American Association of Variable Star Observers (AAVSO)
Sat, 01/08/2022 - 14:39

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.


American Association of Variable Star Observers (AAVSO)
Good news Arne!

Funny how…

Good news Arne!

Funny how people obviously not AAVSO members can post spam in the forums but legit members can't post images...

If you want the images available I'd happily post them on my website.



American Association of Variable Star Observers (AAVSO)
Red leak data

We have begun scans and should have a report this month.

Some strange and surprising results!

We are attempting to make sure some of these strange peaks are not instrumental artifacts.



American Association of Variable Star Observers (AAVSO)
Jim - can you give a teaser,…

Jim - can you give a teaser, i.e. are the "strange and surprising results" of the 'don't-buy-these' magnitude?  I ask since I'm close to clicking the 'buy' button but will hold off if so.


American Association of Variable Star Observers (AAVSO)
Red leak data

I am also waiting for Jim's Red leak data of Optolong report. Is it archived somewhere?

I have asked OD data ,and plot to Optolong, and got reply as shown bellow.

"the Optical Density is OD3. Spectrum 300-1100nm."

"Sorry we have no data and plot at present." 



I am not a member of AAVSO, but will join you after I get filters.

American Association of Variable Star Observers (AAVSO)

Im about to pull the trigger on the optolongs.

Wondering if there was any sort of update that would make me hold off and maybe by one Chroma instead of the optolong bundle?😊

American Association of Variable Star Observers (AAVSO)
Optolong filters

I haven't seen or heard anything new that would worry me about the Optolongs.  

Last time I looked at the retailers' web sites the Optolongs were still only avaialble as UBVRI sets (probably still cheaper than only two or three of the more expensive filters).  I'd use the B and V fitlers for starters and see what kind of results I got, i.e. how my measured magnitudes compared with those of experienced observers.  

I wouldn't consider buying another B filter until I had plenty of experience with the B in the Optolong set, and until I had learned to transform the measurements.   That may be all that is necessary.     


American Association of Variable Star Observers (AAVSO)

Thought I'd share this if useful to anyone.

  • C11, Starizona Large Format corrector (2064mm focal length)
  • Optolong BVRI
  • M67
  • Transforms generated using TG v6.9a (with manual outlier rejection)
  • Average of 4 transform sets (acquired over 4 nights, 5 exposures per filter per night)



description= TG - Version 6.9a, Telescope= C11 F7.4 Optolong, Time created (UT) = 2023_05_28_21:00:52
Tbv= 1.214
Tb_bv= 0.202
Tbr= 1.120
Tb_br= 0.133
Tb_bi= 0.108
Tbi= 1.141
Tv_bv= 0.018
Tvr= 0.964
Tv_vr= 0.032
Tr_vr= 0.067
Tri= 1.219
Tr_ri= 0.086
Ti_ri= -0.098
Tvi= 1.067
Tv_vi= 0.018
Ti_vi= -0.043
Tr_vi= 0.039
Tbv= 0.032
Tb_bv= 0.021
Tbr= 0.018
Tb_br= 0.014
Tb_bi= 0.014
Tbi= 0.017
Tv_bv= 0.006
Tvr= 0.011
Tv_vr= 0.011
Tr_vr= 0.014
Tri= 0.031
Tr_ri= 0.016
Ti_ri= 0.019
Tvi= 0.011
Tv_vi= 0.006
Ti_vi= 0.008
Tr_vi= 0.007
[R Squared Values]
Tbv= 0.988
Tb_bv= 0.822
Tbr= 0.993
Tb_br= 0.766
Tb_bi= 0.807
Tbi= 0.992
Tv_bv= 0.101
Tvr= 0.981
Tv_vr= 0.082
Tr_vr= 0.250
Tri= 0.939
Tr_ri= 0.282
Ti_ri= 0.182
Tvi= 0.984
Tv_vi= 0.096
Ti_vi= 0.173
Tr_vi= 0.279