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Observing >> Variable Star Observing and Radio Astronomy

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ASTERON
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photometry questions
      #4311186 - 01/13/11 07:59 AM

Is it possible to do Science quality photometry from a medium light polluted area ?
Is there any narowband filter which may be used for cutting off the sodium lamp spectrum which is the main light poluter in my back yard so that photometery can be done, or would any type of filtering be self defeating because I do not integrate the entire spectrum coming from a star ?

I apologize if the question is dumb but I'm new to this and need to make some decisions.


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Mirzam
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Re: photometry questions new [Re: ASTERON]
      #4311307 - 01/13/11 09:18 AM

In general, you typically will be making short (~1 min or less) exposures and will be measuring differences between target and reference stars. The sky background light level is subtracted from the target and reference stars as part of the calibration process.

The answer to the filter question depends on the type of photometry you are performing. I'm not an expert so perhaps some others here will comment.

JimC


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rutherfordt
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Re: photometry questions new [Re: ASTERON]
      #4311462 - 01/13/11 10:31 AM

Asteron:

It is definitely possible to do photometry from a light polluted area since you normally do use filters that exclude most light pollution (the exact filters will depend on what you are actually doing).

With a CCD camera (mono) most people use a "V" filter (the "V" stands for "visual so it is a sort of yellow-green). There are also "R" (red) and "B" (blue) filters as well as others. These are not the same as the RGB filters that are used to take "pretty pictures."

I am currently using an infrared photoelectric photometer using "H" and "J" filters-- both are in the near infrared. They are very much immune to light pollution-- I have had car headlights sweep across the telescope, outside house lights turned on, etc. all to no effect-- in fact, I could do it from a mall parking lot and would have no issues.


Tom

Edited by rutherfordt (01/13/11 10:36 AM)


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kohudson
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Re: photometry questions new [Re: ASTERON]
      #4311519 - 01/13/11 11:00 AM

You can also do spectroscopy. Check out this link: http://valerie.desnoux.free.fr/paris%20saint-charles/Paris_obs_en.html

I think this is SO cool!


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Hubert
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Re: photometry questions new [Re: kohudson]
      #4311544 - 01/13/11 11:18 AM

Hi,

You can even do unfiltered photometric observations from light polluted area. It's all about how bright your target is.

Clear skies, Hubert
http://www.vvs.be/wg/wvs/


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ASTERON
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Re: photometry questions new [Re: Hubert]
      #4312137 - 01/13/11 03:34 PM

Hi Tom,
that's very encouraging.
What is the IR photometer you are using and at what wavelength does it have peak sensitivity ?
I thought that most infrared is absorbed by water vapor and other junk in the atmosphere.
What type of observations do you do in IR ?
Thanks


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ASTERON
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Re: photometry questions new [Re: ASTERON]
      #4312176 - 01/13/11 03:47 PM

Ken,
the spectroscopy data from the heart of Paris is very impressive.
Thanks for pointing this out and the link.


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ASTERON
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Re: photometry questions new [Re: ASTERON]
      #4312200 - 01/13/11 03:55 PM

Hi Hubert,
good to know. It seems that once i jump into the water I will most probably start with the brightest stars before I try very faint ones.
It Seems that for real faint targets it will make sense to select targets that show very fast variability so that one can do some work from a few hours at a dark site and still get some meaningful data.
slower brighter targets will probably best be done from my home which is more light polluted but I will be able to collect more data at higher collection rate (more nights).

Edited by ASTERON (01/13/11 03:57 PM)


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jgraham
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Re: photometry questions new [Re: ASTERON]
      #4312452 - 01/13/11 05:51 PM

No problem! I do my photometry from my back yard usng a V filter. My skies are pretty bad with the faintest stars I can see wirh my unaided eyes being about magnitude 3-4ish. However, I can easily do very accurate photometry down to about magnitude 17.

Have fun!


