Variable Star Observations using the SBIG ST7
There are as many was of observing variable stars as there are observers.
Differences in equipment, experience and software make using someone elses
methods difficult, and I am not suggesting that you should use just one method.
All I can say about my observations is, 'This is the way I do it'.
I am sure that over time I will adjust my methods, as experience grows.
My aim is to produce useful observations for reporting to the AAVSO.
I am currently observing the Long Period and Catyclismic Variables in the
Project of the AAVSO.
I use a CCDV filter obtained from SBIG
I use the F10, 10" LX200 with a Meade F3.3 reducer.
In fact with the camera setup it results in a F4 focal length.
This allows me to get the star in the field of view, after a 6 point T-Point
mapping run, on every occasion. I have tired F10 and find that I often have
to hunt around for the variable.
The length of time I have to image to get a reasonable image for estimates,
at F10, shows up the errors in my alignment and tracking.
I use CCDOPS......in Medium resolution........that is, I bin the image 2 x 2.
I would like to image in High Res...but again...some of the problems that occur at longer focal lengths, also occur if I use High Res.
Procedures are somewhat time dependant......If I have a lot of time available
during an observing run I work through the list from West to East. I take
a 30s image, to see if the star is in outburst and get a quick idea of how
long I might have to image to give myself every chance of observing a star
at minimum. I then usually take a 300s image for the observation. This usually
gives me an image with a reasonalbe chance of picking the variable up, well
above the background level.
But depending on conditions I find that even at 600s I often cannot get below
about mag 17.0.
If time is restricted, I skip the variable unless it is seen on the 30s image. In my mind it is better to get an obsevation of a star in outburst, rather than send in a <17.0. This allows me to move rapidly through the list over a period of say 2 hours.
I have a folder, containing a chart of every variable on the list, and opposite it a printed image of the area at F4, showing the variable. This image is handy as many of the charts do not include all the stars that can be seen in the area of the variable.
I also have a printed sheet, listing all the variables, with spaces for JD
time, and estimates, based on various comparisons listed for the project stars.
And for the CV and LPV...you must use the listed comaprisons.
When completed it allows for a written record of the observations and it is
easy to later transfer the observations to KSOLO, the AAVSO reporting program.
I haven't used the new online report form yet....my Internet connection is still too expensive to do that during the day. And after a long observing sessions I just want to hit the 'sack'. I still have to work for a living.
Before starting a session I create a new folder on my desktop, using the
date as a name ...eg 000926
Into this folder I transfer another folder, containg Dark frames. These I
obtained previously, and for various lengths of time, at different resolutions.
If the camera setup has remained unchanged I transfer the Flat fields I used
on the previous night into the folder as well.
Once an image is downloaded I subtract the dark frame and flat field the image and then save the image to tonights folder with a file name such as AY_Lyr01b.......the name, the first image, in Medium resolution.
This format allows me to take many different images for storage.
I normally note the length of each exposure on the record sheet....eg 60/180/600....
I now move on to the next star, using TheSky's find function, which allows me to name the Variable using the GCVS and then slew the telescope to the location, and start with a 30s image again.
I nearly always calibrate the guiding system.....and tend to use SelfGuide on 300s images or longer or if I am taking a Hi resolution image. Another reason is that I often come across an interesting object...galaxy or PN or something I would like an image of and have been known to stop and catch an image just for fun...it is a hobby after all.
While one image is being taken, I open CCDOPS a second time and upload the
saved image.
I check the time of the image and note it...changing it to JD....using the
AAVSO JD calculator which I saved as a WebPage and have open on the desktop....
I generally crop the image, and then enlarge it twice to give me a reasonable
sized screen to work with.
I set the background and range...to give me a generally flat background.....inverting the image....to a white background often helps to see if the background is flat.....
And then I use the crossshairs function of CCDOPS.
I adjust the size of the 'box' to include all of the 'brightest' of the comparison
stars....and leave it set like that for all the estimates. I click on an empty
part of the sky to set the background and the set the crosshairs over the
first comaprison and set the magnitude using the - and + keys.....(Shifting
and -/+ makes the magnitude change greater).
I then make an estimate of the variable....and record it.
I set the crosshairs over the next comparison...adjust...if necessary...and it usually is necessary to adjust it slightly....as the comparison are to 1/10 of a magnitude and CCDOPS operates to 1/100....and then I take a second estimate.
And then the third......
From these I get an average.......and record it.
I usually get the estimate done in 5 minutes....and now another image is
ready to be saved....
And so on.....
The Miras...LPV's....don't generally need as long as the CV....maybe 60s
is good enough.....so I often have to do a few of them one after another........It
is at these times I tend to image for 600s on something interesting....and
see what I get!.
Also I only observe the Mira's weekly....If I get more than 1 session in a
week, I skip the Mira's and work on the CV's
I intend to image the CV's in outburst, more than once a night...maybe hourly....in future and leave the really faint or difficult ones to the expert's.
I have also downloaded the other variables which have PEP(V) comparisons and am building these into a Non Project list sheet....
I have checked my results against the data on the AAVSO site and I am happy I am getting useful results...
Here
is an image of the area around AY Lyrae......S up/ W left
Thh position of the variable is slightly to the right of centre.....but the variable is not really visible in this image.....and the magnitude goes well below 15.5, the lowest comparison shown in this image.
Granted the image is a bit noisy....and I would do some adjustments to the background level to achieve a cleaner image for estimation.
But compare it to the image below.....
Can
you see AY Lyrae now......
It is not on the left hand side.....that's problems cropping the image....
It is above and very slightly to the left of the 15.5 magnitude comparison star......and would appear just from this to be around magnitude 13.1...13.2 or thereabouts...but CCDOPS does a much more accurate job.....in fact the average was 13.28
(the 12.9, 15.5, 14.2 stars are the comparirsons used by AAVSO
The 11.8 is not used...it's just marked for interest)
Now have a look back to the previous image..
If you look carefully you will see AY Lyr...as a very faint star...
barely above the background level......
But I wouldn't try an estimate on the upper image....
I would try for a longer image and hope to get well above the background to
make an estimate.
So try it youself and see how you get on.....a little organization is required before you get started....start with a few and build up a library yourself....