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Fourth Photometric FIlter

tcalderw
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I am doing Photoelectric Photometry, and am adding U and B filters to complement my V.  It is easy to add one more filter to the collection.  Would people recommend: Rc, Ic, or ?

Tom 

I'd recommend I
wel
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Hi Tom,


For most purposes, R is too close to V to contain much extra information, so I would recommend an "I" filter!

Cheers,

Doug

Fourth Photometric Filter
Ken Sikes
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Hi Tom,

I am taking part in Jeff Hopkins Orion Project. I have a Optec SSP3a Photometer and and taking BVRI readings. Jerry Persha, the Optec designer of the Optec Photometers, has said that the Optec Johnson I band  filter responds very well to the Cousins transformation values. There are a couple doing J & H band readings and some doing spectroscoptic readings in low, medium and high resoultion work. Here is the link to Jeff's webpage.

 http://www.hposoft.com/Orion/Orion.html

Ken Sikes

 

 

Fourth Photometric Filter
tcalderw
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Ken:

Can you point me to any information about the transformation Gerry refers to?

Fourth Photometric Filter
Ken Sikes
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Tom, go to   http://www.aavso.org/ssp3 about half way down look for gpersha

Ken

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HQA
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Tom, what photometer are you using?

Generally, Doug is right:  if you only have 4 slots, and you already have UBV, then Ic is the next best filter to add to your complement.  Ic (Cousins) is better than Ij (Johnson) as it is the standard bandpass; Ij requires transformation to match the Cousins bandpass, and is wider and redder than Cousins.  If you have a choice, always get Cousins Rc or Ic filters rather than Johnson Rj and Ij.  If you have no easy choice, then you can get Ij and transform.

i will be very interested in your U-band results.  The obvious star is EE Cep, which will be undergoing its next primary eclipse this summer, if you can go that faint, but there are many other bright stars where U-band photometry would be nice to have.

Arne

which photometer
tcalderw
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Arne:

I have an SSP-3.  A 24" telescope is available to me so I should be able to go fairly dim!  Any target suggestions would be welcome.

which photometer
Ken Sikes
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Tom,

As I mentioned I have the SSP3a with the UBVRI Johnson filters. I have not had any luck doinig U band photometry as the photodiode is not sensitive enough in that band. What results have you obtained with the U filter.

Ken Sikes

U Filter
tcalderw
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Ken:

 

I haven't actually taken delivery of the U filter yet, so I have no results.  I have access to a 24 inch telescope, so I presume that the aperture will overcome detector sensitivty limits.

U Filter
Ken Sikes
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Tom,

Do you have a copy of the Manual for the SSP3 ? Here is a link

 http://www.optecinc.com/astronomy/catalog/ssp/pdf/ssp_3_generation2.pdf

Note that on page 7 is the responsitivity of the SSP3. The U band is the far left and the response is low.

Here is what Jerry Persha, the designer of the Optec Photometers, told me about the SSP3a

 The U data is barely above the sky readings. The SSP-3 is not very good when it comes to U. I even have problems using my SSP-5 with U. My observatory is near sea level and the extinction so bad around here that the U band just doesn't come through. You would get better results in New Mexico at 7000 feet.

Jeff Hopkins, of HPO,  also stated that the SSP is not that responsive to the U band. I tried several times to take transformation star readings in the U band, but did not get any acceptabile results.

U band
tcalderw
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What aperture did you use and did the instrument have glass (eg: a corrector plate) in the optical path?

U Band Photometry
HPO
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I used my 1P21 UBV photon counting unit on a C-8 with the enhanced mirror coating for excellent U band work for over 30 years. I tries the same photometer on an 8" LX90 Meade with enhanced coating and got half the U band reading for the same star about the same time. I do not think the corrector plate will make much if any difference. The problem is the PIN diode detector is just not sensitive in the U band. Sure if the telescope is big enough you might get a good U band signal on Siruis. Best not to waste your time trying to do U band photometry with an SSP-3. Also, most CCD cameras are also pretty insensitve in the U band. I tried one on my C-8 and the U band was the same as dark counts.

Jeff Hopkins (187283)
Hopkins Phoenix Observatory 

U Band
Ken Sikes
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I specified the 1MM aperture, and I am using a Meade 10" SCT. I know the corrector absorbs some of the U band waves, but the diode just is not sensitive enough in the U band.

 

Ken

U band difficulty
tcalderw
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Well, I can easily do 10th magnitude in V, and SSP3 U band sensitivity is about 1/3rd of V.  Allowing reduced filter efficiency and increased atmospheric scattering in U (I am at 6300 feet, however), it seems like I should be able to reach 7th magnitude in U.  Certainly nothing to crow about, but not useless, either, especially if little U band data are presently being collected.

I'm sure I can do better than mag 10 in V, but PEP program stars are bright and I haven't been exploring the limits.

U band difficulty
Ken Sikes
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Sounds like you have a lot going for you with the aperature and elevation. Let us know how things work out on determining transformation coefficients.

