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Second photometric filter

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Second photometric filter

Hello all,

This is my first post in this forum. I have a question and I think people here can help.

Me and some friends have a telescope at E-Eye, which we use primarily for astro photography. We also have a photometric V filter, which we lately have been using for performing measurements of variable stars (in particular V1391 Cas). In our filter wheel, we have an empty slot, and we are planning to purchase a secondary photometric filter to fill the last slot.

We have read some threads in various forums, and it appears that a common approach is to use B-V filters. In other threads, we have seen that the R filter is good. The other photometric filters appears to be less important. My question is:

What would you recommend us? To purchase the B or R filters? What are the pros and cons of each filter? Or should we go for someting else i.e. U or I?

Note that we are new in this field, but we find it exciting and plan to start measurements as long as we have our remote observatory in Spain.

Thanks in advance.

I'd suggest either B or Ic. R

I'd suggest either B or Ic. R is definitely an option, but it's bandpass is wavelength-wise relatively close to V filter bandpass. Therefore it's ability to measure continuum tilt is not that good as with V-I or B-V.

Best wishes, 


spp's picture
Second filter

Yes.  Get the B filter next.

If possible, try to get a B made by the same manufacturer as your V.  Filters from different manufacturers may have different thicknesses and can focus at a different points.  In that case you may need to refocus between filters.   If you can't get a B made by the same manufacturer, at least try to get one with the same thickness. 

You can learn more about filters (and other topics) in the AAVSO Tools of the Trade free webinar on September 19.  There will be a discussion specifically devoted to filters.



Eric Dose
Eric Dose's picture
V & Ic hard to beat

V1391 Cas is a fairly red star, so if you're considering stars like it, your CCD/CMOS exposures in B will be quite long, and your transforms using B-V will be poor. And you do need to transform any photometry of stars that are as red as or redder than this one.

By contrast, exposures in Ic will be much, much shorter (more data per night), and for red stars transforms in V-I are superior to those in B-V. Rc is a compromise, useful for red stars when exposure times in Ic are simply too short.

So it's up to you, but for red stars, V and Ic make for a very hard combination to beat.

Second filter: B or IC

Thanks for your recommendations and comments. It sounds like we shoulds either go with the B or Ic filter then. Thanks!

spp's picture

 If you continue your interest in photometry you will soon want both of them.  Baader Johnson/Cousins filters seem to be available, individually, from some EU and UK retailers.


spp's picture

accidental repeat.

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