GT Sgr

[Terry B]

Dear All
I had a request to take some images of the dim mira variable GT Sgr to attempt to make a visual sequence for the star. It is quite close to M28. As it turned out this was a bit unneccesary as there had been a typo in the original request and a different variable star was actually needed.
Before I knew this I had taken a series of images throught B,Vand R filters.
Exposures were 120 sec x 3 for the V and R and a single 300sec exposure for B.
I decided to measure the region anyway. I used a modified all sky photometry technique with comparison stars from nearby standard regions at 2 different airmasses and measured the magnitude of a selection of stars near GT Sgr. These are marked on the image below. It is a V image inverted to show the stars more clearly.
I have also made a colour image using the photometry filter images as BGR. These are pretty close to the normal imaging colour filters but have more overlap. It has produced a reasonable image of M28. It is a very busy region of the sky. The Mira variable is clearly a very red star and the colour of the comparison stars can be seen as well. There are quite a few blue artifacts from cosmic ray hits that I'm too lazy to try and remove. The other colours have less because the stacking method removed them.
The images were taken with my VC200L at 1800mm with a ST10XME. The field is about 30arcmin x 20arcmin.

[Rob_K]

Originally Posted by Terry B

As it turned out this was a bit unneccesary as there had been a typo in the original request and a different variable star was actually needed.

Eek, apologies Terry, my fault (confusing a Roman "I" with a "T" ). I'm reminded of the quote in your signature. Nice job anyway.

Yes, it's a very busy piece of sky. Here's an animation of an arbitrary selection of higher-amplitude variables picked up in blinking two images, one from May 2011 and the other from August 2011. The variables are in the centre of each frame (mostly Miras). It doesn't take account of the huge number of minor variations visible, and as you could imagine, a different pair of images from the library I've built up on this area over the last 18 months would produce a different set of variables. And the shots are not terribly deep.
http://i727.photobucket.com/albums/w...-20aug2011.gif

The existence of obs refers to the AAVSO database - most have no observations recorded. However there would be a long history of observations to be gained from automated surveys of this area. Each frame is a bit over 50' square, and north is roughly to the left of each. Please excuse the dithering introduced by the animation program - drives me spare!!

Cheers -