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Observing >> Variable Star Observing and Radio Astronomy

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jgraham
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R Gem – 2/26/2013 1h45m UT - DSLR Photometry new
      #5719501 - 03/07/13 10:14 PM Attachment (38 downloads)

Telescope: Meade SC8 @ f/10, Orion Atlas EQ-G
Guider: Orion Deluxe Off-axis Guider, DSI Pro III, PHD
Camera: Canon Rebel T2i
Exposure: 16x5sec, ISO 400 saved as RAW
Darks: Internal
Flat: Synthetic
Software: Backyard EOS, Nebulosity, Deep Sky Stacker, Autostar Suite Image Processing, Photoshop

I’ve been using a monochrome camera fitted with a V filter for the past couple of years as my photometric camera, but unfortunately it is often tied up as a guide camera these days. I was very interested to follow the recent work on the AAVSO web site about using DSLRs for photometry and this is from my first night giving this a try. It needs a bit of work yet, but the initial results are encouraging. One thing that I immediately found interesting about this method is that I’ve never seen these stars in color before, and the long-term variables that I usually observe are the most amazing deep red. No wonder they are a challenge to estimate visually, particularly for someone like me who is red/green colorblind.

For the purposes of photometry you want to use the green channel (reporting the filter as TG for the AAVSO) and as with any camera you want to stay within the linear response region (or use an appropriate transfer function). In this specific case R Gem is near its peak magnitude shining at about magnitude 7.8 and I’m a tad outside of the linear region giving an ensemble estimate of 8.3. I’ll use a shorter exposure next time and see if I can get it a bit closer to the reported value.


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jgraham
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Re: R Gem – 2/26/2013 1h45m UT - DSLR Photometry new [Re: jgraham]
      #5719504 - 03/07/13 10:15 PM Attachment (34 downloads)

This is the same image annotated with the magnitudes of selected reference stars based on the AAVSO reference chart. I give 8.3 as the ensemble value for R Gem, though 7.8 is closer to the true value. It was interesting to see that I could reach magnitude 14-ish stars using 5 second exposures at f/10 with my 8” SCT. This is a great time for visual observers to take a peak at R Gem as it is just passing maximum and is about to begin its slow decline to magnitude 14, that is from being the brightest star in the field to near the faintest!

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jgraham
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Re: R Gem – 2/26/2013 1h45m UT - DSLR Photometry new [Re: jgraham]
      #5721315 - 03/08/13 10:20 PM

I just finished taking my second set of photometric images with my DSLR. I haven't reduced them yet, but they look much better. Seeing these stars in color is wonderful. Since our weather here is so spotty I concentrate on long-period variables and they are the most amazing shade of red. I also follow a small set of cataclysmic variables and these strike me as blue, but I need to take a closer look at them to be sure.

Neat stuff.


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jgraham
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Re: R Gem – 2/26/2013 1h45m UT - DSLR Photometry new [Re: jgraham]
      #5722331 - 03/09/13 01:25 PM

I'm in the process of grinding through a set of photometric images from last night. The work flow is a little different than what I used with my monochome CCD, but it is quickly becoming a smooth routine. One interesting data point is that I'm reaching down to about magnitude 17.5 (green) using 16x60 second exposures at ISO 800 with my 8" SCT at f/10. The image scale is also a good match for AAVSO E charts.

Neat stuff.


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John Miele
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Re: R Gem – 2/26/2013 1h45m UT - DSLR Photometry new [Re: jgraham]
      #5722998 - 03/09/13 09:10 PM

Hi John,

Neat work there! What software do you use to do your magnitude estimates? At what point do you feel you need to guide? I am just getting going in this area and I'm hoping I can do 60 seconds subs unguided and keep tight enough images for photometry work...John


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jgraham
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Re: R Gem – 2/26/2013 1h45m UT - DSLR Photometry new [Re: John Miele]
      #5723020 - 03/09/13 09:21 PM

I've been using Autostar Suite Image Processing to obtain my magnitude estimates. I've been using this for several years with excellent results. I'm interested in taking a look at Canopis, but I'm not sure yet how it would use the AAVSO reference data sets.

As for guiding, back when I was using an LXD75 I'd guide when using exposures longer than 30 seconds. With my Atlas, 60 seconds is probably doable, but since it guides so easily I go ahead and autoguide all of my images.


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Rutilus
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Re: R Gem – 2/26/2013 1h45m UT - DSLR Photometry [Re: jgraham]
      #5726411 - 03/11/13 05:31 PM

John - Excellent work.

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Astrodj
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Re: R Gem – 2/26/2013 1h45m UT - DSLR Photometry new [Re: Rutilus]
      #5730506 - 03/13/13 02:50 PM

John,

I hope to be visiting some Carbon Stars in Gemini tonight so I will add this (nearly Carbon Star??) to my list of stops.

I am visual only but I appreciate the views and heads up! Thanks.


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jgraham
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Re: R Gem – 2/26/2013 1h45m UT - DSLR Photometry new [Re: Astrodj]
      #5730554 - 03/13/13 03:34 PM

Thanks! I also thoroughly enjoy observing visually, but I've never been good at visually estimating variable stars and I'm amazed at people who can do this. Many years ago when I built my 16.5" f/6.5 Newtonian one of the first things I noticed was the color of faint stars, particularly faint red stars. I'm hoping to get a peek at R Gem if we ever get a clear night! In the mean time, I've taken a second set of photometric images including R Gem and I've think I've got a better estimate for it. I'm looking forward to getting a third set before too much longer. Once I get 3 good estimates (based on values being reported on the AAVSO web site) I'll start reporting my data.

Fun stuff...


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Astrodj
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Re: R Gem – 2/26/2013 1h45m UT - DSLR Photometry new [Re: jgraham]
      #5732020 - 03/14/13 10:59 AM

I viewed R Gem last night. It is still pretty bright, with very nice color to it. I would say it is still brighter than mag 8 visually. I didn't try any serious magnitude estimates though. Tricky stuff with red stars.

The color last night was very similar to TU Gem, a Carbon Star near M35. Maybe not quite as bright as TU though, and a little lighter red, where TU was a dusky red.

Still, R Gem was brighter and therefore had more color evident than the other two carbon stars in Gemini I located last night (VW and BM).

I was using a C5 at about 60x, which under my skies has a limiting magnitude of about 11-12 at lower powers. Perhaps more aperture would show the color of these somewhat fainter Carbon Stars better. Next time I will try them with the 10 inch dob to compare.


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jgraham
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Re: R Gem – 2/26/2013 1h45m UT - DSLR Photometry new [Re: Astrodj]
      #5732192 - 03/14/13 12:45 PM

Coolness. Photometric measurements are often taken in a specific color region, most often green using a V (visual) filter. The AAVSO recently began accepting photometric meaturements using green RGB and Bayer matrix filters designated as TR, TG, and TB filters (tri-color R, G, and B), though I believe only the TG data is actually being used. Since long-period variables are usually (always?) deep red it is not surprising that their true visual appearance is brighter than their photometric estimate. This shows very well in the image I posted of T Gem. The TG photometric estimate was 15.0, but it clearly looks brighter than the nearby 15.1 magnitude reference star.

Neat stuff.


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Astrodj
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Re: R Gem – 2/26/2013 1h45m UT - DSLR Photometry new [Re: jgraham]
      #5733375 - 03/15/13 01:22 AM

Very interesting. I'm going to go check out that T Gem post. I don't think I'll be seeing that one visually anytime soon. Thanks.

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