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Hi All,

With the appearance of SN2011by, I really would like to use my equipment to do some real science and learn how to monitor the brightness of the outburst over time.

I currently have various scopes, and an SXVR-H18. Normally, when taking deep-sky shots, I pick a relatively long exposure (say 5 minutes) and take lots. I am wondering how different things are for photometry.

Specifically, as the H18 has anti-blooming gates, and a relatively shallow well depth, I guess it is critical to keep the exposure length shorter so that the chip doesn't become saturated?

The full specs on the chip for the H18 are here: Starlight XPress SXV series cameras Page

Also, do you still need to shoot mulitple exposures and stack? I am guessing that raw frames (no calibration) should be used as well?

Any advice gratefully recieved on how to get the most accurate results out of this sort of kit.

Thanks in advance,

Richie

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Hi Richie,

Glad to hear you're interested in doing real science with your kit :D I had a student doing photometry of supernovae with a 16-inch telescope and an ST8 camera over the winter -- the project worked fantastically well and we got some great light curves!

The observations you need for photometry are not that different than for general pretty pictures. You're absolutely right though that you must avoid saturating the star on the detector. Take a few test exposures, and see what the pixel values are getting up to. As well as the supernova, you'll want to have a few (3--4) other stars in the image that you can use as references. Check they're not saturated either. Ideally you will also need to take a set of exposures (3-5) so that you can correct for defects in the camera (hot/dead pixels, cosmic rays, etc).

Calibrating the images properly is vitally important. The processing for 'science' images is pretty much the same as for pictures -- but you need to worry much more about maintaining the integrity of the data rather than making it look nice. You'll need dark exposures as normal, and flat fields as normal. Then align and stack the images (need to be a bit careful about the combining algorithm here -- a median is not optimal, but usually pretty robust). Don't do *anything* like scaling the image/changing the levels etc.

What software do you have available to you? MaximDL has photometry algorithms built in to it, if you have that. Otherwise I'm sure there must be free and relatively easy to use "aperture photometry" programmes out there. I can recommend free professional ones, but they have a significant learning curve.

To do the photometry, you basically measure the brightness of supernova by summing up all the counts in a defined aperture size, and comparing it to the brightness of nearby stars. That will be enough to let you see how the SNe brightness changes over time and make a lightcurve. You're making the assumption of course that the nearby stars are constant intrinsic brightness -- which is usually OK. If you want to measure the actual magnitude of the supernova (rather than just it's change), you'd need to make observations of known 'standard' stars to calibrate your photometry onto a standard system. That's a bit more involved, so I'd just try to get a lightcurve first.

To get a nice SNe lightcurve, you want a measurement every 3--7 days. This is usually the limit in the british climate :p

sn2011by is probably already past its peak brightness. You could measure the decline of course, or keep an eye out on the supernova pages for new ones coming along.

I'm sure there must be some guides on how to do photometry around on the web? if not, I'll try to write one for here...

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Hi,

Thanks for the info. I use MaximDL - i've had a play with the astrometry stuff, but my check stars are not coming out at the right mag value. Not sure what I am doing wrong!

I guess just a light curve for a first pass would be nice, although having a calibrated system sounds like the way forward - is there a set of standard calibration stars that I should use?

Cheers,

Richie

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The AAVSO on their website have a good introduction to CCD photometry - The BVIR filters are not cheap!!

BTW standard photometry can't be done, in most software programs (AA4 etc) with one shot colour CCD's.

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Hi Merlin,

Thanks for the info - luckily I use a Monochrome camera (the SXVR-H18) - Can you tell me what BVIR actually means?

Cheers,

Richie

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BVIR - are the standardised bandwidths of the photometric filters Blue, Visual, Red and Infra Red.

They have different bandwidths from the usual imaging RBG filter set.

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Thanks for the info. I use MaximDL - i've had a play with the astrometry stuff, but my check stars are not coming out at the right mag value. Not sure what I am doing wrong!

That's because you haven't (I assume?) calibrated the system. Maxim is just summing up counts on the detector -- it has no clue how to transform the number of counts into fluxes and hence magnitudes. You need to observe a star of known brightness and work out what your "zero point" is. That is usually given as the magnitude of a star which will give 1 count per second -- i.e. our system has an R band zero-point of 19.6; a star of mag 19.6 produces 1 count on the detector per second. Each telescope/CCD/filter combo is different of course. Once you have that value, you know how bright a star is that gives you N counts per second. I can't remember the details of how Maxim defines the zeropoint (there are other ways to what said above), but I'm sure it is in the manual.

To get this value, you typically look at a "standard star". These have been observed regularly to make sure they are not variable, and their brightnesses calibrated to that of Vega (which has a magnitude of 0.0, by defintion). I use the Landolt standard stars, which are pretty much the industry standard for calibration of UBVRI data. You can get a list here;

Finding Chart

As Ken says though, without proper photometric filters, it gets trickier to do this calibration (not impossible, just a lot more work) and a lot harder to compare your results to other peoples taken in standard filters.

You'll also see around these days the "Sloan" filter sets; ugriz (small letters). That is another option if you're thinking of buying filters -- but less commonly used amongst 'small' telescopes than the UBVRI (capital letters) system.

If I were you just now, I'd look at a couple of Landolt standards with my RGB filters, and use them as a proxy for proper BVR filters. That will be good enough to get you sensible zero-points so that your photometry looks reasonable. It won't be quite correct of course, but you'll probably get within 0.5 mag or so.

Oh -- make sure you observe the standard star on a very clear night. You need to make sure there is no cloud/haze getting in the way, or your zero-point will be off!

Edited by FraserClarke

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