Ultraviolet photometry

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Hello there !

Someone doing UV photometry around here ?

I could not find any topic on U photometry on the new forum, so I opened one. Such a photometry looks like to be very exotic, and promising at the same time.

I’ve got a Baader MPCC coma corrector, which is made of BK7, so it could help. BK7 is quite transparent in UV.
The modern CMOS cam are quite good at the relevant wavelenghts. Sure, capturing UV could be not so easy, unless you live in the mountains, but some LC and studies seen on the aavso server suggest that it is feasible.

Also, I’have been in touch with an optician designer from a well known company who seems to be interested to help amateurs which are off the beaten track ; resulting on a possible collaboration to design a specially UV transparent corrector for telescopes.

So …

I was about to order a Sloan u’ filter. I was told that the Baader U filter had some red leaks. Or was it the Optolong ? I can’t remember.

Anyway, the Sloan and Bessel are almost sold at the same price, but the Sloan has a better efficiency, at first sight. The Sloan seems better centered around 350, while the Bessel seems to capture a significant portion of deep blue.

As there are very few amateur doing such observations, I guess not sticking to the Bessel filter is not a problem.

The area concerned by UV photometry includes flare stars, novae, SNs, Be stars, symbiotic and hot stars, WD and many other celestial bodies.

Do you have any anecdote, suggestion, advices ?

It could be interesting to share our practices.

Looking forward to reading your comments.
Cheers,
Christophe

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One could argue that the violet (I wouldn’t call it ‘ultraviolet’ despite the name-usage) is the most important part of the entire electromagnetic spectrum as regards stars. We’re talking from the atmospheric cutoff up to where the Balmer decrement is, and for just the reasons/targets you suggest, and I would include pulsating variables and more. Do bear in mind that exposures are likely to be about 10x longer in u/U compared to what one is accustomed to in order to get similar S/N.
With just about any detector+filter, expect that a proper transformation to a standard system will require nonlinear coefficients, and likely two-color indices U-B and B-V, or u-g and g-r, to get things to work over a wide color range. Even Landolt had to do this for his work on UBVRI standards.

\Brian

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You must not have searched very hard here on the forum or your AI searcher isn’t very good. We have had several discusions on U filter observing and filters.

I had my Optolong Johnson U and B filters scanned by Ray Tomlin a few months ago. The B filter scan looks great. The Optolong Johnson U filter appears to have a slight turn on above 1 nm but the filter scan measures only go to 1.04 nm. Very small contribution when combined with almost all silicon-based sensors. You might get some bias for extremely red stars, of course.

T CrB AAVSO Johnson U filter observations and my own compared to published pro U filter measures agree quite well. Some observers with lots of observations are still not transforming… very frustrating.

if you only look at transformed Johnson U in the AAVSO db, at least since 2460700… the U filter data is pretty good. Some spread but that is probably some real flicker and variation etc. from the system in the Johnson U bandpass.

Jim (DEY)

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Here are some plots of the transmission scans that Ray Tomlin performed of my Optolong Johnson U and B filters. Scans run from 0.350 to 1.020 (edit: nm changed to micron).

Ray Tomlin Optolong U B scans
Ray Tomlin Optolong U B scans509×667 36.2 KB

Jim (DEY)

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The plots show 350 to 1020 nm or 0.35 to 1.02 micrometers.

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Jim, Brian,

All,

Ok, Jim, you’re right, I did some search on the new forum, but didn’t go deep enough to find your topic on “Optolong UBVRcIc filters”. One good point for you ! Sorry for that. The answers I found were on the old forum, on which it is impossible to add new posts.

Well, thank you for the help and advices. So, it was the Optolong U filter which casts some leaks in far red, not the Baader Bessel U. I read this a few years ago, but cannot remember and neither found it right now. Your topic add a confirmation. Thanks for the plots. What I read about the optolong U filter could be problematic for observing UV flares on M stars, as an example, but as you said, the far red contribution is small. That said, the transmission seems very good between 350 and 400nm.

You noted thas the exposure times were very long (as confimed by Brian). What scope/optic are U using, Jim ? I understand that RC scopes are a “must” for UV observations, because of the design : native flat and corrected field without any need of additionnal corrector. The SC, and most of the usual coma correctors for newtonian are not suited for UV observations (or only marginal), cause they are made of glass which absorbs UV, unless it is made of BK7 or fluorides. The Baader MPCC is such an exception (BK7). You most probably know that, just in case.

Now, here are the transmission plots for the Baader Bessel U and Sloan u’. The Sloan seems better at first sight, but there are some leaks in the far red too.

