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Bright nova photometry

HQA's picture
HQA
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Joined: 2010-05-10

After talking with several observers, and submitting three nights of BSM_South data, I'd like to remind observers that saturating 5th magnitude variables with regular telescopes is REALLY easy to do.  There are lots of V-band measures for Nova Del 2013 that are ~0.2mag fainter than they should be, and lots of others with way too much scatter for a 5th magnitude star.  Look at the observations from observer HQA and you will see what I mean.  This star really doesn't change on a minute time scale; if you are seeing 0.1-0.2mag scatter, it is all due to the way you are collecting data.

With BSM, a 6cm telescope, saturation sets in at V=6.5 for a 30second exposure.  You can scale to your own situation.  With a 16-inch (40cm, 44x more collecting area) telescope, for example, you will saturate in less than one second for V=6.5, and for Nova Del 2013 at V=5, the exposure will be in the 0.1sec range before saturation.  Don't assume that 65535 is the saturation level for your sensor; many SBIG cameras, for example, start saturating at 45K ADU.

I'd keep the peak ADU in your image to be half-well (30-40K ADU).  If your field of view is small and you don't have good comp stars, then stack as many short-exposure images as needed to build up the signal/noise.  Stacking doesn't hurt in any case, because anything less than about 10 seconds will also suffer from scintillation, but stacking effectively increases the exposure time while retaining the dynamic range.

We want quality data - the pros are waiting until the nova fades, and this is a chance to show what the amateur/small telescope community can provide!

Arne

Bright Nova Photometry: visual control with binoculars
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SGQ
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Joined: 2011-06-08

To support the notes reported by Arne

I can show what I experienced with Delta Scorpii (mag. around 1.8) observed with a CMOS camera at 0.5s of automatic pose and about 2cm of objective. The saturation affected indeed the results.

 

As a result of that I decided always to parallel the differential photometry calculated upon the digital photo with traditional visual observations with binoculars.

 

SIGISMONDI, C. . Differential photometry of delta Scorpii during 2011 periastron. Il Nuovo Cimento della Società Italiana di Fisica. C. Geophysics and Space Physics (Online), v. 36, p. 243/11498-1-247, 2013. 

Link to the text on arxiv: http://arxiv.org/pdf/1112.2356v1.pdf

That paper was also presented as a poster in the 100 AAVSO annual meeting in october 2011:

text on

http://arxiv.org/ftp/arxiv/papers/1109/1109.5865.pdf

 

Costantino

Arne, Amen!!! I've taken
daveh's picture
daveh
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Joined: 2012-10-08

Arne,

Amen!!!

I've taken several images of N Del 2013 over the last few days.  I've used 4 different iTelescope systems with a variety of filters and time exposures from 300 seconds (you commented on this one the other day) to 15 seconds. EVERYTHING was saturated!!

So,as you say, let's wait until the nova fades and try again.

Dave

Thanks for the tips...
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avdhoeven
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Joined: 2011-10-06

I have did some reprocessing on my data according to your remarks. I was convinced the data was good (not saturated and so on) but had still quite some 'noise'. Stacking made a huge difference. Now I have very nice continuous measurements with 0.02 mag range... I wonder should I delete the old observations and replace them with these?

Bright nova photometry
HQA's picture
HQA
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Joined: 2010-05-10

Hi avdhoeven,

I would delete your old observations and replace them with the newly stacked values.

Arne

Thanks will do that...
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avdhoeven
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Joined: 2011-10-06

Thanks will do that...


Edit: just replaced my data (HAGA). It is much more consistent now. And now going outside to get more :)

Defocusing?
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WGR
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Joined: 2010-08-17

Arne


Would this be a good time to defocus with larger telescopes to keep from saturating, and still be able to take longer exposures, thus avoiding scintillation?   I know that the early exoplanet guys had this same problem with saturation vs scintillation. 

 

Gary

defocus
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HQA
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Hi Gary,

There are many ways to keep from saturating bright stars, including defocus and neutral density filters.  I've written a couple of emails detailing the various techniques for Bright Star Photometry.  It can certainly be fun to give them a try and see how bright you can image and still get good photometry. However, you have to be careful and check everything along the way.  It is usually safer to just avoid stars that are too bright for your normal exposures and let some smaller telescope take over, than to work hard to lose photons!

Arne

AAVSO 49 Bay State Rd. Cambridge, MA 02138 aavso@aavso.org 617-354-0484