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Photometry Project

PostPosted: Sun Jul 27, 2014 7:19 am
by DaveMcC
Well after last weeks talk I decided to see what could be done with my own basic kit. First off I found that the OU project star I spoke of. V829 Her, too dim and hard to find from Skellingthorpe. Yes, I got some images at 30 and 60 seconds with DSLR, but of VERY poor quality. The Alt Az goto mount I used is no good for even at 30 seconds. Its constant jigging about was noticeable. Next I found, the hard way, that Maxim DL photometry options as demonstrated last week only works when using a FITS format file, not only that, it needs specific data in the file header that a 'conversion' of a JPG to a FITS format does not supply. However, basic comparison to a known reference star is still possible using the Maxim DL information window, just not the automatic CSV file creation for N frames at one go. Time to get out paper and pen. I will post a set of step-by -step instructions - when I work it out myself.

Stacking gave a brighter image (if you want a pretty picture) but the relative brightness of target and reference seemed to get offset in differing directions invalidating the combined frame as a 'science' frame. It may be me and perhaps somebody else who is better at stacking could confirm this or otherwise.

OK so the plan now is to use two easy to find brighter stars, Eta and Sigma Aquila (Aquila is due south at midnight, 8th Aug) . I will use either the 110 Vixen on an old equatorial mount fitted with a basic 'clock' drive or the C8 on its fork & wedge and see how that works. I have yet to work out what my reference star will be in each case, I will wait to see what is available in the image first. Maybe there is an AAVSO / BAA chart available. A possible cheaper (better in this case ?) software option is AIP4Win as referenced in the BAA paper "Variable Star Photometry with a DSLR Camera" by Des Loughney, ... ughney.pdf , anybody used it ? It comes 'free' with a book - that costs about £50.

Watch this space.

Re: Photometry Project

PostPosted: Sun Jul 27, 2014 8:00 am
by DIYastronomer
I have AIP4WIN2. I have never used it for photometry but it is supposed to be good for variable stars. I use it for planetary and deep sky stacking and enhancing. The book runs to 700 pages and the software needs registering. I have an older version somewhere which you can borrow.

Re: Photometry Project

PostPosted: Thu Aug 07, 2014 9:00 am
by meteorgw
Hi Dave,
I met Des Loghney at the BAA VSS meeting a few weeks ago and I have his contact details and am sure he would be pleased to offer some advice. If you are studying brighter stars the exposures can be short and there is no need to bother tracking as a small amount of drift does not matter. They even recommend using slightly defocussed images, but this could cause problems if there are other stars close to the target or ref stars. Would using the dslr to image in RAW mode get around the FITS problem?
Regards, Graham

Re: Photometry Project

PostPosted: Thu Aug 07, 2014 11:04 pm
by DaveMcC
Well its time to write up progress so far. mmm not a lot.

Monday 28th /29th July.
The plan was to use a simple set-up with short exposures in area of Aql. My targets are eta and sigma. So starting at 22:35 BST took about 100 frames using an auto-timer. First with the DSLR on the goto Alt/Az mount with tracking ON then OFF. 55mm lens fitted, f5.6 and times from 30 to 15 seconds. Next repeat the run with a telephoto lens at 75, 100, up to 300 mm. Then repeated again with camera fitted via a T2 mount to an 80mm refractor that I had reasonable results with when using film. In this experiment it was on an un-guided equatorial mount, but for 3 to 5 seconds @20 shots per run.

Best results using camera on its own was with tracking OFF, as Graham had suggested, with the Telephoto lens at its minimum of 75mm , 15 seconds. Only a small trailing was seen on the PC screen on checking next day.

Of the 4x20 shot runs with the 80mm telescope. 3 seconds gave a total image time of about 60 seconds when stacked. And that is another story. Registack 5 would not play ball with the jpg's. Probably me but in end used deep sky stacker, another free bit of software. Was able to save final stacked image as a Fits file. When viewed in MaximDL - the best laid plans .... fell apart, see below

Tuesday 29th/ 30th July
The plan on this night was to repeat last nights work but use the fork & wedge mounted C8 (unguided) as a platform to piggy-back the camera on. Camera fitted with 75 mm at f5.6 and 4 second exposures taken in sets of 20 frames again. Took 3 sets like that and one set at 300mm at 1 second. Remembered to take some Dark frames at the end of the night this time.

Covington's book on astrophotography suggests the maximum exposure times you can use with no tracking at various declinations for various lens. For OK images t= 1000/ (F * cos stars declination) so at 0 dec where I am looking and with a 75mm lens this works out at 13 seconds. For sharp stars its t= 343/ (F * cos stars declination) giving 5 seconds at same declination. In practice the 75mm lens piggy-backed on the C8 at 4 seconds did give quite a good final stacked image, in my view. The C8's mount was unguided and polar alignment basic and so between frames there was some drifting south but no trailing within the image.

Deep Sky Stacker seemed to ignore my Darks and as for Maxim DL, well, without the fits data file image data, its useless (you can have fits format, but the meta data inside will still be blank if your camera / CCD does not provide it) So I knew I had to pick a known reference star and measure and record with pen and paper the relative brightness of any target. What I found was the relative brightness of 'known' stars to the reference did not match my measurements. So this is work in progress and I suspect the AIP4WIN software may be better for what I want to do. As I said, work in progress.

Re: Photometry Project

PostPosted: Mon Aug 11, 2014 2:09 pm
by meteorgw
Hi Dave,
I have been having a brief look at the Maxim DL tutorials. From what I can gather if you use MDL to caputure your images then you will end up with FITS image files. It does have the facility to control DLSR cameras as well as the expensive CCD cameras. MDL though is rather complex looking and all the writing and buttons are so small which is not great when you are using it in the field. I bought a great little program, Backyard EOS, that is so much easier to use for image capture but does not do the photometry bit. Just connects with the usb cable that came with my 1100D camera so no extra gear to be purchased. There is a 90 day free trial and it is not very expensive to buy. I have found that the files I have saved (RAW) can be opened in MDL, so could be used for photometry or other processing. So far I have only used Backyard EOS for running a series unguided 30 second exposures in the hope of capturing some meteor images. None yet though! Need some input from Komet for expert advice. Graham

Re: Photometry Project

PostPosted: Thu Aug 21, 2014 12:25 pm
by DaveMcC
I'm in contact with Des L now for his advice too. I will try again with RAW frames next time I am out.