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rutherfordt
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Re: photometry questions new [Re: ASTERON]
      #4312792 - 01/13/11 08:23 PM

Asteron:

The photometer is an Optec SSP-4. It comes with two filters, H and J. Peak response is around 1250nm (J) with still good sensitivity at around 1650nm (H).

You're right about the water vapor-- it stops most IR, but these two wavelengths (and a few others) can sort of get through-- they are not immune, though. I have to keep an eye on the water vapor maps and fog or high cirrus will put a stop to the photometry.

You also need as large a telescope as possible-- I'm working with an 8-inch SCT and its almost too small (I've also used a C14 and its much better :-)). I am restricted to bright IR targets (about mag. 2.5 and brighter). Fortunately, there are lots of IR bright stars out there-- R Lyrae is faint visually, in the IR it shines at mag. -2.

Tom


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ASTERON
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Re: photometry questions new [Re: rutherfordt]
      #4313559 - 01/14/11 07:50 AM

Tom,
Thanks for the details.
Did you start doing IR photometry because your favorite targets were bright in IR, or for other reasons ?


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Hubert
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Re: photometry questions new [Re: ASTERON]
      #4314035 - 01/14/11 12:35 PM

Hi Asteron,

You don't have to choose faint stars with fast variability. There are plenty bright stars (7 - 10 mag) with periods ranging from 2 till 4 hours.

Clear skies, Hubert
http://www.vvs.be/wg/wvs/


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rutherfordt
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Re: photometry questions new [Re: ASTERON]
      #4314466 - 01/14/11 03:54 PM

Actually, a local university asked me if I was interested in doing a project for them-- IR photometry of the epsilon Aurigae eclipse-- and I said yes. I've found that I really enjoy doing it, too.

Another bonus, at least for me, is that very few other people (amateurs, at least) are doing this. I appear to be the only person submitting H and J data to the AAVSO on Eps Aur-- there are probably other people doing it, but they're sitting on their data, probably so that papers can be written from it.

Tom


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ASTERON
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Re: photometry questions new [Re: rutherfordt]
      #4315499 - 01/15/11 04:14 AM

Hi Hubert,
Thanks, thats good to know.
I dont really know much about variable stars and photometry.
I am basically just researching my options before I buy any specific equipment.
While I may have the patience to do objects which have long periodicity, I just thought that it may be more prudent and stimulating initially to start with objects which can give results within a period of hours or days, so that I can get exposed to the entire data acquisition, reduction and processing and plotting within a short time so I can get a better notion of what this is all about.

Edited by ASTERON (01/15/11 05:55 AM)


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Hubert
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Re: photometry questions new [Re: ASTERON]
      #4316170 - 01/15/11 01:27 PM

I do photometry on High Amplitude Delta Scuti-stars (HADS) and some eclipsing binaries. You can read more about the HADS here http://www.vvs.be/wg/wvs/vvs_wvs_HADSproject.php

but it's in dutch. Maybe you can let it translate by google or something. I think it's better that I make a English version to put in on our English section of the website. Anyway, its' a project that were are running in our Belgium variable star section and every year we make a publication as an IBVS.

There are some bright stars in the 8 - 10 mag range and you can see that a lot of them have periods of 1.5 - 3 hours. Perfect for a short night.

Clear skies, Hubert
http://www.vvs.be/wg/wvs/


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jgraham
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Re: photometry questions new [Re: Hubert]
      #4316367 - 01/15/11 02:59 PM

When I first started with photometry I also looked for stars with short periods, but I found that long period variables where actually better suited for my environment. Clear weather here is very spotty (my last notebook entries were in November) and with long period variables its okay to only get a look at them once a week or so. They also don't tie up my scope all night. I'll usually make a sweep through my current observing list before switching over to deep sky imaging. Another nice thing about long period varibles is that they typically span a very wide range in brightness. I also take a peek at a set of cataclysmic variables. Now those are fun! Night after night these little gems will sit just at the edge of where I can see them hiding around magnitude 15 or 16, and then bam! They become the brightest star in the field!

Fun stuff.


