I’m starting to use a remote telescope that has a large filter wheel that includes UBVRI filters. I’m probably the first observer to use these particular filters. How can I remotely verify that the filter wheel position numbers I was given are correct?
I guess you should ask the provider of the remote telescope about the position of the different filters.
Josch
Lane:
Image some reasonably bright targets (10-12?) with extreme color (both blue or red) in all the available filters for perhaps 60 sec. An incorrect filter is going to yield poor agreement with the expected magnitude. Visually,a very red star will look much fainter in a B filter compared to an R or I filter. The U filter is certainly going to show low flux compared to other JC filters because of atmospheric absorption. The V filter is going to yield the correct/expected V magnitude. It may not be perfect but give it a try. I suspect an incorrect filter will stick out. For these five JC filters, you should be able to identify which is correct with this effort.
Ken
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Ken, THANKS! You’re my guru, as always…Lane
Josch, I did get the position numbers from the provider but they are a brand-new, unused install and I would rather not depend on the accuracy of the installer. Just being careful…Lane
Hello Lane,
You would get a decent idea of the filters position using this sort of color test, imaged from some distance of the telescope (20m should be enough) in broad daylight:
https://www.cloudynights.com/uploads/monthly_12_2017/post-253189-0-61083500-1513647580.png.
Christophe
Lane, check one of the recent novae and check if there are BVI photometry and take images in those filters and check whether your photometry is comparable with the data in the AAVSO database.
Josch
I agree with Ken - image a field with both blue and red stars, and compare the differential photometry. Where I might differ slightly is that I recommend a field where there are VSP magnitudes for those stars (so they are constant in brightness) and actually use differential photometry instead of a visual comparison. For example, if you have a blue star A and a red star B, then, for example, using the R filter, calculate the (RA-RB) catalog difference and the (rA-rB) instrumental difference. These should be nearly identical (usually within 0.02mag or so). The best choices for such fields are the AAVSO Standard Fields like NGC7790 or M67. Then you have multiple pairs of stars that you can use to confirm the results. I had to do this numerous times with APASS, as we had various equipment failures over the years, especially with the filter wheels.
You might try imaging a standard cluster with all the filters and then go through the process of determining transformation coefficients. If you don’t have the filters properly identified, it should be obvious.