Narrowband filters: which ones and are they parfocal with LRGB?

[kirkster501]

Astronomik or Baader?  And since I have Baader LRGB - perfectly parfocal with the Tak - will Baader NB be parfocal with the LRGB?  I am not buying anything else for a bit,  I have enough learning to be getting on with but just wondering?  I may ask for a Ha for xmas

[swag72]

Why not try an Astrodon? The 3nm is a VERY nice filter indeed.

[narrowbandpaul]

I'm fairly sure that it's the filter thickness that really determines parfocality. Read an interesting analysis by Don Goldman who showed that typical filter thickness tolerance and easily exceed the critical focus zone. So even if they are meant to be the same thickness there still may be a small focus shift. I don't know if the Baader filters are all the same thickness. They are a good quality though. I agree with Sara, the 3nm astrodome are nice!

[Gina]

Yes, the filter thickness determins the increase in back focus you get by adding filters into the light path.  You add one third of the thickness to the back focus.

Baader filters are 2mm thick and Astrodon are 3mm thick so a considerable focus shift between Baader and Astrodon but since I don't use NB and WB imaging in the same session it doesn't bother me.  I think WB and NB filters of the same make as reckoned to be parfocal but I haven't tested it.  For my main imaging rig I use Astrodon NB filters as they are narrower than Baader but Baader appear to be as good as you get for LRGB so I didn't spend the extra for Asrodons.  Everyone agress, I think, that Astrodon make the best NB filters.  Actually, I am using Baader NB filters for my second, widefield rig as that uses 36mm unmounted filters to cover the APS-C size sensors and I couldn't afford the larger Astrodon versions.  If the widefiled second rig proves very successful then I might just fork out for Astrodons otherwise I have better uses for the funds.

[Gina]

Seems Ian King doesn't do the 36mm unmounted Astrodon filters and 50mm are too big (won't go in) as well as being extremely expensive   So 36mm ones would need importing, adding to the cost.

[kirkster501]

Thanks for your inputs. The Astrodon ones are too expensive for me for the use they'd get, I am struggling to use LRGB with our climate of late!   Gee whiz, the 3mm is £360 - over three times as much as the Baader!  I have done some reading and the Baader Ha is very respectable by all accounts.  So that one is the one I will go for I think. 

I imagine the 7nm is the one to go for?  I will ask Madame Kirkster501 to buy it me for xmas

Thanks, Steve

[Sp@ce_d]

Thanks for your inputs. The Astrodon ones are too expensive for me for the use they'd get, I am struggling to use LRGB with our climate of late! Gee whiz, the 3mm is £360 - over three times as much as the Baader! I have done some reading and the Baader Ha is very respectable by all accounts. So that one is the one I will go for I think.

I imagine the 7nm is the one to go for? I will ask Madame Kirkster501 to buy it me for xmas

Thanks, Steve

I've 2 sets of Baader LRGBHaSIIOIII + an Extra 2" Ha. There is a very slight shift with B & OIII on my doublets. The Esprit seems better but I'm still faffing around with it so not 100% yet. I'd expect the Tak to be perfect tho

Yes, I'd recommend to 7nm I have 3 of them... Would love to get a 3nm though..

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[Chris Rowland]

I think the Baader filters are very good value for money.  I've had good results from the Ha and OIII filters.  I've got he SII but don't get enough signal.  I've also got the RGB filters and they also seem good.  They are all close to parfocal and parfocal with the UVIR filter.

Chris