Measuring variable stars with a DSLR

[Tom Moran]

Hi.

Looking at the members recent images I noticed James produced a light curve of FN Cam using a DSLR. I have always been interested in this but have been unsure how to go about this. Anyone got any pointers on how to approach this?

Regards.

Tom Moran.

[Jeremy Shears]

Hello Tom,

there is info on DSLR photometry on the Variable Star Section website. Under Observational Programmes, click on Eclipsing. On the page you will see a link to a JBAA paper by Des Loughney.

All the best

Jeremy

PS I enjoyed visiting your own BAA Members Page

[Tom Moran]

Thanks Jeremy.

Is the data suitable to be submitted to the VSS?

Regards.

Tom

[Jeremy Shears]

Yes, absolutely Tom. All the data can be sent to the VSS. One of the main programmes is to measure accurate times of minimum for eclipsing binaries, this in turn shows whether the period of the system is evolving. All interesting astrophysics!

I hope you’ll give it a try.

All the best

Jeremy

[Gary Poyner]

Hi Tom,

I’ve recently added a link from the BAAVSS home page to the updated AAVSO DSLR manual.  You might want to look at that as well as Des’s paper…

Good luck,

Gary

https://www.aavso.org/dslr-observing-manual

[Tom Moran]

Gary.

Looking forward to the learning curve and contributing to the VSS. Thanks for the encouragement.

Regards.

Tom

[James Screech]

If you want more details on how I produced the FN Cam measurements (as well as many others) I can provide you will more details.

James

[Tom Moran]

Thanks James that is much appreciated.

I am currently working through the IRIS tutorial. I am familiar with calibration frames but the Iris interface is somewhat strange to my eyes.

Regards.

Tom

[Tom Moran]

James

Looking at calibrating the images using Iris seems a little long winded. I currently use Nebulosity4 to calibrate my raw CR2 files into FITS with dark, flat and Bias calibration (post processing). Can I use these post processed fits files from Nebulosity in Iris?

Tom

[James Screech]

I don’t see why you couldn’t use the calibration in Nebulosity and then do photometry in Iris, though I’ve never used Iris myself. Below are details of the hardware, software and workflow that I use.

Hardware:
Canon 500D (modified) with red dot finder mounted on the hot shoe
Bahtinov mask
Canon 200mm f2.8 lens
Celestron SE4 alt/az mount
Bracket made from a binocular L mount and dovetail bar (to connect lens collar to mount)

Software:
APT
Fitswork
Nebulosity 4 (demo)
AIP4Win

Capture workflow:
Perform two star alignment of mount using APT’s liveview feature with crosshairs on.
Manually focus lens using mask using 1 – 2 sec exposures,I find APT’s bahtinov aid doesn’t work very well, so just look at the images on screen.
Capture 20 flats (same ISO as required for variable)
Capture 20 darks (same ISO and exposure as required for variable)
Use APT’s Goto++ feature to get within 100 pixels of the target star
Use APT to capture images

Notes:
I have previously ascertained the correct ISO/exposure for a variety of star magnitude ranges so as not to saturate the variable or comparison stars.
APT’s Goto++ is a great way to centre an object and saves a log of time.

Processing workflow:
Use Fitswork in batch mode to extract 3 colour planes into individual fits files for flats, darks and lights.
Delete the red and blue files as they of no use.
Use Nebulosity demo to preview the files and rename/delete those that have excess trailing or cloud, so these files are not used for photometry.
In AIP4Win setup basic calibration (darks and flats).
Use AIP4Win MIPT to analysis images (calibrating at the same time).
Import the AIP4Win data file into the BAA VSS spreadsheet to produce BAA database submission format files.
As well as submitting the photoemtry to the BAA VSS I also measure the timeings of minimums (using software I have written) and submit the times to the Krakow University database.

Notes:
I have previously determined the read noise and gain of the ISO settings I use as these are needed by AIP4Win
I have a small utility to calculate the heliocentric offset, again used in AIP4Win.
Personally I don’t think much of AIP4Win and only use it so that I can use the VSS spreadsheet. It contains a lot of bugs and the author is not interested in fixing them.
For comparison stars I use the AAVSO VSP to produce charts and tables of suitable stars.

The above works for me, I’m sure a lot of other software and workflows will also work. Good luck if you decide to give it a try and feel free to ask if you wat any help / advise.

James

[James Screech]

A few other things I missed, I keep the ISO to a maximum of 800, so that noise does not become an issue and restrict the exposure to between 10sec and 30sec. Less than 10sec and scintillation in the atmosphere will reduce the accuracy of the measurements and longer than 30sec field rotation  & tracking become an issue on an alt/az mount without guiding (its not possible to guide in alt/az mode anyway). I also avoid a 15degree radius of the zenith also due to field rotation due to the alt/az mount.

James

[Tom Moran]

Thanks James that is very useful.

You have probably saved me some money because I was considering purchasing AIP4WIN. I am currently working through Muniwin which seems much easier than IRIS and will produce data that is compatable with AAVSO and BAVSS. 

Have you any suggestions as to some relatively easy targets to get started with?

kind regards.

Tom

[James Screech]

I would go for a EW type as they are continually changing in brightness and generally have short periods. Possibly VW Cep (mag. 7.2) if you are using a camera lens, this has a period of just 6h 40m so a full orbit can be captured in one night, it is also circumpolar from the UK so can be imaged any time of the year. Or possibly NR Cam (mag 10.76) if you are using a telescope for similar reasons, circumpolar, period 6h 8m.

What camera, lens/telescope are you using?

Before you take a time series in anger you will need to determine the exposure/iso you need for different magnitude stars, making sure you allow some spare space at the top end to take account of non-linearity in the camera, different star colours and atmospheric transparency.

James

[Tom Moran]

Hi james.

I intend to use an old 350D (modified) with a fixed 200mm lens.

This will be attached to my imaging set up. The idea is to capture the variables at the start of the night then set away imaging.

However my 100D is attached to my 80mm ED refractor so I can catch fainter variables with that.

I like the idea of VW Ceph. It should allow me to develop the skills and maybe get enough data for a light curve in one night.

Regards.

Tom

[Kevin Toogood]

In relation to James’ point about “leaving some space at the top end”, is it the case that this can be checked just by looking at the histogram on the camera whilst working out the correct exposure time? You’d be looking to check there wasn’t any point of the histogram which hit the very top of the graph? Thanks

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