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B vs V filter for testing the waters of phtometry?

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#1 John Miele

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Posted 15 February 2021 - 03:40 PM

I'm interested in doing some photometry with my ASI1600mm camera. Filters are hard to find, but I found a used Johnson/Cousins B filter. Would that be ok to start with instead of a V filter? Would I need longer exposures or some other drawback? Are comp star B magnitudes as plentiful and easy to find as V magnitudes?

 

Thanks!

 

cs...John



#2 ButterFly

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Posted 15 February 2021 - 05:19 PM

Choose both if you can.  Two filters lets you make an H-R diagram.  If you only have one, that's the one you start with.  There is nothing not okay with that.

 

You can look at the spectra here if that is your concern.  There are plenty of stars visible in any spectral band, and sure, a particular star can be fainter in one band than in another (the basis of H-R diagrams).  The drawback of only one filter is no H-R diagrams.



#3 555aaa

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Posted 15 February 2021 - 05:22 PM

You can't substitute B for V. B and V are pretty far apart, but you can do B only, but you won't have any color correction terms (meaning catalog B-V vs your B-V) which is your color calibration correction. You also won't have a color index vs airmass correction.  B is also down in the not very sensitive part of most camera's color response. Red stars have B-V of more than 2 magnitudes (meaning B is 2 mag fainter than V, and for very "blue" stars B-V is slightly positive, maybe -0.5.)

 

What you can do is this:

 

1) take test exposures of a field of catalog stars hopefully close to zenith (airmass near 1). You want a wide range of color index from B-V being over 2 down to about 0.

 

2) Subtract your instrumental mag from the B mag of the catalog stars.

 

3) Plot that vs. color index (B-V). That plot is the error of your B measurement vs. the color of the star you are measuring. Since you aren't measuring the color of the star with a V filter, you don't know how the star's color is affecting the result. If you have the V filter, then you at least know the color of the star (color changes as the star changes brightness) and so that gets corrected. The whole point of needing 2 filters for one magnitude measurement is that you are calibrating the effect of the unknown star color on your single filter measurement, so you need a second filter to get a measure of star color.

 

eta...

You could in a pinch substitute a green visual (non-photometric) for V and use that to measure an instrumental color index and then use THAT to correct the B measurement you make. You just need something that is giving you a metric of the color of the star you're measuring, to correct the B measurement you make. The reason you need that is that everybody's system has some color sensitivity even with only one filter.


Edited by 555aaa, 15 February 2021 - 05:30 PM.

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#4 MCinAZ

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Posted 16 February 2021 - 05:27 PM

  Agree with everything 555aaa wrote, though I'll point out another option since the original poster wasn't explicit about stellar photometry. A single filter (J-C V is a good choice) or even no filter at all can be used for several types of observations. Examples are measurement of asteroid rotation light curves, monitoring flare stars and exoplanet transits.

 

  Richard Berry's recent article in Sky & Telescope provides a good introduction to this topic. Recommended if you haven't read it.



#5 Ed Wiley

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Posted 17 February 2021 - 05:43 PM

Stat with the Johnson V, if you can find one. Then add the B later if you want to do two-color photometry. Lots of AAVSO members use only the V.  Make sure to pick up the AAVSO CCD Guide. CMOS is a bit different than CCD but the principles are the same. The main trick is setting your gain to best dynamic range and your offset high enough to avoid clipping. (gain = 0, offset = 10 on my ASI 183mm). AAVSO even offers two CCD/Photometry courses  in the Fall.

 

Ed


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#6 GaryShaw

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Posted 17 February 2021 - 07:16 PM

Stat with the Johnson V, if you can find one. Then add the B later if you want to do two-color photometry. Lots of AAVSO members use only the V.  Make sure to pick up the AAVSO CCD Guide. CMOS is a bit different than CCD but the principles are the same. The main trick is setting your gain to best dynamic range and your offset high enough to avoid clipping. (gain = 0, offset = 10 on my ASI 183mm). AAVSO even offers two CCD/Photometry courses  in the Fall.