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ASTERON
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Re: photometry questions new [Re: jgraham]
      #4317543 - 01/16/11 05:43 AM

Hubert,
Thanks for the tip - I may try the google translation but I dont have good experience with it (at least not on German and Japanese).
I think after I figure our what projects I want to do It would be possible to better focus on what equipment I need.
I think currently the weakest link in my equipment chain is the mount (Atlas EQG). I may be able to get away using a 8" scope with Imaging gear on it but it definitely would not be good with my 40 pound 12" LX200R OTA +Camera on it.
If I get a good mount I may not have enough $ left for a good camera.
I am still looking for the best solution (If I stay with the Atlas mount I would be restricted to a lighter OTA - I was thinking about maybe a 8" AT8RC with Carbon Fiber OTA which may be light enough to sit confortably on my Atlas.
I was wondering if my FSQ106 refractor being a widefield 4" will be good enough for this kind of work - I have both the focal reducer and the focal extender for it so I can have three different focal lengths to select from (if so I would only have to buy the CCD so I can start working on this.

Edited by ASTERON (01/16/11 05:49 AM)


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ASTERON
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Re: photometry questions new [Re: ASTERON]
      #4317547 - 01/16/11 05:53 AM

John,
Thanks for sharing your experience.
I understand your reasons for choosing long period variables. If you observe regularly and have an ongoing program it does make a lot of sense to operate as you do.
Cataclismic variable sound exciting - are there any good ones with brighter baseline magnitudes such as 8-10 ?
Thanks


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Hubert
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Re: photometry questions new [Re: ASTERON]
      #4317625 - 01/16/11 08:16 AM

Asteron,

I use a C8 on a EQ6 with a SBIG ST7. Works very good for me.

A 4" refractor will also do the job. Very soon I'm going to do tests with a 10 cm refractor equipped with a DSLR.

Clear skies, Hubert
http://www.vvs.be/wg/wvs/


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Hubert
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Re: photometry questions new [Re: ASTERON]
      #4317626 - 01/16/11 08:18 AM

The famous ones becomes as bright as magnitude 8-10.

SS Cyg, R Gem, RX And, RU Peg ....


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rutherfordt
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Re: photometry questions new [Re: ASTERON]
      #4317701 - 01/16/11 09:15 AM

Asteron:

Concerning your equipment, one of the nice things about photometry is that the image quality is not as critical as if you taking a "pretty picture." It doesn't matter if your star images are smeared out over a few pixels (in fact, that is what you want-- you don't want the image of a star to be so tightly focused and tracked that its light falls onto a single pixel).

Go ahead and try what you have, you may be surprised at how well it does. I have a friend who carries a C14 with on his Atlas and it does really well-- it is a visual only set up, but I'm really impressed with how well it works.

Tom


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ASTERON
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Re: photometry questions new [Re: rutherfordt]
      #4317748 - 01/16/11 09:47 AM

Tom,
Thanks for this comment. I didn't know that you can do photometry with less well focused images. That seems very interesting.
I have used my 12"LX200R to visually observe with my Atlas and it works real fine (if there is no wind). So I already know that the Atlas can take the weight. What I did not know was that you do not need perfect tracking for photometery.
I have a (non-cooled) Meade DSI IIPro CCD camera. Do you Think I can use it for photometry ? I have no way of controlling its temperature so any temperature changes during an observing session may change the pixel response to light. (however, I may be able to build a Temperature regulating enclosure for it).
If it is not usable - what would be a good starting level CCD camera for work with the 12" LX200R F/10 on an Atlas something which can yield Science grade S/N levels and can be gotten used for a reasonable price.

Edited by ASTERON (01/16/11 09:49 AM)


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ASTERON
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Re: photometry questions new [Re: Hubert]
      #4317777 - 01/16/11 10:01 AM

Hubert,
Thanks, this is great. I also have an Explore Scientific 127ED triplet refractor (5"). I may give it a try on the Atlas.
Now all I have to do is get me a decent Camera.
By the Way, If you are going to be using a DSLR, How are you going to get the data for the relevant pixels from the Raw File ? can you actually know the light intensity from the Raw file, or are you going to be visually comparing it to comparison or calibration stars visually from the ficture ?
How would you know that the CCD of the DSLR does not heat up during use ? and how are you going to deal with pixel sensitivity changes during the night.
I have a Canon 40D - thats why I am interested to know how you are going to do photometry with a DSLR - because if it is possible, then I already have all the equipment that I need for starting.


