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Are Astrodon and Baader any where near parfocal?

MartinB

By MartinB
in Imaging - Discussion

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MartinB

MartinB

Posted
Posted

There have been some images posted here comparing 3nm astrodon with 7nm baader ha filters. I've been impressed with the apparent improvement in contrast and am thinking of splashing out, on an ha filter at least. Does anyone know whether they are close to being par focal?

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Sp@ce_d

Sp@ce_d

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I got my 3nm OIII on Friday so I spent last night generating new filter offsets with the baader LRGB & Astrodon 5nm Ha SII & new 3nm OIII. ACP has a script that takes an hour or so to do this & even having run this 3 times last night my tests with Ha alone still came back with better results focussing with focus max than using the offsets generated only an hour or so before. Even when I used all Baaders they were not parfocal. I think the astrodons seem more critical. I'm inclined to accept the hit of focussing per filter unless I can get better results generating decent focussoffsets.

Sent from my iPhone using Tapatalk

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opticalpath

opticalpath

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Martin, I had this problem in spades when I tried to mix an Astrodon Ha filter with Astronimik RGB filters (also on a MN190!)  The problem is indeed due to the difference in thickness.  The Astrodons are 3mm thick and my Astronomiks were only 1mm.  Your Baaders are 2mm, I think.

A little bit of refocusing between filters may not be the only issue, though.  If you're using an OAG, the guide camera image will go out of focus when you refocus on changing filters.  If the focus change is not great, you may be able to find a compromise focus position for the guide camera that still gives good enough star images to guide on with either filter.  The situation was so bad with my mix of Astronomik and Astrodon that I could not get sharp enough guide stars to guide on  at both focus positions. It's one thing to have to refocus the imaging camera back and forth ...... quite another to have to change the OAG guide camera focus!  I ended up having to bite the bullet and buy a set of Astrodon RGB filters too :eek: .  (Which are great, by the way, and the pain soon passes.) 

Adrian

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davedownsouth

davedownsouth

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As has already been said, loads of difference between the 3nm Astrodon and 7nm Baader.  With my Lakeside focuser on the Star 71 scope, focus with the 7nm Baader is achieved at a setting of 2180, with the 3nm Astrodon it needs to be set to 2310.  NB.  When switching between all of my baader filters (Ha, OIII, SII, R, G, B and L), I pretty much using the same setting and get focus; they really are all parfocal  :cool:.

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ollypenrice

ollypenrice

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Hmmm, I don't find the AD3 all that different from the Baaders in the FSQ106N but it certainly is different. I don't focus robotically so maybe my judgement of difference is different. I certainly need to refocus but for me a narrowband run will be a night's worth of work anyway. I wouldn't scroll the filterwheel to include NB even with the Baader Ha. 

Olly

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swag72

swag72

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That is a very good point (sadly!)  which I hadn't considered.  Thanks for that Adrian

I've got three different filters in the mix.

My luminance is a Hutech IDAS, my RGB's are Baader and the Narrowbands are Astrodon. I use an OAG and have found that the focus changes between all of the filters. The Hutech seems to give me the middle ground so to speak, so my OAG is focused on that and then I had to give it a little tweak for the Astrodons but from there, thankfully it's been OK.

There's a definite difference in my OAG star shapes and focus between the 3 different filter makes, but nothing that PHD2 can't deal with.

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opticalpath

opticalpath

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From what Sara said, Martin, sounds as if you may be OK with Baader and Astrodon.  The difference in thickness is only 2mm versus 3mm so a compromise position for the OAG focus could work out if the guide stars are just a little out of focus in the two positions.

I didn't think of this potential problem when I purchased my first Astrodon NB filter.  Sadly, the difference in focus between the 3mm Astrodon and my 1mm Astronomiks was too much for the OAG; I could not find any compromise position of the OAG focus that would allow reliable guiding at both focus settings for the different filters.

I agree with Olly about completely separating runs of NB and RGB to reduce (or eliminate) the need to refocus constantly.  But that still leaves the considerable inconvenience of having to adjust the OAG camera focus between sessions, if an in-between position isn't good enough for both. I realised very quickly that that was too much faff and swallowed the expense of replacing the RGB filters with 'matching' Astrodons. Now all is calm again, and as I said, the pain soon passes :smiley:.

Adrian    

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ollypenrice

ollypenrice

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From what Sara said, Martin, sounds as if you may be OK with Baader and Astrodon.  The difference in thickness is only 2mm versus 3mm so a compromise position for the OAG focus could work out if the guide stars are just a little out of focus in the two positions.

I didn't think of this potential problem when I purchased my first Astrodon NB filter.  Sadly, the difference in focus between the 3mm Astrodon and my 1mm Astronomiks was too much for the OAG; I could not find any compromise position of the OAG focus that would allow reliable guiding at both focus settings for the different filters.

I agree with Olly about completely separating runs of NB and RGB to reduce (or eliminate) the need to refocus constantly.  But that still leaves the considerable inconvenience of having to adjust the OAG camera focus between sessions, if an in-between position isn't good enough for both. I realised very quickly that that was too much faff and swallowed the expense of replacing the RGB filters with 'matching' Astrodons. Now all is calm again, and as I said, the pain soon passes :smiley:.

Adrian    

I hadn't thought about the OAG issue. When imaging with the 14 inch (and, obviously, an OAG) we only had a Baader filterset so the question didn't arise.

Olly

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ollypenrice

ollypenrice

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Yes, I have an OAG with my Atik one 6 and if you want pin sharp stars in the guider, you have to refocus it when changing from the Baader to Astrodon.

But accordng to Craig Stark, author of PHD, you do not want pin sharp stars in the guider. Guiders work on tracing a stellar 'centroid' which is a notional point derived from the whole stellar image. He suggests that soft focus actually helps define this centroid. (This is why autoguiding can work well below the level of a single pixel.)

Olly

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