RGB parfocal filters

[Ken82]

Finally a little bit of respite from work and cloudy weather ! 
 

Rather than just auto focus as normal I wanted to see how parfocal my new RGB filters were. 
 

I set up and focused on red (filter 2) at which point I got a reading of 0.5 FWHM. Then without refocusing I moved to blue (filter 4) and the value jumped to 3.3 FWHM. The same was also true of the green (filter 3). 
 

Actually you can visually see the star bloating/out of Focus when it’s changed to blue. 

This is not something I’ve looked at before and I normally refocus between filters anyway but a difference this high. 

I need to check out the same filters with a different scope next time out as it may well just be the scope. 

What would others expect to see for FWHM between filters in a very high end setup? 
 

Ken 

[tooth_dr]

I have noticed this too recently, and it's real problem when you try to take RGB data but dont have a (reliable) autofocuser.  I suspect it is due in some extent to the ability of the telescope to correct the light?  You havent said what scope you use, but in my doublet it a problem, but it isnt a high end setup.  

[Ken82]

9 hours ago, tooth_dr said:

I have noticed this too recently, and it's real problem when you try to take RGB data but dont have a (reliable) autofocuser.  I suspect it is due in some extent to the ability of the telescope to correct the light?  You havent said what scope you use, but in my doublet it a problem, but it isnt a high end setup.  

Yes I think you are right, it may be the telescopes ability to correct the wavelengths. I’ve had a previous issue with this even though it’s a 5-6k scope. I was hoping these filters would solve the problem. 
 

I’m using a takahshi fsq 106 and chroma rgb filters.
 

It’s best I test the filters  with a different scope first before making conclusions. 
 

To be fair even if the filters aren’t parfocal they are excellent for zero halos! 
 

What doublet are you using ? 
 

 

[tooth_dr]

7 minutes ago, Ken82 said:

Yes I think you are right, it may be the telescopes ability to correct the wavelengths. I’ve had a previous issue with this even though it’s a 5-6k scope. I was hoping these filters would solve the problem. 
 

I’m using a takahshi fsq 106 and chroma rgb filters.
 

It’s best I test the filters  with a different scope first before making conclusions. 
 

To be fair even if the filters aren’t parfocal they are excellent for zero halos! 
 

What doublet are you using ? 
 

 

Having no haloes is great.  I’m just using an ED80 to gather the RGB data, hard to justify an expensive focuser for it. 

[ollypenrice]

I tend to think that decent filters are indeed parfocal. It's the optics which are not. Since I'm currently using good optics (Tak FSQ106N and TEC140) I find more or less full parfocality. However, that doesn't mean that the FWHM is consistent across the filters.

You say your R gives O.5 and your B 3.2.  The first question is, how low can you get your B by refocusing?  The original difference can have several causes, viz

- non parfocality of the filters. 

- non parfocality of the optics across the visible spectrum.*

- imperfect blocking of the UV by the blue filter.*

- camera's sensitivity varying with wavelength.

- microlensing artefacts where there are microlenses on the chip.

- spectral class of the star itself. The best test would be on a G2V close to the zenith.

* These two will contribute to blue bloat which is really a separate issue from parfocality because, even in the best possible focus, the problem will persist.

Olly

[Whirlwind]

As noted above you really need to go through a focus routine with each filter to see whether they are each at best focus rather than just flit between the three and assume they are at focus.  Also as highlighted stars are different brightness at different colours so more flux will arise in the brighter band.  Although it is noted you could observe a G2V star you don't need to use this type of star to see the effect (G2V just represents a solar type star so generally you balance to white using this spectral class as it makes colours look 'natural' to our eyes).  

Most refractors (even the high end ones) become less corrected towards the near-UV and near-IR anyway.  If you want optics that are perfectly colour corrected you need a mirror only system.

[Ken82]

Just wanted to add a quick video to confirm that these filters are in fact parfocal. In fact I would go as far as say these are one of the best Astro purchases I’ve ever made. 
 

As normal I’d still expect to do a slight refocus between filters. 
 

With a few problems I’ve had elsewhere recently these have been a real find 😀
 

Absolutely zero halos even on vega etc 
 

asi 6200+chroma filters+fsq 85 

ken 

[retret66]

I have Astrodon 3nm SHO and LRGB, even though they say it is parfocal, I find it still slightly out of focus and I think this is probably due to manufacturing tolerance. This is why I turn to a tri-scope to eliminate changing filters many times during a session.