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Photometry with a CCD Camera. Filters?

solarblast's picture
solarblast
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Rookie.

Does one need filters to do photometry with a b/w ccd camera? Are there benefits to using color filters? 

Photometry Filters
CTX's picture
CTX
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Joined: 2010-07-08

Rookie,

Welcome to the world of variable star observing and please remember that there is no such thing as a dumb question.

To directly answer your question the definite answer is that YES you need to use photometry filters, and at a minimum a Johnson prescription "V" filter, which will surfice for the majority of your needs for some time to come.  NO to the "color" filters if referring to the RGB filters intended for making a color compositie image.

"Different models of CCD chips have different spectral responses. Because of this, it is vital that one use proper photometric filters when observing variable stars:"

Please read section 2.3 of the CCD manual regarding filters!

http://www.aavso.org/ccd-observing-manual

Also pay special attention to section 2.2 since sampling tends to be an issue with new observers plus a really smart idea to read the whole manual.

Tim Crawford

 

Hi, thanks. I'll download the
solarblast's picture
solarblast
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Hi, thanks. I'll download the manual, and take a look at the manual section cited.

What do photometry filters cost?

What I know about phtometry is this from CCDSoft manual.

Isn't there some color aspect of photometry--maybe depended upon the spectral color of a star?

I did a MP light curve using CCDSoft, and I'm assuming the result was OK.  It was taken from an example in the book. I guess the results had nothing to do with color? Is color somehow relevant to variable star measurements?

photometry observing...
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GFB
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 Hi,

Photometric filters can run $200-$300 each depending on the size.   

 

Yes, there is a color aspect to photometry.  Red stars will be brighter(higher magnitude) in the red band than the visual band for example.  By measuring two different bands on a star such as the B band(blue) and the V band(visual) and calculating the difference, one can characterize the color of the star, which is referred to as the Color Index.  If the B-V is low, such as 0.01, the B mag and V mag are similar so the star is consider blue or white.  If the B-V is high, such as 1.0 it mean much less B is seen and the red is consider more red.  B-V is a very common Color Index and V-I is sometimes used.

 

For most photometric measurements the color index isn't the biggest worry.  Things like signal to noise ratio and exposure time are bigger considerations.  Normally you want exposure times long enough for good signal levels, yet short enough to catch any rapid changes in brightness.  Some stars show brightness changes in the 60 second range or less. 

 

Color differences can also show up in how a star is observed.  A very blue star changes in brightness due to it position in the sky.  It will be brighter overhead compared to near the horizon due to the changing thickness of the atmosphere, just like the sun is as it sets.   This extinction has effects all stars of course, but blue stars show the effect more.

 

Using a standard photometric filter reduces some of these effects.  And has the advantage that the band you are observing is much closer to the band that others are observing also.

 

Bill Goff

if I choose not to use any
solarblast's picture
solarblast
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if I choose not to use any filters., can I still do, say, variable star work?


I've practiced doing asteroid light curves with CCDSoft5, and the manual does not discuss filters. If anyone goes that way, and submits their data to the AAVSO, then is it somehow modified to correct for no use of filters?

observing unfiltered
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clittlefield
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Hi Solarblast,

With some types of variable stars, you can observe unfiltered and use the V magnitudes of the reference stars when you're calculating the magnitude of the target star. This bandpass is known as "unfiltered with V zeropoint," often abbreviated as CV (not to be confused with the abbreviation for "cataclysmic variable"). It is critical to select reference stars which have approximately the same color as the target star; this is where knowing the B-V color of a reference star is handy.

However, it is critical to emphasize that this technique produces scientifically useful data only for certain types of variables. It works well for time series of cataclysmic variables, but it would almost certainly produce useless data on Mira-type stars (for example). Since I specialize in cataclysmic variables, I can't say with certainty how useful CV-band data is in other areas of variable star observing--except that a filter will almost always make your data more scientifically useful and easier to compare with that of other observers.

Best Regards,

Colin

Hi, thanks. If I wished to
solarblast's picture
solarblast
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Hi, thanks. If I wished to tackle certain variable stars is there some  sort of guide as to what filters or nonfilter methods can be used to provide good info?

...Filters?
GFB's picture
GFB
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Hi,

There is a CCD Manual on the Observing tab of the AAVSO web site.  It has a section on filters, though it isn't a step by step guide on filter usage.  It shows that when using a filter your observations will be similar to the observations of others that are stored in the AAVSO database, therefore they are more useful to someone using the data.  Remember, your observations might be there for many years!

 

That said, there are occasions when a filter is less important.  If you are observing an eclipcing variable and just want to establish the time of minimum, using a filter is less important.  Some interesting targets are very faint.  In this case, since you want to collect every photon you might observed unfiltered.

 

As a general guide you might start by using a V filter.  You will have V magnitudes for comparison stars on target charts and you'll learn with experience how long the exposures are required with your equipment to get a good signal so that your data is useful.

 

Bill Goff

 

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