Ken

U band limits
tcalderw
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Perhaps someone from HQ could chime in: would U band photometry down to mag 7 or 8 be productive?

U Band Photometry
HPO
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U band photometry is important because many stars are much more active in the shorter wavelengths.  Some variable stars may only show a decease of 0.5 mag in the V band but 2 or 3 mag in the U band. This was one of the sad things I have seen with the popularity of the CCD photometry. The old single channel PMT photon counters not only shined with higher precision, but also opened the U band window. I know some CCDs can do U band work, but they are expensive and in the minority. I used the SRO 50 AAVSOnet system for AZ  Cas for UBV data and that worked well, but again it is hard to beat the PMT based single channel systems for U band data.

Jeff Hopkins (187283)
Hopkins Phoenix Observatory

U photometry of bright stars
HQA
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Hi Tom,

Certainly U magnitudes in the 7-8mag range are useful.  With a photoelectric photometer, you have to move between target and comparison stars anyway, so there are no restrictions about having the comp star in the same field of view.  U-band is probably the most astrophysically interesting band, containing the Balmer jump and emphasizing the influence of metals on the spectral energy distribution.  One obvious target is VZ Cet, Mira's companion.  This is presumably a white dwarf (though there is some controversy about it), and can be measured when Mira is at a faint minimum - like all hot stars, VZ Cet is quite bright in the U band.

U has multiple disadvantages too.  It is where the atmosphere starts cutting off, so the bandpass shape is influenced by the transmission.  It suffers the most extinction, so is harder to do from sea level.  It is where the QE of the detector is decreasing.  Finally, because it straddles the Balmer jump, the flux from most stars is also decreased, making measurements more difficult.

If you can do it, I highly recommend U.  Be aware that your star choices will be more limited and that you will always want more aperture/better detector QE.

Arne

Arne

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FJQ
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Going back to CCD photometry, would you say Arne that a U filter is more "useful" than a Rc filter if you already have a set of BVI filters?  I heard others mention that the Rc is less useful than the Ic because of its bandwith proximity to the V filter. 

http://www.aavso.org/lcg/plot?auid=000-BLG-310&starname=SN+2014J&lastdays=32&start=&stop=2456713.40692956&obscode=FJQ&obscode_symbol=2&obstotals=yes&calendar=calendar&forcetics=&grid=on&r=on&uband=on&iband=on&bband=on&v=on&pointsize=1&width=800&height=450&mag1=&mag2=&mean=&vmean=

I did a run of BVRI photometry of SN 2014J (M82 SN) from 26Jan14-24Feb14 (see link above) but I wonder if I would have provided a more useful data set if I did UBVI series instead.

James

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HQA
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Hi James,

As always, the answer is "it depends".  For SNe, U-band is quite useful because red-shifted cosmologically important SNe may only have the UV present in the IR bandpass of interest, so knowing what they look like in the rest frame is important.  For other stars like novae, Rc contains Halpha, and in fact gets dominated by Halpha and doesn't reflect the underying continuum.  If you want to measure the Halpha strength, then Rc could be important.  If you want to see the continuum, another bandpass might work better (such as Ic for example).

The ideal is always spectroscopy.  Broad-band filter photometry is like coarse resolution (R=5) spectroscopy, where instead of having great spectral resolution you have the multiplexing advantage of great angular resolution.  Both work; it just depends on the scientific project.

For BVRcIc, the Rc filter is the least important under most circumstances.  Still nice to have, but U-band, if the star in question is not too faint, is probably better.  However, that "faint" part is what usually limits the usefulness of U-band photometry.  At USNO-Flagstaff at 7000ft elevation and an R/C telescope along with a thinned, backside illuminated CCD, the U-band exposures were typically 20x longer than V-band.  I was always working on some faint target (say, 18th mag at V with 10-minute exposures), so U-band was out of the question for those targets.  Probably 10% of my research included U-band photometry.

Arne

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FJQ
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"U-band exposures were typically 20x longer than V-band. "

Thanks for the quick response!  Perhaps I was being more enthusiastic than warranted because of the long integration times needed for such a short wavelength filter.  Using an ST-10XME and shooting from only 150 meters elevation will certainly entail long exposures on objects other than a 10th magnitude V object like SN 2014J!

James

U-band photometry
HQA
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I mentioned the factor of 20 for a well-exposed U-band image.  That said, the science might dictate that you spend that factor of 20 on the good image!  For example, on the 18th magnitude object I mentioned, where I took a 10-minute V-band exposure, it would only be about 3.5hrs of U-band imaging to get to the right depth.  If the target does not rapidly vary in brightness, sometimes it is worth spending half the night on a single multifilter dataset.  I usually recommend that you work on targets better suited for your equipment, though!

All of the AAVSOnet telescopes have an R-like filter (usually Rc or SR), and it gets used a lot.  I can't think of a single AAVSOnet filter that hasn't been used for one scientific project or another.  That said, if you have limited space in your filter wheel, then some filters will get used more than others and you will have to compromise, or swap filters/filter trays when necessary.

Arne

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