Do I need a V filter?

[daz]

So, starting to take steps in to variable star measurements,... I have downloaded the AAVSO CCD guide and will be working through that to measure my systems parameters (gain, e/adu, etc) and it comes to the question of filters...

The Photometric filters are expensive, but also seem to be rarer then rocking horse poop....

It seems Baader have stopped making all their Johnson -type filters and are working on a set of sloan filters, but I can't see any info on when they might be available.

So, is there an alternative to the V filter I can use while I sell my remaining kidney and good eye to pay for one??

I have the standard RGB/Ha/Hb/O3/S2 filters for astro-imaging at present...

 

Cheers!

[andrew s]

I seem to recall that you can transform G to V. Also you can use clear for some projects.

@JeremyS is your man as he is director of the BAA Variable Star section and I am sure he can point you in the right direction.

Regards Andrew 

Edited January 28, 2021 by andrew s

[JeremyS]

Yes, I hear that getting hold of V filters is challenging and/or expensive atm. I hope this will change.

whether you need a filter depends on the type of star and observation you are making. I rarely use a filter as I specialise in cataclysmic variable stars where sensitivity and time resolution are paramount (a V filter will knock about 1.5 mags off the detection limit). Use V band comparisons and report as CV.

For highly coloured stars like long period variables (red) a filter is important 

For some targets you could use a green (G) filter and submit as TG (which is really meant for Bayer matrix  Tricolour Green in DSLRs). At least until you can get a V filter. Or you can transform the G measurement to V, but this takes quite a bit of work to calibrate. Either way use V band comps.

 

[daz]

Thank you both!

Found this interesting discussion - https://www.aavso.org/use-baader-rgb-non-photometric-filters as well.

So, at least I can start measurements and keep the kidney!

Thanks again!!

[Whirlwind]

On 28/01/2021 at 09:14, daz said:

So, starting to take steps in to variable star measurements,... I have downloaded the AAVSO CCD guide and will be working through that to measure my systems parameters (gain, e/adu, etc) and it comes to the question of filters...

The Photometric filters are expensive, but also seem to be rarer then rocking horse poop....

It seems Baader have stopped making all their Johnson -type filters and are working on a set of sloan filters, but I can't see any info on when they might be available.

So, is there an alternative to the V filter I can use while I sell my remaining kidney and good eye to pay for one??

I have the standard RGB/Ha/Hb/O3/S2 filters for astro-imaging at present...

 

Cheers!

It depends on what you want to do.  AAVSO prefer to have calibrated results so they can compare to all other results.  However, if you are doing things for your personal interest then you don't need to have a V filter to do photometry.  In fact a Lum/clear filter is just fine, that will show up variability in some ways better than a V filter as you are allowing more light in.  In the astrophysics world different colour filters are used to infer spectral types of objects or how to classify any variability.  For example the depth of an eclipse in a binary will be different with different filters because each star will have its own temperature (barring two similar temperature stars).  On the other hand planet eclipses will be the same depth in any band - in this case because of the shallow depths more flux (i.e. no filters is better).  So don't think you have to have specialist filters to do photometry.

[CraigT82]

Would it be possible to use the Baader Venus filter in lieu of a photometric V filter? The passband of the Venus filter is shifted slightly to the lower end compared to photometric V.

 

[andrew s]

1 hour ago, CraigT82 said:

Would it be possible to use the Baader Venus filter in lieu of a photometric V filter? The passband of the Venus filter is shifted slightly to the lower end compared to photometric V.

 

 

 

Not advisable.  The pass bands don't  match and the UV is heavily attenuated by the atmosphere.  K for a bright Venus but difficult for most stars.

Better to use a non photometric V filter.

Regards Andrew 

[JeremyS]

1 hour ago, CraigT82 said:

Would it be possible to use the Baader Venus filter in lieu of a photometric V filter? The passband of the Venus filter is shifted slightly to the lower end compared to photometric V.

 

 

 

No, this filters transmits UV light. It’s a long way from V

[Merlin66]

I think the transmission is a poor comparison to a proper V photometric filter.

[CraigT82]

Ah I've misread the UBVRI chart... I thought V was the shortest wavelength but it's actually green! V for Verde? My question should have been:

Is the Baader UV filter a substitute for the Photometric U filter? 