Both are not so good between 380 and 400 nm, but the Sloan go down to 330nm, which is probably the extrem limit for our cameras/BK7 and the atmosphere transmission at ground level.

Hummm. It’s hard to make a choice, but surely the optolong is a good option, all in all.

Cheers !

Christophe

I can’t upload images today.
Here are the liks to the Baader filters.

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Greetings,

I wasn’t trying to score points… searching our old and new forums is difficult was the point…

I have only used the Optolong U filter with this system: Explore Scientific “Essentials” Triplet 127mm + ES 0.7x reducer + Atik490EX CCD f/6.67.

As mentioned before the Optolong U is not even close to parfocal with the above OTA. A auto-focuser I’m sure would fix that problem… I used the focus scale on the 10:1 to adjust focus manually. The Ic filter was also slightly non-parfocal. Good focus could be had for B, V & Rc… Ic no adjustment was necessary for photometry.

I suspect the ES127 “Essentials” optics, a triplet lens, was designed and optimized for visual use and not such wide wavelength use. A mirror only OTA maybe improve things.

I took some test images with a Sky-Watcher Maksutov-Cassegrain 180 with the Optlong U but that was useless. Too small of a FOV, f/ratio of f/15 plus the U filter factor equals too much exposure time.

My transformation coefficients for U and the other Optolong B,V,Rc,Ic filters are stable and seem to work fine. For U the transformation coefficient is moderately large so transforming U is highly, highly recommended.

Ray caught my typo (or the AI did it!) above. I meant to type microns not nm.

Oh, well, I’m just an amateur!

Jim (DEY)

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Hello Jim,

Thanks for the reply.

I had a quick check of your optical system.

Here are the relevant documentations :

https://www.ximea.com/products/usb-vision-industrial/xid-usb3-scientific-sony-ccd-cameras/sony-icx814al-usb3-mono-scientific-camera

Your triplet is made of H-FPL51, I assume this is the same glass or very near the Schott FPL51.

The reducer is made of FDC100.

All in all, your system is quite good.

Transmission at 350 nm is about 0.947 (triplet) * 0.944 (reducer) * 0.75 (filter) * 0.4 (sensor) = 0.26

You may improve the transmission, potentially, by removing the reducer/corrector. After all, the U filter will cut other wavelenghts. The Atik sensor is not very efficient in UV band. This is the main weakness in your system. Cmos camera can achieve better transmission, specially those made with the IMX585 chip. (mine is asi2600 = about 40% at 350 nm is the same than yours about transmission in UV).

Well well, UV astronomy in not easy from the ground.

I had a quick check about the optolong U filter in Europe, and as far as I know, it is unavailable currently, unless one buy the full UBVRI serie.

There is also the chroma Bessel U filter, which has a near perfect transmission : 97% at 350nm.

See for exemple :

This filter is very expensive : about triple price than the Baader filters. I can’t buy this one, unfortunately.

Finally, I noted than the optolong is cuting quite abruptly around 340 nm.

You’are most probably right when you say that a mirror OTA would be a better choice. UV satellite are RC basically RC scopes. But those scopes are not particularlly open at f8. I think thing would be better at f4, ou f3 with a very good coma corrected (UV transparent).

More when I can buy the long dreamed U filter !

I’m just an amateur too :wink:

Christophe

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The high frequency, low wavelength, cut-off at 0.350 microns, 350 nm, is from the spectrophotometer that Ray used. 350 to 1020 nm was the range the device is capable of.

However, the U filter, according to the Optolong published response curve, does drop quickly below 350 nm compared to a standard Johnson U bandpass. However, CCDs and CMOS (silicon-based) sensitivity also drops quickly there. Cost I’m sure is a factor in the decision by Optolong in manufacturing. Probably accounts for the moderately large transform coefficients I have gotten as well. Not really a problem.

As \Brian has mentioned “anything one can do in U is good”–kinda rhymes–I think that is called an internal rhyme! Please transform if you aren’t already and especially when go over to the U-side. :star:

Jim (DEY)

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All,

I began operating with the U filter (optolong).
Things are interesting. I’ll report later when I’ll get a larger vision of this new area.

One of my targets was 000-BQH-858, which very opportunately was in outburst recently, and came from mag 18.25 to 15.4 in CV. (now 17+).
I got a few time series between dec 5 and dec 13 in CV, B and U.

Were can I get some comps with U mag at this level ?

Any advice/help would be very appreciated.
Thanks in advance,
Christophe

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Oh, well.
It seems the catalog II/355/galex_ais on VivieR will do the job, with NUV mag.
Also, those data are available in I/353GCS 242.

