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)