 

Ed

Hi Ed

Do you know when  the AAVSO will be offering instruction based on cmos cameras rather than ccd? Going forward, few new observers will want to or be able to use ccd cameras for variable star photometry so it seems overdue for the organization to start catching up with the times and revamp the outdated ccd training manual and video tutorials to be cmos-based. I just noticed that a series of training videos are still based on something called Photometrica rather than VPHOT - I found that confusing at minimum. 
 

Do you know also whether the AAVSO is doing any industry coordination to address the complete unavailability of photometric filters as you mentioned above? I would think the AAVSO has a lot at stake in making sure observations can be based on these standardized filters. I see no indication in the market place that these filters will be available in the foreseeable future so, as a new cmos observer, I have to use a OSC with UV/IR cut filter and do tricolor conversions and submit my observations as ‘TG’ rather than ‘V’. I can’t wait for the day when I can use an ASI 183 or 178 MM with proper V and B filtration. I hope you don’t mind my questions and thank you for anything you can offer on the above.
cheers,

Gary



#7 Ed Wiley

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Posted 18 February 2021 - 11:33 PM

Gary:

I teach CCD1 for AAVSO. The course will be in September. There has been talk about CCD courses and CMOS. I don't think any decision has been made to create a separate CMOS course. One of the problems is that we volunteers who teach the CCD courses have not had as much (or any) experience with CMOS as with CCD. Speaking for myself, although I do have experience with an ASI183 using BVI filters on a 80 mm APO, I have no experience with tiny-pixel cameras on larger scopes. So while I can teach the basics of calibration and some of the other basics, I might not have answers for all the questions that pop up. Never-the-less I have signaled that CMOS is welcome in CCD1 so long as CMOS photometrists understand my meager knowledge-base. 

 

I have no first-hand knowledge of the efforts to get photometric filters back on the market, but I understand that such efforts are being made. One thing for sure, expect price increases. Its a tiny market.

 

Ed


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#8 dhkaiser

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Posted 19 February 2021 - 08:05 AM

Ed long time since we have talked.  You mention BVI, no R filter photometry?



#9 Ed Wiley

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Posted 19 February 2021 - 11:36 PM

Hi Dan,

 

I have BVRI in the C11. I feel very lucky to have picked up a BVI set for the little refractor. Given one can is adding filters as they can find them or afford them I like BVI as the 3-filter step from BV, then add R. Just my opinion based on wider sampling of the spectrum. But I might not make that choice if I was specializing in Miras or if I was an asteroid photometrist.

 

Good to hear from you Dan,

 

Ed



#10 freestar8n

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Posted 20 February 2021 - 01:38 AM

I don't think the OP has followed up so I'm not sure where he is at in his thinking.

 

I'm not sure what kind of photometry he wants to do - but a good practical guide is by Brian Warner: Lightcurve Photometry and Analysis.  You can use a photometric filter in combination with C or L filter to correct for extinction.  I think you would map L to V and B to B and do the calibration accordingly - and thereby improve the accuracy of your B estimate - plus have a basis for an estimate of the V mag.

 

Frank


Edited by freestar8n, 20 February 2021 - 01:38 AM.


#11 Ed Wiley

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Posted 21 February 2021 - 09:24 PM

Frank and John:

I have always admire MPO Canopus and Brian's thought on transforming to the standard system. Much of this is asteroid strategies which I can appreciate given one is trying to do something with very faint objects. Actually, you can transform a single filter without using a second filter, but it depends on accepting the default data for the standard stars rather than collecting the two filter data directly. I have never done this. There is a discussion, I think, in Brian's book (or Canopus manuals?) and (I think) in the AAVSO guide.

 

John: By all means buy the B, especially if it is a Astrodon with *** on the filter. Avoid it without the 3-stars as the early ones have a red light leak.

 

Ed


Edited by Ed Wiley, 21 February 2021 - 09:26 PM.

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