Now, I’ve got some pretty uncertain measurements using those Galex NUV mag on 12/12 evening, 120sec exposure on a 12’'newtonian :
CV=15.55, B=15.67, U=19.3 (error 0.3).
The target was not visible in U on the other evenings.

The challenge will be to understand what is lurking behind all of this stuff.

Edit : I’ve read in this paper that the FUV and NUV band are respectively in the ranges 1344-1786A and 1771-2831A, which is not the UV seen from the ground. So, the problem remains about getting some reference magnitudes for our targets.
(The Calibration and Data Products of GALEX - ADS)
Is there another UV catalogue ? IUE maybe ? Could not locate it for the moment.

Humm.
Time will say.
Christophe

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All,

Sorry to be so insistent :wink:
That is now two days I spend on the question on finding the good U reference mags for the comps.
I had searched on various catalogues, but wasn’t able to find any star with U or u Sloan mag, even bright stars. Most probably SDSS have the right data, but I couldn’t find any convenient way to access them.

Searching on the old AAVSO forum is difficult because you only have one or two letters (u, U, UV), the word magnitude is too common in our business to be usefull ; moreover, stars like U Gem or UV Ceti are rendering things more difficult.

So …

If any UV filter user know how to … I would be glad to discuss with her/him how to find the comps U/u mags.

Thanks in advance,
Christophe

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The main problem with finding Sloan u or Johnson U photometry for general field stars is simply that the data do not exist, particularly for stars between, say mag 7 and 12 or so. I would avoid the GALEX ‘nuv’ magnitudes since they are not really anything like U or u. Funny stuff happens at the shorter wavelengths due to the chromospheres and coronae around even ordinary stars. Thus a cool, red star can be very bright in the near-uv.
South of about +15 Dec, one can have a look at the SkyMapper survey (VizieR item ii/379). This gives uvgriz photometry, where ‘v’ is an intermediate passband between the nominal Sloan u and g. Be sure to look at the numbers of ‘good’ measurements and the errors shown for each entry. Use the psf magnitudes. The full VizieR output also shows basic color-indices (g-r, r-i etc), so you can avoid stars of extreme color. The source paper for SkyMapper DR4 is well worth reading (and re-reading) not only for finding out how the data were taken/reduced, but also for seeing the ways in which big survey projects can go awry.
If a field of interest is covered by the SDSS footprint (northern, high-latitude regions mainly, plus a strip along the Equator), you can use those data as well. Generally the photometry is saturated brighter than mag 15 or so, at least for g and r magnitudes. Note that generally n=1 for SDSS data, and cirrus-y nights etc are not accounted for — some of the data are crap. Try to find some confirmation from other sources. The various versions of the SDSS catalgues are at VizieR; I’d recommend using either DR7 (VizieR ii/294) or DR16 (v/154).
Finally, although it is almost never suggested by AAVSO, you can derive new photometry on a standard system by observing standard fields (Landolt or others), doing the transformations for extinction, zero-point, and colors, and obtaining new results for the fields of interest. After getting at least two or three nights of consistent results, please publish your findings so others will have them!

\Brian

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Christophe,

Have you first asked the sequence group to see if they can generate U or any other missing catalog magnitudes. Not always possible but usually they can generate something, if not they will let you know.

Below is a link to a previous post. You probably want the “Request Comparison Stars…” link.

OBTW, I found that AAVSO forum post via a google search! :smiling_face_with_three_hearts:

Jim (DEY)

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Hello everyone, Brian, Jim,

Thanks for the suggestions, Brian and Jim. I really appreciate your help. This new field isn’t easy to grasp. There are quite a few difficulties to overcome, and finding suitable comps is apparently not the least of them.

It makes you wonder if it wouldn’t be worthwhile to dedicate a small instrument (an apochromatic refractor with fluoride lenses, for example) for a year or so, to create a basic U magnitudes catalog for bright stars, say between magnitudes 6 and 12. Just an idea. The field of view would be about ten square degrees in such a refractor with a large-sensor camera. This would certainly help make UV photometry more accessible to amateurs. There would be at least one less hurdle to overcome.

I’m going to read the suggested paper: SkyMapper Southern Survey: Data release 4. Brian: I’ll try to report in this topic all stuff that could be of interest to newcomers, as much as I can.

Jim, thanks for the suggestion and the link to the query document. I could ask the team for sequences, but if, as Brian says, the data simply doesn’t exist, it will be difficult anyway. Also, I can’t request sequences for every observation I’ll make, or AAVSO will have to hire a full-time staff on this!

