Alt-Az mount for DSLR photometry?

[James Screech]

Is there any reason why a Alt – Az goto mount cannot be used for DSLR photometry? I know there will be field rotation but for short exposures (30sec) I don’t think it should have a detrimental effect on the photometry.

The reason I ask is that I have just ungraded to a CCD camera on my telescope / EQ mount and was thinking about using both this and my DSLR (with telephoto lens) at the same time, to take time series of eclipsing binaries. I can get a new alt – az goto mount for about £220 much cheaper and smaller than a second goto EQ mount.

James

[Grant Privett]

No reason at all you cannot do photometry this way. Keep the exposure short, position the target at the field centre and use a comparison star close to it and if the flat fielding and dark subtraction is okay you will get good results.

Of course DSLRs are a bit noisy, so combined with the use of a short xposure it can mean a lot of exposures are needed.

[Andy Wilson]

Hi James,

Along the lines of what Grant says, good flat fields would be very important for this to work. Otherwise the shifting position of the stars would introduce variability due to vignetting, dust motes, and any other variation in sensitivity across the image.

Assuming you’ve got good flats then I think you would get good results. You can check how well it is working by including a non-variable check star in your processing, to check that it stays roughly constant rather than varying.

Andy

[James Screech]

Thany you for confirming that. I didn’t think there would be a problem, however I had come across a reference on the internet that said it wouldn’t work which set seeds of doubt and so didn’t want to spend the money on a mount only to find it was of no use. Might even try the DSLR with my old ST80.

James

[David Perkin]

Several people using the Deep Sky Stacker software report success in stacking frames taken using Alt-Az mounts. I would guess that other stackers which, like DSS, use triangles of stars would also work. The triangle-based stackers can accomodate frame rotation. DSS requires star trails to be short so you still need short exposures. Stackers intended for planetary work, such as Registax6 and Autostakkert!2 would probably not work well (although Registax6, I believe, also uses triangles) and those based on fourier transforms could not handle the frame rotation.

Regards,

David Perkin.

[Robin Leadbeater]

Hi James,

 I would recommend checking out the tracking capability of the the  Alt Az mount you are considering.  This is much more demanding for Alt Az mounts as, unlike equatorial mounts where all that is needed is a single axis running at siderial rate, both axes have to run simultaneously at varying speeds under computer control. For low cost mounts you may find this rather than field rotation limits the useful exposure time.  I have a vague memory of  a low cost Alt Az mount (Celestron I think) which could be run in equatorial mode using a wedge which might be a better solution but this would depend on the mount drive software being able to cope with this configuration.

Cheers

Robin

[Des Loughney]

I use an undriven DSLR on a standard camera tripod which can cost less than £100. I use the camera with a Canon 200mm lens attached. This means that you can do

satisfactory measurements down to magnitude 10. If I am doing an eclipsing binary series I will take 20 images at 4 secs per exposure. The images will be stacked using AIP4WIN and the stacked image analysed.

if the sets are ten minutes or fifteen minutes apart then the target system will still be in the field of view and can be quickly centred for each set.

Des Loughney

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