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Old 26-03-2010, 11:35 AM
Karls48 (Karl)
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Photometry

[FONT="Arial"]I have been trying photometry for some time now using Wratten filters or just V- filter with my achro refractor. The results are not very encouraging so far. I can get magnitude varying +/- 0.1 to 0.5 from catalogue magnitude. Even so it lets me to plot light curves, although magnitudes are not correct. I’m thinking about purchasing proper Photometric filter – Astrodon Johnson/Bessell V. At over $200 for little piece of glass (ouch) I would appreciate any comments about what can I expect from this filter. Also if you could post some images taken with this filter.[/FONT]
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Old 26-03-2010, 01:22 PM
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lepton3 (Ivan)
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Photometry

Are you observing stars? Catalogue magnitude is notoriously inaccurate for photometry -- be careful with the source catalog, as many (most?) are not accurate better than 0.5 mag anyway.

A more important question in some ways is what is the variation in your measurements? If your stdev is 0.1 or 0.5 mag, then you need to improve your equipment or technique. I assume you are processing carefully with darks and flats? Also beware of "twinkling" when using a small aperture scope -- you will need longer exposures, which means better guiding.

From what I understand, using a Johnson-Cousins filter, along with some careful measurement and accounting for atmospheric extinction will allow you to get accurate absolute magnitude measurements. That is a reasonable amount of effort, but necessary if you really want absolute magnitude measurements.

As an alternative, you might consider differential photometry? I have had good results with this technique, even without filters. If I can use a comparison star with similar colour index (B-V), I can get around 0.02 mag error using my Canon DSLR. In fact, I have managed to capture an exoplanet transit light curve (Delta mag around 0.025) this way.

Ivan
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Old 26-03-2010, 05:44 PM
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Terry B
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Dear Karl.
You should be able to get good non transformed measurements with a V filter that are within 0.1 mag error.
This depends on the camera you use as well as the filter and how rigorous you are at taking your measurements.
What camera are you using?
How linear is its response?
How are you measuring the mag?
Are you properly reducing the imaged with flats, darks etc?
There is lots of info on the AAVSO site and other good info about filters here
http://reductionism.net.seanic.net/C...ternative.html
The doc is a bit heavy going but it explains well how the filters work and how colour changes your measurements.
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Old 26-03-2010, 07:49 PM
Karls48 (Karl)
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I’m using GStar camera with SkyWatcher 150x 750 refractor from very light polluted Sydney suburb backyard. The camera uses ½” Sony Ex view chip with pixel size 8.6x8.3um. AD converter is 8 bit only and frequency response is within 5% linear from 480 620nm. But it is very sensitive - 0.0001 lux at f1.4. Images are taken as 2.56 seconds FITS exposures stacked on the fly with dark (bias corrected) subtracted from each exposure by capture software. Darks are generated for every 1degre C of temperatures drop by stacking 250 exposures, bias subtracted and scaled 0.004. I do not use Flats as it used to generate more problems the it solved. The 25 or 50 exposures make my “Sub”. Three to four such subs are taken of each object of interest and then processed in Maxim DL. Firstly one of the subs is plate solved with PinPoint (to make sure that I did got stars I intended), then Fits header is modified with object name and exposure time and filter used. Capture software I’m using doesn’t do this automatically. Then subs are flipped or mirrored to have North up and right way around (helps me with orientation in the planetarium software) and bad pixels removed (by using bad pixel map) I’m aware that if I use Maxim for capturing, I could have images plate solved and facing right way around in first place, but I would also have up to 200 FITS files for every object I image. Then I align and stack Subs. Except of screen stretch I do not do any other processing. Then I plate solve final image. The residual is between 0.2 and 0.6-arc/ sec., depending on seeing and general conditions that night.
I have a GSC2, Tyco2 and UCAC2 catalogues installed but use UCAC2 most of the times. I can do photometry on stars from Mag about 7 to about 14. Any stars brighter then this are saturated and fainter one have S/N ratio too low. Although I can see to mag 18 on occasions.
I imagine to about 30 deg or less from zenith otherwise the sky glow from surrounding suburbs become too bad. For photometry I use only about 1/3 of central image area. I believe that PinPoint automatically compensates for atmospheric extinction, but I better check it up.
Any comments as what I’m doing wrong appreciated.
How do you do “differential photometry” ?
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Old 26-03-2010, 08:27 PM
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Dear Karl
Thanks for the reply to my questions. You have certainly explained what you do.
When I mentioned linearity I didn't mean colour linearity but linearity to increasing exposure time. This applies to CCD cameras but I don't really know how to apply it to your camera. What is the range of exposure times you can do?
Is 2.56sec the maximum?
If so you need to to a test.
Find a brightish non variable star that is near to the zenith. It needs to be bright enough to register with a 0.5 sec exposure.
Take 5 frames at 0.5, 1.0, 1.5, 2.0, 2.5 sec and dark subtract them all. Don't do any stretches or other processing.
Measure the magnitude of the star in each frame using a single star tool using an annulus that includes most of the star image. (that I hope maxim has. I don't use maxim so I'm not sure of the name).
It should give you a total fux for the star and maybe a max pixel level and a magnitude conversion.
Import the data into a spreadsheet.
Average the results for each exposure time.
Work out the flux/sec if maxim didn't give you the magnitude conversion.
If the camera has a linear response then the results should be the same in each exposure.
If the camera has some sort of antiblooming, it will not be linear above about 1/2 the saturation level.
Once you know the level that you camera has a linear response to you can know what data in the single frames you can use.
You can stack images for photometry but only "average" and "sum" are valid. These will increase your signal to noise but nothing else.
The catalogues you mention are all very crude for photometry and can't be relied on at all for measurement.
You are better using the values from the AAVSO by producing a chart here. http://www.aavso.org/observing/charts/vsp/
An easy star to try to begin with is T Aps deep in the south. It is currently about mag 11.5 in V.
You need to use the comparison values from the chart not from the other catalogues. The technique is called differential photometry. Measuring the magnitude of a known star and your variable star in the same image.
You then can work out the correct magnitude of the variable star from the measurement you took of the known star.
Hopefully this will help.
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Old 26-03-2010, 08:31 PM
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lepton3 (Ivan)
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Karl,

