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Gasman

Photometry software?

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Hi chaps

I`ve been taking a lot of spectra of Nova Del since its discovery and been trying to estimate its magnitude with not much success. I thought I would be able to simply load an image click on maybe 5 or 6 stars in the fov with known magnitude and then click on Nova Del to get its magnitude but it doesn`t seem to be that easy?.  I tried Astroart 5 which came up with a value of 8.5 for Nova Del when the AAVSO observers were making it 6.7-6.9. I think I may have saturated the image which might account for the difference with AA5 but surely all the stars would be saturated in the fov the same so a reference star of say 8.5 would still be 8.5 for the photometry calculaton regardless of the saturation or have I got that totally wrong?. Any other photometry apps out there please?

Cheers

Steve

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Unfortunately once you start to saturate any part of the required data for your measurements you've lost the data you need to do magnitude measurements.

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Ah! thanks Cath I thought I was in the wrong ball park with that, any photometry apps you recommend at all? Not quite sure how accurate these computer photometry apps are if/when I get the saturation right?

Regards

Steve

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The AA5 photometric set-up seems to work best with Mono images.

Just make sure the aperture setting is > x3 FWHM image size and that the star images are not saturated.

MPO is the other one to try....

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Hi Steve

You can get down to milli mags with good data .. Maxim will let you do photometry with good reference stars that are not saturated.

Non-saturated stars mean stars with signal below anti-blooming gate activation levels, say (full well depth)/3.

It also depends on what type of camera you are using - monochrome or colour, ccd or DSLR, since mags are given for colour bands and images must

be calibrated for your instrument response.

All this being said I recall single shot colour ABG SXpress CCDs being used for sensitive photometry. 

Apps I know of that will do this for you include IRAF, IDL, IRIS, Maxim. Can't say for Pix, AA, etc.

hth

Mike

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Thanks for the info guys. Its this saturation/photometry/magnitude thing I`m trying to get my head round. My 16bit Atik 314 ccd has max well level of 65536 adu  so I`ve been told to aim for about 20000 to 30000 adu as an average for my lights so presumably saturation will occur when it starts going above that?.

Before I started reading up on Photometry I mistakenly thought Photometry=magnitude which is maybe why the Photometry I did with AA5 (albeit with saturated images) gave the wrong values which leads me to question how an image taken with a well depth of 20000 can give the same photometric values as an image taken with a depth of 30000??, I`ve got a photometry script in Pixinsight churning out some data from some Nova Del images but I`m not sure if/how to convert them to magnitudes yet!

My head hurts :huh:

Steve

Edited by Gasman

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Steve,

If you reduce the exposure, then all the star images including the reference stars will have a smaller ADU count.

The sensor response is almost linear, so the calculated magnitudes work out the same.

HTH

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That helps a lot Ken rhanks. I initially thought even if the image is saturated then all the stars would be saturated by the same amount even the reference stars which would cancel out the saturation Doh!. I know better now!

Steve

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So long as you keep all the stars used in the magnitude/photometry measurement below the maximum 65535 value you should be fine.

If the sensor/camera really is linear in brightness then you can use the entire 65535 range, although some pixels will saturate before others (manufacturing inaccuracies) so probably best to keep your measurement stars below say 60000.

When a star image becomes saturated, it's peak/top gets flatten/chopped off, much like cutting the top/peak off a mountain and so leaving a flat top to it, in which case you can no longer tell high tall it was, or in the case of a star how bright it was at it's peak value.

Edited by Cath

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Thanks for that interesting way to describe it! I did have the dilemma last night of reducing my shots until I got the stars below 65535 but some stars I clicked on still showed a value of 65535 even with an exposure with my ccd (cooled to -15) of 0.01 seconds?. That was the fastest exposure I could try so I'm not sure what to do there it seems the camera is too sensitive to get some stars less than 65535 adu? Maybe I should have turned the cooling off to make it less sensitive?

Regards

Steve

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Cooling a camera doesn't affect it's sensitivity, but it does reduce the amount of noise in the image. I'd definitely carry on cooling the camera to keep the noise down.

I guess your only way then is to reduce aperture (also reduces resolution) or use an ND/Moon filter or variable polariser filter to cut the amount of light down reaching your sensor.

Some examples shown here ..

http://www.telescopehouse.com/acatalog/Online_Catalogue_Lunar___Planetary_40.html

Edited by Cath

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A saturated star will reduce in intensity ( ADU count) as you reduce the exposure. Hot pixels or stuck pixels won't. Ignore these.

If you take a sequence of images with doubling exposure times each time of the same stars and plot the count vs time you will notice linearity starts to drop off well before 16bits (65K ADU), and often below 20K. The noise floor will reduce by a factor of 2 roughly every 6 degrees for silicon, hence the power of cooling.

rgds

Mike

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One thing I'll say is dont think that as long as the value is less than 65535 then it wont be saturating. 

Most cameras have anti blooming gate sensors that means that as the "buckets" get full, the charge gets dissipated. These kick in well below the maximum level. So, whilst CCDs and CMOS are very linear, the relationship changes to a curve that becomes an asymptote - ie you never copmpletely hit the buffers.

So - stick to stars less than 2/3rd full sigmnal and you will still be linear....

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Thanks guys for all the helpful info much of which is new to me!. So in a nutshell to keep my images from saturating I need to keep the adu count well below 65k on the brightest stars and with my Atik 314 setup which is giving 65k even with its fastest setting I have to either reduce the aperture and therefore reduce the resolution or filter it somehow. I have my Atik filterwheel setup with the R,G,B and UV filters in BUT when doing spectroscopy I use the Star Analyser which fits nicely in position 6 in the filterwheel but this creates the problem of not being able to add any more filtering to the image as the filter slot is in use with the Star Analyser and the Atik 314 screws directly into the filterwheel unless I can place a filter somehow over the end of the scope. (120mm diam??)

A lot easier just looking through an eyepiece isn`t it :grin: 

Steve

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Well I think I`ve just answered my own query. I only need the magnitude estimate of the star and not its spectrum which I can do after or before any spectra has been taken so my thinking is to get one of the darker filters in a spare filter slot and take an image with that afterwards which hopefully will reduce the adu to a useful level!. Colour doesn`t come into spectroscopy so as its all Mono stuff that should work with the right filter!!

Steve

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Steve,

Remember what Cath said....

It's ONLY the target and reference stars which need to be un-saturated.

If you have one or two brighter non critical stars saturated it doesn't matter.

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