How to image Doubles and Variables?

[Jove]

Hi all,

Has anyone tried imaging doubles and variables? I think it would be interesting to take images to see how things change over time - and even do some CCD photometry on variables maybe. Does anyone know if the webcam-like CCDs (lime the DMK cameras) work for this kind of project? I'd be very interested to hear if anyone has done anything like this before and what kind of set up you used.

Thanks!

[AztecastroMcJ]

u can u simple point and shoot cam

or mob phone cam

or webcam at least with the easier brighter doubles, never tried Variables

heres Albireo with my Mob phone Cam and webcam

think i prefer mob phone cam result tho having both is good

http://www.flickr.com/photos/39852236@N05/4440885156/sizes/o/

i'm sure you'd get really nice pics and better result

with DSLR or CCD cooled cam aswell

though they are much more expensive

James

[Jove]

That's quite an effective image for such a simple technique! You just pointed your mobile camera/webcam down the eyepiece I guess?

[AztecastroMcJ]

yep handheld using 25mm ep

pic obtained was very noisy and stars were tiny

so i croped pic, zoomed in, lowered brightness, increased contrast, (to hide noise)

[lunator]

I have taken pics of about 50 doubles using a compact camera and bracket.

If you look down this section of the forum you should see them

Cheers

Ian

[brianb]

I'd be very interested to hear if anyone has done anything like this before and what kind of set up you used.

I actually do a fair bit of photometry with nothing more complicated than an ordinary DSLR, a very ordinary 50mm f/1.4 lens and a fixed tripod. I get best results on variables in the mag. 4 - 6 range; on a good night (nice even transparency) I can get results accurate to better than 0.01 mag. The essential thing to do is take lots of images & play clever with the stats. I actually throw away the blue & red layer information, the spectral response of the green layer is similar to the visual / V measure which is what we're mostly interested in.

You must never apply sharpening, noise reduction or any form of processing other than contrast stretch if you want to obtain good photometry.

I never post the images I take because, when taken properly for photometry, they're really ugly & boring ... and they always come in big sets - I usually take 15 at a time so that defects in the sensor chip are minimised (they all have them) & to provide a statistical measure of the accuracy of my results.

[Jove]

Thanks for all the replies... I have so many questions!

Ian, is the bracket you use used to attach the camera to a telescope allowing you to do afocal photography more easily?

Brian - very interesting to hear about your photometry. What kind of field of view do you get with your DSLR - I would have thought it would be quite wide, which would help make sure you capture the stars you want, but sometimes make it difficult to separate stars? When you have acquired your images are you using some software to perform aperture photometry? It's interesting you trow away the Red and Blue data, guess this would mean a monochrome webcam would be more appropriate than a colour camera?

Sorry for all the questions... this area of astronomy really intrigues me and is something I would love to try myself. I have been reading the AAVSO's guide to CCD Photometry [PDF] as an introduction to the field, but that assumes access to a decent CCD and skill in using it - so being able to use a DSLR would be a boon. I was thinking I could use eyepiece projection at a low magnification to take some images of starfields with variables in shot, and repeating at intervals to build up the light curve (I would use some kind of aperture photometry software as described in the AAVSO manual). Does that sound like a reasonable idea to you? Does photometry require long exposures, such that I would need an equatorial mount, or could I take many short exposures and stack them?

Thanks again!

[lunator]

Hi Jove

Yes I use a bracket. It cos about £15. There are better ones available now.

It does allow the exposure time to be around 1 second with allows fainter stars to be picked up.

Cheers

Ian

[brianb]

I would have thought it would be quite wide, which would help make sure you capture the stars you want, but sometimes make it difficult to separate stars? When you have acquired your images are you using some software to perform aperture photometry? It's interesting you trow away the Red and Blue data, guess this would mean a monochrome webcam would be more appropriate than a colour camera?

Yes, the field is quite wide. Just about covers the "W" of Cassiopeia with the 50mm lens. I use the central part of the field, this avoids issues with vignetting / illumination falloff towards the edge of the field. No real issues with seperating stars, if there were I'd use a longer focal length ... or work on different stars where the issue doesn't arise ... there are plenty of underobserved candidates to choose from!

Yes, a mono CCD would be a good choice - but you do need a filter of some sort, CCDs are very sensitive to infra red light & many long period & semiregular variables are several magnitudes brighter in IR than they are in the visual range ... green #58 plus an IR blocker is a reasonable approximation to Johnson V. A CCD with no antiblooming gate, or an ABG that can be disabled, is useful as it means that the response is linear up to saturation - with an active ABG (or my DSLR) you have to keep exposures down to at most 1/3 full well to maintain linearity.

I use the aperture photometry too in AIP v2. Works real good ...

[Jove]

Thanks for the info chaps. My first step will be trying to get some sort of reliable (focusable!) Scope -> DSLR setup going, hopefully the moon will oblige this evening.

Brian, one further query - is the AIP software you use the same as comes with the Willman Bell book "The Handbook of Astronomical Imaging" ? (which looks well worth looking at). Do you know if the software is available separately or if the book has a distributor in the UK?

Again, thanks from an (over-) enthusiastic beginner (really, I feel like a puppy bouncing around)

[brianb]

Brian, one further query - is the AIP software you use the same as comes with the Willman Bell book "The Handbook of Astronomical Imaging" ? (which looks well worth looking at). Do you know if the software is available separately or if the book has a distributor in the UK?

That's the one. Make sure you get the 2nd edition ....

AIP software not available seperately. Mind you the book is a damned good manual.

I got my copy through SCS Astro ... not sure who else stocks it in the UK.

[Jove]

Thank you for the information Brian, it's very helpful

[FraserClarke]

this area of astronomy really intrigues me and is something I would love to try myself. I have been reading the AAVSO's guide to CCD Photometry [PDF] as an introduction to the field, but that assumes access to a decent CCD and skill in using it - so being able to use a DSLR would be a boon.

It's certainly possible with a DSLR, or anything which can take an image -- your eye included!!

The reason there are "specs" on the equipment from AAVSO etc is to provide some kind of standardisation and let you compare your results with others more consistently. In particular, standard filter curves are well defined, and won't be properly replicated on a DSLR. However, that's only relevant if you want to compare with others in a standardised way. If you're doing it for yourself, or to learn the techniques, a DSLR will do just nicely.

Good luck!

[Jove]

For anyone else reading the thread and interested, I found this published article on DSLR Photometry which discusses the strengths and weaknesses of DSLR versus 'science' CCDs. Click "Free Full Text Article" for the, err, full text, free.