About 3 yrs ago Arne Henden posted comments on the Optolongs. There is a link to this comment somewhere in this thread.
The gist:
large red leak in U, making this filter unsuitable for most photometry.
similar but smaller issue in B. Fine for most photometry except, perhaps. for very red targets, but probably also okay for red stars if you transform. You should be transforming anyway after you have some experience.
V, R, I all good.
I did my own testing of the Optolongs using a C5/ ST-10 CCD system, comparing the B,V,I color transform coefficients versus the transform coefficients produced by the Astrodon and the early square shoulder Chroma filters. I used a Landolt field without very red stars.
The Astrondons produced color transforms quite close to ideal (color transform close to 1, magnitude transforms very close to 0). The Optolog and Chroma filters gave good usable results (but not nearly as good as the Astrodons).
I think the red leak in U should not be a problem for most of us. Our systems are relatively insensitive in U and we mostly observe from low altitudes where the much of the UV is absorbed by the atmosphere. There are plenty of projects for us to do that donāt need U observations.
I was thinking of doing that but stacking two filters in my FWs wonāt allow rotating the wheel. I might try putting one in front of the LED flashlight to be sure there is no emission in the U. A few more things to try.
Initial U transform via U-B for the Optolong U filter and based on only five stars from Landolt SA-110 is 1.00488. Now that I know that I need to focus U Iāll do that and gain more stars to use in the L-S fit.
Perhaps I dd put a bit too much of my own interpretation in the gist list. Here is Arne comment of the U filter:
*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."
If a filter only āmightā be okay for blue stars but is clearly not usable for red stars to me it is not a good choice for most of us.
Arne on the Optolong B filter:
āIād 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.ā
I will try to state more clearly my main point.
We shouldnāt be fussing about the Optolong U filter, since most of us have telescope/camera systems that are particularly unsuited to making U observations. Iād say pick targets and projects for which you can make good transformed magnitudes measurements in B,V,R,I. Sell the U filter to astrophotographers who want to take pictures of Venus.
Roger all that. The U came with the set just putting it thru its paces to see what could be done with it. As I mentioned beforeā¦ too long exposure times mostly, uncertainties in general. Good discussion by everyone.
Iām afraid looking at the U flat images that the interference coating is not close to uniform and other strangeness. I have never seen flat-field image structure like what this filter produces in any other filter I have used over the years.
I ordered the Baader 0.610 nm long-pass filter to try as suggested by Robin. Should be here in a few days, perhaps in time for some clear weather! Some simple A, B comparisons on some test stars will tell a lot.
I have more tests to run here if we ever get āclearā skies again. The standard fields should be enough to characterize the U and B I think, just need to get some good quality photometry weather.
I wonder how many photometric filters are just sitting unused in former observers drawers, retired observers who have stopped observing, etc. Certainly those filters could be put to use by others. Iām guessing the management doesnāt want to get into the filter trade/loan business like they used to do with telescopes!
OBTW, I did one test image of T CrB using all the Optolong U, B, V, Rc & Ic filters before the T-storms, cloud and rain returned. The U photometry was a bit high, the error bar very large (the out-of-focus issue) and so not statistically an outlier even at 1-sigma compared to other T CrB U phometry in the dbānot reported to the db. The B, V, Rc, and Ic was fine.
I wonder if what youāre seeing in the U filter flats is related to the annealing pattern in the detector. In using any filter in the violet (narrowband comet filters, Sloan u, etc) with the thinned CCD I have access to, the flats are leopard-spotted (from the annealing pattern) and have various other āfunniesā, but the images flatten-out very nicely.
Application of the flat fields to science images certainly produces flat science images! However, the raw flats just look strange! Normally all my other filters have a few % drop off around the edges from normal vignetting. The image attached shows the U flat. I used two lights sources and LED lamp and normal sunlight bright sky both thru a milky diffuser. The flats are basically identical. The brightest areas are around 50K DN and the lowest are around 12K DN. The raw flats just look strange but the science images are just fine as mentioned.
Any chance the filter wheel stopped between two filter positions? I had a similar image the other night. Only happened a couple of times, after which the images all looked normal.
It does look like that doesnāt it but Iām pretty sure the filter was centered in the optical path. To be sure Iāll try to take some new flats and look down the OTA to confirm centrality of the filter!
With such are large range of DNs if filter positioning were a problem the science images would not be flat but they are OK.
Iāll do an eyeball confirmation just to be sure tho. Maybe Iāll get the new flats today to check.
Apparently, Roy was correct. I took some U twilight sky flats last night and they are pretty uniform. So the stuck filter wheel seems to have been the problem. Never done that before. Reprocessing and redoing photom. More laterā¦