Thanks for bearing with me! 

[JeremyS]

1 hour ago, CraigT82 said:

Ah I've misread the UBVRI chart... I thought V was the shortest wavelength but it's actually green! V for Verde? My question should have been:

Is the Baader UV filter a substitute for the Photometric U filter? 

Thanks for bearing with me! 

V is for visual. It approximates to what our eyes see, which are most sensitive in green.

the Baader UV is pretty close to U band. But this is ultraviolet and as Andrew points out, most of this is stopped by our atmosphere. So I don’t really see a role for this filter in amateur stellar photometry. Some amateurs use a B filter (“blue”), at the shorter wavelength end, but even that generally has less applicability- likely much longer exposures than V. 

Edited February 8, 2021 by JeremyS

[CraigT82]

Thanks Jeremy.  The Baader Venus filter popped into my head whilst watching Stargazine Ep.35 when Dr Froebrich mentioned they are short of U and B contributions but it sounds like that's because they're not really achievable with backyard scopes

Cheers

[JeremyS]

13 minutes ago, CraigT82 said:

Thanks Jeremy.  The Baader Venus filter popped into my head whilst watching Stargazine Ep.35 when Dr Froebrich mentioned they are short of U and B contributions but it sounds like that's because they're not really achievable with backyard scopes

Cheers

B is within amateur grasp, Craig. It’s usually used for specific projects where “ B minus V” colour index data are helpful. 
Another reason why B is a bit harder is that CCDs tended to be less sensitive at that end of the spectrum. By contrast they are more sensitive in R.

[CraigT82]

Thanks Jeremy.  Apologies to Daz for hijacking the thread!

[Whirlwind]

8 hours ago, CraigT82 said:

Thanks Jeremy.  The Baader Venus filter popped into my head whilst watching Stargazine Ep.35 when Dr Froebrich mentioned they are short of U and B contributions but it sounds like that's because they're not really achievable with backyard scopes

Cheers

Well they are 'achievable' but you do need the correct set up.  Firstly you need a reflector, many refractors aren't well corrected in the near UV. Secondly many commercial CCDs aren't that sensitive in the near UV (CMOS can have advantage here).  The issue of attenuation has already been noted but you can still get some signal.  

However you also need to think about the stars you will observe.  Objects bright in U or B are very hot and hence conversely few and far between relatively.  Many stars will not be that bright in U and B.  As such you need to make sure you have a large aperture to be able use these type of filters.  If you are just starting out V is definitely the way to go as you will get the most out of it and then you can diverge from there.

[BinocularSky]

Another thread-hijack coming in 🙂  (apols, @daz )
@JeremyS Now that it's clear that Baader have stopped doing Johnson-Cousins photometry filters and are moving over to Sloane DSS production (although this is taking its time!) and the only potentially available Johnson-V are from Astrodon with a delay of around 3 months, in your opinion which would be more useful to get in the long term to get: Sloane DSS UGRIZ or Bessell UVBRI, both of which are available (at a price!), or hang on for the Astrodon?

[JeremyS]

1 hour ago, BinocularSky said:

Another thread-hijack coming in 🙂  (apols, @daz )
@JeremyS Now that it's clear that Baader have stopped doing Johnson-Cousins photometry filters and are moving over to Sloane DSS production (although this is taking its time!) and the only potentially available Johnson-V are from Astrodon with a delay of around 3 months, in your opinion which would be more useful to get in the long term to get: Sloane DSS UGRIZ or Bessell UVBRI, both of which are available (at a price!), or hang on for the Astrodon?

As long as you record the filter type when you submit data you can use which you prefer (or which are affordable/available). Arne Henden of the AAVSO said years ago that the Sloane filters would become the standard for amateurs, but this hasn’t happened yet. Maybe it will now. I see a few people submitting G band data.

[BinocularSky]

3 minutes ago, JeremyS said:

Arne Henden of the AAVSO said years ago that the Sloane filters would become the standard for amateurs, but this hasn’t happened yet. Maybe it will now. I see a few people submitting G band data.

Thanks, Jeremy. ISTR that the AAVSO has a filter code for Sloane filters, but not for Bessel. And I suppose the demise of Baader J-C filters might hasten the change to Sloane.