Quote:
I do not use Flats as it used to generate more problems the it solved.
I have found the use of flats is very important, as the illumination varies across the frame. Perhaps it is not such a problem with a small region of a small sensor, but I expect still a problem, as there can be dust, etc.

It is important to take the flat of the exact optical configuration that was used for the science images, so that means no changing filters, removing the camera, or anything, until the flats have been taken. I use a "T-shirt" diffuser and a LCD panel computer monitor for flats, and it has been working well for me.

Quote:
For photometry I use only about 1/3 of central image area. I believe that PinPoint automatically compensates for atmospheric extinction, but I better check it up.
Atmospheric extinction is a significant factor, it must be taken account of for absolute photometry, so yes, do check.

Ivan
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Old 26-03-2010, 10:16 PM
Karls48 (Karl)
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Thanks Ivan and Terry. You have been very helpful.
Yes, my camera’s maximum exposure time is 2.56 seconds. Frame integration = 1/25fps = 0.04 * 128 (maximum frame integration) = 5.12/ 2 (interlaced camera) = 2.56 seconds. The shutter can be set from 1/12000 to open (but for 2.56 sec only) and its CCD chip got anti blooming feature. The Maxim Dl got variable anulus.
I have been stacking subs with median feature – that may be part of the problem. But I can select sum or average or sigma. Next imagining session I will find the linearity of my camera.
Software I’m using displays a lot of information about the star in the anulus. ADU (analogue to digital conversion – multiplying this number by gain gives number of photons per pixel), SNR, flatness, star magnitude, catalogue magnitude, catalogue identifying number and few more parameters that I don’t understand.
And I will revisit Flat frame acquisition. It may work this time.
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Old 01-04-2010, 09:10 PM
mikelangelo11 (Mikel)
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I used flame photometry just for once, when I determined the sodium content in beers. In that session, we used KCl & NaCl to create our standards.

In setting up an external standard for Ca in water, what compounds are best to be used? I know CaOH is not a good one as it precipitates out.
I am considering CaCl2 - but wondered as Calcium compounds seem to precipitate out in most cases.
Are there any Ca compounds that would not precipitate?...
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Old 10-04-2010, 09:04 PM
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You can do 1% photometry (+/- 0.01mag) with a clear filter. You just need good technique, good images and a good catalogue.

SNR is perhaps the most important factor. Unless you are shooting bright stars, 2.56 seconds is FAR TOO short an integration duration for accurate photometry. Anything less than 30 seconds is likely to introduce a lot of systematic errors in the process (particularly scintellation errors). Try stacking a set of images to build up the integration time (ie stack 15 images). If the target is very bright then stop down your apperture or use any one of UBRVI filters.