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ASTERON
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Re: photometry questions new [Re: ASTERON]
      #4317789 - 01/16/11 10:06 AM

Hubert and Tom,
Is there a single high level text for photometry that can be used by a newbie but is very concise and at the same time practical ? I have no problem with graduate level math and I am not afraid of technical language and/or equations.
What should i buy or download ?
Thanks


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Mirzam
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Re: photometry questions new [Re: ASTERON]
      #4317817 - 01/16/11 10:20 AM

I don't know about concise (the book is over 2" thick) but this contains chapters on photometry as well as software that is widely used.

http://www.willbell.com/aip/index.htm

I would be interested in others responses to this question as well.

JimC


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ASTERON
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Re: photometry questions new [Re: Mirzam]
      #4318190 - 01/16/11 01:36 PM

Hi Jim,
Thanks for the link. looks like an interesting one ( especially as it includes the software as well). Do you know when the book was first published ? (I have some books from Willman-Bell that while being very good for their time are a little antedated because they dont cover the most recent stuff on the market.)
I am still interested in in other responses by other variable star obdservers.

Edited by ASTERON (01/16/11 01:37 PM)


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gavinm
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Re: photometry questions new [Re: ASTERON]
      #4318418 - 01/16/11 03:27 PM

You could do exoplanets - short period, there's plenty of them, they aren't too challenging (although it is challenging to get really good data). I image from the middle of a urban environment using a range of filters. LP has not been an issue.

Try this book by Bruce Gary (exoplanet specific but v/ good)
http://brucegary.net/book_EOA/x.htm

This one is also quite good.
http://observatory.ou.edu/wrccd22oct06.pdf


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Mirzam
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Re: photometry questions new [Re: ASTERON]
      #4318569 - 01/16/11 04:45 PM

Hello Asteron,

I know what you mean about some of the W-B titles being out of date.

However, the Image Processing handbook is the second edition from 2005. I have the 3rd printing of the book from 2009, and the AIP4WIN software included with it is version 2.3.1, which is fairly recent (I think).

The book includes a good discussion of many basic principles of image calibration, signal-to-noise, characterizing your CCD performance, etc., that are applicable to both astrophotography and photometry. I like it but I still have a lot to learn.

JimC


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ASTERON
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Re: photometry questions new [Re: gavinm]
      #4318588 - 01/16/11 04:53 PM

Hi Gavin,
Thanks for the links.
Exoplanets are interesting - I will look into this


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ASTERON
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Re: photometry questions new [Re: ASTERON]
      #4318593 - 01/16/11 04:56 PM

Jim,
Thanks. I will probably order it directly from W-B
too bad they rip you off on international shipping rates.


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btieman
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Re: photometry questions new [Re: ASTERON]
      #4318813 - 01/16/11 06:55 PM

You might also look up Brian Warner's book "Lightcurve Photometry and Analysis" Covers most topics relating to general photometry--variables, binaries, asteroids, etc...

I got my copy direct from: http://www.minorplanetobserver.com/ but I image Amazon carries it as well.

Brian


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ASTERON
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Re: photometry questions new [Re: btieman]
      #4319710 - 01/17/11 03:20 AM

Brian,
Thanks - looks like a very useful one I will order this soon.


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ASTERON
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Re: photometry questions new [Re: ASTERON]
      #4319722 - 01/17/11 03:48 AM

Brian,
Just ordered Brian warner's book from Amazon.
Supposed to reach me only on mid February
In the meantime I will try reading the PDF books recommended by Gavin (already downloaded).
Thanks everybody for the links and recommendations
Any other intresting primers or more advanced references or books ?

