all of this will have to be found. We get an internal value
of +/- 0.01 or so on the unfiltered runs. Based on conventional UBV pe -
where we would expect <0.007 down to about V = 10 it should be better than
Brian's suggested 0.03. But even that would be good enough for visual
comparison sequences. I'm not really internet oriented - after three
decades of computers I stay away from them as much as possible - but we're
setting up web pages and will try to make this material available widely.
There are two other Bessell filter sets available but not yet in use. We
should begin to get results by the end of this month. I'm fitting the
filters to the ST6B wheel today and will try out the whole assembly again
tonight if fine. 

The only problem with all of this is that it begins to show up the
limitations of the telescope and charts. After locating V803 last night in
about 30 seconds I then spent an hour trying to get the star on the CCD
frame. Both the field orientation and the eyepiece had been changed to
improve matters- which confused the mind a little! It doesn't help that the
observatory is now an attic at the other end of the house from the computer
(which will change soon) but it's becoming clear that the camera needs to
be set up like the conventional photometer with a field eyepiece,
retractable mirror and all that! The eyes are never dark adapted - you have
to look for faint objects on the computer screen - and there is not usually
a bright star nearby. Even the CCD responses differ - mine produces images
which don't entirely match the Taichi Kato and the CBA charts. So it's not
always as simple as it sounds. I can see why many amateur CCD people stick
to easily identified fields. It's something of a change from a 50cm Zeiss
or a 41cm B & C to a backyard 10" Meade LX - the first of the LX series!

The urgent things now are some faint and reliable CCD calibration
sequences. Could Brian Skiff help with these? I'm at 35S so can see to the
north a bit - my NE horizon is good. But if there are southern ones so much
the better. We'll look at a few of the well known southern CV fields
initially but would appreciate knowing what the real problem fields are.

Regards.

Stan Walker
Photometry Section, RASNZ

----------
> From: Brian Skiff <bas@lowell.edu>
> To: JRSanf@aol.com
> Cc: vsnet-chat@kusastro.kyoto-u.ac.jp
> Subject: [vsnet-chat 880] Re: Magnitudes, sequences, etc
> Date: Friday, May 01, 1998 2:31 AM
> 
>      To determine V magnitudes (say), one needs to observe in two
filters,
> V and some adjacent one (B or R usually) in order to correct for the
color
> term in the system (there's no such thing as a set-up with a zero color
> term).
>      Mike Bessell has published in several places (PASP, and the
erstwhile
> CCD Astronomy magazine) recipes for filters that very nearly establish
the
> bandpasses using the transmission of the filters alone---one says the
> response of the system is "filter-defined".  Thus one needn't be
concerned
> about the varying response of different CCDs, as John suggests.

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