Optolong UBVRI filters

Phil Sullivan loaned to me his set of 1.25" Optolong filters.  I know that James Hamilton (Wisconsin) is going to scan a set with better equipment than I have, so I did not do any spectral scans other than to make sure the bandpass looked reasonable.  I look forward to Dr. Hamilton's report!  The Optolong filters are mostly interference/dielectric and match the Johnson/Cousins bandpass central wavelength and shape.  Since attachments are forbidden on AAVSO forums, I can't include the normal images and plots that I would normally do.

The filters look nice.  The cell is thin (5mm) and will not be a problem in thin wheels like the ZWO versions. I think the Rc and Ic filters start with colored glass; the others look like clear glass that is either dielectric or AR coated.

For on-sky testing, I always choose a very red star, usually a carbon star.  For Fall observations, I choose T Cas, which has (B-Ic) of about 7.5.  For spring observations, I choose R Leo, with a (B-Ic) around 8.  These two stars are on Frank Schorr's program, and so are monitored pretty often with several BSM sites.  In this case, the observations were made on October 7 and used T Cas.  For the 30-day period centered on the observation date, T Cas increased in brightness from B=10.65 to B=10.50.  I looked at raw data from BSM_TX, BSM_NH and BSM_NM for T Cas.  Over that period, there were 21 observations.  Plotting the untransformed results gives a good idea of how various systems compare using the same comp stars.  The untransformed scatter is about 0.05mags from the linear trend line.  The check star, labeled 104_1 on a VPHOT chart, shows B=10.87 +/- 0.03 for all systems including the Optolong dataset, where the Optolong measure is about 0.03mag fainter than the trend line.

When plotting the Optolong B magnitude for T Cas, it is slightly brighter than the other systems (0.07mag), but pretty close.  This deviation could be due to a slightly longer red tail on the Optolong B filter, or it could be due to a small amount of red leak.  Certainly there is larger scatter between all systems for this red star, so small differences in the bandpass are probably present for other filters/telescopes.  Perhaps James' scans will give us more information.  I'd say the B filter is perfectly usable for most projects, and might be fine even on T Cas when transformed.

For V,Rc,Ic, the plots all look good.

For U, there is definitely a red leak.  This shows up both as a diffuse halo around T Cas, as well as an untransformed magnitude that is much brighter than it should be.  The U filter might be ok for blue stars (say, a CV or a delta Scuti variable), but I would not use it for red stars.

I did take a dataset for NGC7790 that could be used to determine transformation coefficients, but I did not do this step.

I think the Optolong UBVRI filter set is a good one at a great price, with the caveat that you shouldn't look at red stars with the U filter.

Arne