Edited by ASTERON (01/17/11 03:49 AM)


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Hubert
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Re: photometry questions new [Re: ASTERON]
      #4319823 - 01/17/11 07:16 AM

Hi Asteron,

Have you read the AAVSO CCD manual?

http://www.aavso.org/sites/default/files/CCD_Manual_2010_revised.pdf

clear skies, Hubert
http://www.vvs.be/wg/wvs/


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ASTERON
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Re: photometry questions new [Re: Hubert]
      #4319843 - 01/17/11 07:43 AM

Hubert,
No - thanks for pointing this out. I will read it.
Incidentally I thought in order to use AAVSO website you need to be a member ?


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Hubert
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Re: photometry questions new [Re: ASTERON]
      #4319856 - 01/17/11 07:57 AM

No, you don't have to be a member.

clear skies, Hubert


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zvaragabor
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Reged: 01/25/11

Re: photometry questions new [Re: Hubert]
      #6265821 - 12/21/13 06:16 PM

My question is, can I use a color webcamera for photometry?

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Ed Wiley
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Re: photometry questions new [Re: zvaragabor]
      #6266041 - 12/21/13 09:03 PM

I used to think that one needed a mono ccd and filters. Not so. Google "webcam photometry." Here is one paper I found, there are others.

http://adsabs.harvard.edu/abs/2009SASS...28..117T

Ed


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Re: photometry questions new [Re: Ed Wiley]
      #6266095 - 12/21/13 09:56 PM

I don't see why not. I would use the green channel and the V photometric reference data. The usual guidelines apply in that you either need to operate within the linear range of the camera or determine the transfer function to linearize the data. One possible challenge in that a webcam might be limited to 8 bit data. Whenever I start using a new camera for photometry I'll take some time to compare my results with data reported on the AAVSO web site.

Give it a try, what could possibly go wrong?


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zvaragabor
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Re: photometry questions new [Re: jgraham]
      #6273682 - 12/26/13 01:45 PM

Can I convert G (RGB) to V (UBVR)?

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Raginar
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Re: photometry questions new [Re: zvaragabor]
      #6274152 - 12/26/13 06:25 PM

No, the V filter is specific. There are some thAt don't care.

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Ed Wiley
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Re: photometry questions new [Re: zvaragabor]
      #6274363 - 12/26/13 08:27 PM

I think you need to start digging into this if you want to do photometry with a color camera. A good place to start is

http://www.citizensky.org/content/about-citizen-sky

and

http://www.citizensky.org/content/dslr-documentation-and-reduction

I think that if you nose around the Citizen Sky site, sign up, do a bit of reading and research you will find most of your answers. I say this not because I am withholding information but because I don't have authoritative answers; but the folks at Citizen Sky probably do.

Ed


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NJScope
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Re: photometry questions new [Re: zvaragabor]
      #6279521 - 12/29/13 04:46 PM

zvaragabor:

In principle, yes you could convert "G" from the RGB system to "V" from UBVRcIc typically used for photometry. For example VT from the Tycho-2 catalog is not exactly equal to V from the Johnson-Cousins system. Nonetheless, when VT values collected with the Hipparcos satellite are compared to reference V values determined elsewhere, within reason a relationship can be established between the two magnitude systems [ie. VT - 0.090 * (BT - VT)]. Note however, that in this case BT values are also required. The formulae are actually much more complicated and depend upon the range of (BT-VT). This should in no way discourage you from using the G channel from a color camera to produce lightcurves from a number of targets including asteroids, variable stars, stars with potential exo-planets, or even some extended star-like objects (eg quasars). This is particularly true for anyone who is simply trying out photometry as an alternate to visual observing or astrophotography. Indubitably if you intend to regularly publish your data, you should consider purchasing a purpose-built, temperature-controlled, non-antiblooming ccd camera equipped with photometric filters. That does not, however, preclude the use of a mono- or color-webcam, dslr or single-shot color ccd camera which can produce publishable lightcurves from minor planets or prove the existence of a new variable star.


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