Personally I like to use MPO Canopus as my software of choice for measureing my images (though I don't know if it supports stacking. I usually stack in somthing like MaxIm DL or AIP4WIN. Astrometrica will do a good job of stacking as well and it has some reasonable photometry tools - but it was designed fro Astrometry so I would expect better than +/- 0.1mag from it due to it's catalogues.

If you want to delve more into the how and why then I highly recommend Brian Warners book - "A practical guide to Lightcurve Photometry and Analysis" now in it's second edition.

Cheers
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Old 25-04-2010, 05:29 PM
Karls48 (Karl)
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Thanks for your input David and all of you who helped me. Few more questions if I can bother you.
Since last post I was lucky to upgrade my telescope and camera. I’m now using 150mm x 750mm Skywatcher achromat. I know that achromat is not generally accepted scope for taking nice images, but I hope that for the purpose of Photometry and maybe Astrometry it will be good enough. I want to get 2 to 2.5X Barlow to increase depth of the image I can take with this scope. I’m looking on GS 1.25” 2.5X Apochromatic Barlow – any comments on this one? I own Meade LX90 8” scope on wedge that would be most likely better for the task. Unfortunately the scope is too heavy for me to set up. The 6” refractor on HEQ5 is on permanent pier in the back yard, covered with trampoline when not in use.
I was also able to buy Meade DSI 2 camera with Outback cooler. The camera uses 16-bit A/D converter and with 8.3um X 8.6um pixels is able of exposures up to 1 hour and it can lower CCD temperature to about 25 deg below the CCD temperature (+/- 0.5 deg). The CCD temperature without the cooling would be around 28-30 Deg C at 20 Deg C ambient. I have found out very quickly that there is no point in lowering CCD temperature below the ambient temperature because CCD sensor fog out. At present I’m taking images with CCD temperature set to 15 to 20 Deg C (depending on ambient temperature) and exposure 42 sec. The image acquisition software “Envisage” supplied with the camera got one unique feature. Not only that it can align ant stack images during the acquisition but you can specify quality on the image that is acceptable. Any images that are bellow the specified quality setting (due to the seeing or the guide errors) will not be stacked. It is not SBIG camera but I could (just) afford it – so it will have to do for now.
The new setup being much heavier then previous one has introduced few problems. GoTo’s become very inaccurate and focusing very difficult. Balancing and flexure in the guide scope mounting and the RP focuser were main culprits. At present I can get 16-arc sec in Dec but over 4 minutes in RA so it needs more work.
The RP focuser is - well RP focuser. I have to swap 3 of them I had until I found one with minimum sag and most square. Unfortunately, it it also the stiffest one of the lot. I have fitted 6 to 1 vernier on it to make focusing easier. It isn’t Crayford but it makes focusing bit easier. Againg unfortunately the Crayford focuser I had on previous scope does not fit, different diameter. Custom made adapter for it would be too expensive.
I have downloaded Astrometica as David suggested and I’m learning how to use it. So far it looks good for what I need and I will most likely buy it. As for image processing and initial plate solving I use Maxim DL and I’m happy wit it.
Bit too long explanations, but necessary if you going to offer any advice base it on the equipment I have. I’m in my sixties and retired. Purchasing of new equipment takes long time and hard saving.

What I want to do.
Monitor and do Photometry on some unusual stars – like vy CMa.
Do the light curves for some nearby G and F stars.
May be, to do Astrometry on some southern binary stars that have not been looked on for few decades. I have tried it before but Web cam I had then and EQ3 mount were not good enough for the task.

Questions:
Is my equipment adequate for the things I want to do?
Is it worthwhile to use Quality settings – that is software rejecting images below the Quality setting? Up side is good images, down side is that 5 of 42 second image that would normally take 3.5 minutes can take 5 to 20 minutes (depending on seeing).
I’m using Baader fringe filler filter. Am I better off by using this filter (frequency response 90 to 98% from 480 to 660 nm) it includes UV and IF block. Would the colour fringing that my achromat scope introduce around brighter stars degrade photometry results without this filter?
Is it better to use single images lets say 42 sec or stack serval images (S/N ratio improves with stacking)

Well that’s about all I need to ask for now. By the way I’m still waiting to capture that elusive meteor from outside of solar system.
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Old 26-04-2010, 10:39 PM
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lepton3 (Ivan)
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Karl,

The equipment you describe is more than adequate for generating some interesting light curves using differential photometry. Far more important is your technique.

I can recommend a good book on the subject. See if you can get hold of "The sky is your laboratory" by Robert K Buchheim. Despite the corny title, this book covers some interesting projects on photometry and astrometry among other things. The projects are ordered roughly by difficulty, so you can work your way through it, refining your technique.

Ivan
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