Okay, so I’ll start by imaging some reference fields. Landolt.

I know there’s also at least one U comp near T CrB at mag 12, a field which I observe regularly. So I could establish some sort of base.

By the way, is there a possibility to retrieve all the U comps that are in the Variable Star Plotter?

Thanks for all,
Cheers,
Christophe

Ps : Jim : “OBTW, I found that AAVSO forum post via a google search!” → One more point, Jim !

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All,

The TAP VizieR Service is definitely the tool to use to find all U/u sources for a given field, used with The Guide Star Catalog, Version 2.4.2.

The table to query in TAP VizieR for the southern hemisphere sky, as suggested by Brian (SkyMapper Southern Survey: Data release 4) is “II/379/smssdr4”.

Also, the table “II/339/uvotssc1” contains some interesting data from the Swift Observatory. The Umag is centered on 346.5nm.
See :

Maybe, setting up a small U/u survey would be worthwhile.
A little apochromatic refractor, or one with fluorid lenses and a short focal length, would be an interesting solution.

The goal would be to have wide fields of view (10-20 square degrees) to maximize the chances of finding comparison stars. We would focus on bright stars initially (magnitude 4-8, with good SNR).
I can access to some mountain areas (in the range of 2000/2800m altitude in summer ; 1500/2000m in winter) to do this.

This would possibly help to unlock the potential of amateur UV astronomy, if needed.

Perhaps this is a pointless, useless idea? I can easily imagine that if it were useful, it would have been done a long time ago… And that this kind of survey isn’t for amateurs.

Some comments, thoughts, ideas ?

Thanks,
Christophe

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All these are good points. For getting new Johnson U or Sloan u data for bright stars, you will almost certainly end up having to get data in at least two filters (u,g or U,B) simply because inevitably your instrumental system will not match the standard ones closely enough. It is common for U-B transformations to need added color terms as a function of B-V and (B-V)^2. Luckily getting the BV data doesn’t add much overhead since the U filter exposures will be 80% of the observing time.
For fairly bright stars one might start building a working catalogue with the Jean-Claude Mermilliod UBV compilation, VizieR item ii/168, which could serve as ‘standards’. I have a fairly large file of UBVRI that includes stuff not in Mermilliod’s lists and otherwise ‘not available in stores’.
There is also a catalogue of very good UBVRj data by Kornilov, that includes all stars north of -15 Dec and brighter than V=7.1. This is not at VizieR (thus not well known), but I have a copy and I’m pretty sure the AAVSO chart team has a copy. I’m pretty sure all four colors need modest transformations to the Johnson/Landolt system (and Johnson R to Cousins R). This would be a valuable contribution to make in itself. Internal errors are very good in this catalogue.
I have also been curious about the Swift + XMM u,b,v photometry. It could also probably be transformed more closely to the standard system, but I get the impression it is maybe too soft (errors ~0.1 mag only?) to use except as a general guide. Sebastian Otero can probably weigh-in on this since it figures in VSX entries.

\Brian

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Unfortunately, the only thing that VSX shows from XMM and Swift are identifiers. They are added in order to support classifications by means of X-ray detections.
I haven’t been experimenting with photometry from those catalogues.
I do agree that you will have U data from the GCPD in the case of the bright stars.

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Brian,

Thanks for those marvelous catalogues !
The Mermilliod is very good on VizieR, with a large mag range.

The full Kornilov catalogue can be found at this adress :

Where the data can be downloaded. A famous one, it seems.

I had a close look at the Lausanne catalogue. It is ubvbBeta, from Strömgren. I could’t derive the U data from this catalogue.
Anyway, it is maybe of little importance right now, we have plenty of reference mag already :wink:

Maybe of interest :
The following catalogue has 3500 sources mostly in the Milky Way:

and this one lists some stars and sources in the northern sky:

Clear skies,
Christophe

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Good to know the Kornilov catalogue is ‘out there’ somewhere! It really does need some work before using it, however. The Palomar-Green survey (VizieR II/207A) is one that I went over to get accurate coordinates about 15 years ago (see the links under the ‘ftp’ tab for the catalogue). Possibly a valuable observing project would be to get UBV data for those stars — though many are variable and there are some quasars etc in the list.
In re transformations from uvby-beta to UBV, a good place to start is the brief paper by Petr Harmanec:

Remember that the Stromgren system is really about color-indices, not separate magnitudes per se. It is perhaps worth mentioning the very nice list of high-weight UBV standards among naked-eye stars from the Harmanec group:

…especially the rigorous methods used in their photometric reductions.

\Brian