Photometric filters

[Dr Andrew Smith]

Since Farpoint Astro enveloped Astrodon their photometric filters seemed to have evaporated. The only ones I can find are Badder. Does anyone know ant other options. What is the current recommendation for photometry?

Regards Andrew

[Grant Privett]

This evening the Farpoint site was still showing them as available – though not always in stock.

[Dr Andrew Smith]

Grant, yes it’s been like that for ages and they are out of stock at dealers except of the odd filter. Regards Andrew 

[William Bristow]

Andrew.

Chroma produce a Bessell photometric UVBRI series in 1-1/4“ or 2” mounted and 50mm round or square unmounted, other sizes can be made to order.

Available direct from Chroma USA or on special order from their UK stockists.

https://www.chroma.com/products/optical-filter-sets-for-astronomy-applications

William.

[Robin Leadbeater]

The response curve of those Chroma filters

https://www.chroma.com/products/sets/27103-bessell-ubvri

looks very different from the Baader (and other) versions I’ve seen

http://astrograph.net/Baader-Planetarium-Johnson-/-Bessel-V-Filter-125-and-50mm-x-50mm

Robin

[Dr Andrew Smith]

Robin, they are different standards. Baader are Johnson-Cousins and Chroma are Bessell.

Regards Andrew 

[Robin Leadbeater]

Hi Andrew,

I thought Bessell was pretty much the same as Johnson-Cousins eg here half way down the page

http://spiff.rit.edu/classes/phys440/lectures/filters/filters.html

Of course the elephant in the room is the response of the sensor which is very different today compared with detectors of previous eras

Robin

[Nick James]

The cost of these photometric filters is pretty eye-watering compared to the bog-standard LRGB filters that people use for imaging. As Robin says it is necessary to calibrate the transformation coefficients for any particular sensor in any case and you then need to image in multiple bands so you can apply those coefficients. It may not be perfect but could you use imaging  RGB filters and transform them to something close to the standard photometric  bands? I know that people use DSLRs for variable work and do something similar with the Bayer RGB pixels. I’ve certainly found that the green pixels of my ASI294 match Gaia G quite nicely and I use a neat tool called rawtran which transforms the RGB into other photometric bands. It seems to work pretty well with reference to APASS photometry.

[Dr Andrew Smith]

I think it’s less of an issue with differential photometry as the camera response impact the target and comparison star equally. However, the filter and camera response will make some difference if the comparison star are of very different spectral classes.

Regards Andrew 

[William Bristow]

Andrew.

Two other Bessel-Johnson-Cousins and Kron-Cousins photometry filter suppliers I forgot about.

Custom Scientific and Andover Corp, both USA based, there are no UK retailers so you would have to contact direct.

https://customscientific.com/astronomy.html

https://www.andovercorp.com/products/astronomy-filters/johnsonbessel-ubvri-filters

https://www.andovercorp.com/products/astronomy-filters/ubvri/

If you particularly wanted Astrodon you would have to place an order with Farpoint and wait. It is unlikely the Astrodon photometry filters will reappear in the usual retailers any time soon, they are struggling with demand after absorbing several other manufacturers last year and don’t have sufficient production capacity. The parent company, Optical Structures Inc, was emailing previous customers at the end of last year offering shares in the company in order to raise funds for expansion. I have colleagues in the US with back-orders in excess of eight months.

Were you intending to obtain a full set of UVBRI filters or just partial?

Last year I looked at adding to my partial Baader set some of the Sloan series. Running the transmission and QE numbers for my current camera and OTA it was apparent that a full set of Sloan’s would be pointless.

For the Sloan series only the g’2, r’2 and i’2 were of any practical use, u’2, z’2, z-s2 and Y2 are all outside the capabilities of my current instruments.

Similarly, for the Bessel – Johnson-Cousins series U is out of range, only B, V, Rc and Ic are possible and in recent years most of my projects have been in B and V only.

William.

[Robin Leadbeater]

I remember Gordon Myers talking about transformations (and extinction)  at the 2018 BAA/AAVSO joint meeting.

https://britastro.org/video/13862/14771

He put some numbers to the size of the effect for different systems he tested (6:39)

At the end of the day though these are still approximations dependent on the actual spectrum and can be way out in some circumstances. As an extreme (though real) example, a Nova spectrum dominated by H alpha. eg

https://britastro.org/specdb/data_graph.php?obs_id=650

A significant fraction of the H alpha flux would appear in the standard Johnson-Cousins V passband but would be completely missed by the Chroma version. Spectroscopy is much more straightforward 😉

Robin

[Robin Leadbeater]

The same would apply if using the G channel from a DSLR to estimate the V mag of the nova as the G channel response at H alpha is different again eg

http://www.astrosurf.com/buil/50d/test.htm

Robin

[Dr Andrew Smith]

Thanks William, not that fussed about brand but wanted high transmission and only the three listed.

Thanks for the info.

Regards Andrew 

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