Photometric filters

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

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.

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.

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.

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