Baader IR-PASS 685nm - Parfocal with CCD LRGB?

[SnakeyJ]

I currently have the Astronomik Pro Planet 742 IR-PASS filter, but finding this is a long way from parfocal with my Baader LRGB CCD filters. I've seen that Baader offer an IR-PASS, starting at the redder 685nm, so perhaps a broader window and will prove less effective at mitigating poor seeing effects. If it was almost as good as the Astronomik, but had the benefit of being parfocal with the other Baader CCD filters, it could save a lot of time doing IRLRGB sets and be well worth another £ 50 - can anyone offer any advise/experience on this?

BTW - I did try Baader's website, but it seems to be in a proper mess at the moment!

On the off chance anyone has a s/h one languishing in there EP/filter case - please feel free to PM

[bobmoss]

I know this is an old thread, but did you ever find out the answer? Did you buy one in the end?

[swag72]

For imaging, believe parfocality claims at your peril My RGB Baaders are not parfocal and neither are my Astrodon narrowband filters.

Only in my opinion, but as focus is THE make or break part of an image, I'd never rely on parfocality claims anyway. I refocus after every filter change, and having just moved to an auto focuser, the computer refocuses for me after EVERY narrowband sub.

[SnakeyJ]

I know this is an old thread, but did you ever find out the answer? Did you buy one in the end?

I didn't ever find out, but suspect it will be parfocal. Perhaps Astrovani could confirm, as he has used the 685 a lot recently. I will get one at some point, but the PP742 is still giving good service and not being parfocal is less of a problem with a stepper focuser - as I can step out 20 steps for IR automatically during sequenced captures.

[bobmoss]

For imaging, believe parfocality claims at your peril My RGB Baaders are not parfocal and neither are my Astrodon narrowband filters.

Only in my opinion, but as focus is THE make or break part of an image, I'd never rely on parfocality claims anyway. I refocus after every filter change, and having just moved to an auto focuser, the computer refocuses for me after EVERY narrowband sub.

Thanks for that, very usefull. I've only just got my filters and a wheel and haven't had chance to use them yet.

[bobmoss]

I didn't ever find out, but suspect it will be parfocal. Perhaps Astrovani could confirm, as he has used the 685 a lot recently. I will get one at some point, but the PP742 is still giving good service and not being parfocal is less of a problem with a stepper focuser - as I can step out 20 steps for IR automatically during sequenced captures.

Thanks. A stepper focuser sounds like a great idea. I've gyoit the standard Meade Microfocusser but may have to upgrade at some point.

[ronin]

Your Baader LRGB CCD filters will be "parfocal" because they are the same glass type and the same thickness, the other one will likely be on a different glass and a different thickness.

The apparent thickness is dependant of the refractive index and the wavelength so at the wavelength scale even the set of Baaders are not parfocal but you cannot tell and cannot adjust the system finely enough.

What I suspect you would want to do is find out the glass thickness and the glass type used on the Baader IR Pass.

[Physopto]

Hi,

Astronomik filters are quoted as 1mm thick. Baader filters are quoted as 2mm thick.

Filters of the same thickness from the same manufacturer made of the same glass should be parfocal. The coating should make no difference to the parfocal ability, if from the same manufacturer. This will only change if they change their manufacturing technique between batches of stated parfocal filters. You cannot mix filters made by different manufacturers normally as they use different thicknesses of glass and possibly different glass types. Each glass type has a different Refractive Index, and hence will refract particular wavelengths of light at different angles. This means that the light will come to focus  at different positions unless it is exactly orthogonal to the filter surface. The way that good filters work is that they employ quarter wave techniques ( destructive interference) to reject unwanted light. Each layer of the lens coating has a particular job to do. Many layers are combined to tailor the filter to allow or reject different frequencies of light, so that only those wanted emerge on the other side of the glass filter and pass on to the CCD or to the eye.

Note: even the quoted thickness of filters are not usually exact. For instance Astronomik quote 1mm but in fact talking to Gerd Neumann of Astronomik informed me that their filters were 1.1mm, so the figure quoted is either rounded up or down for simplicity. These figures are never exact! there are always slight differences as no manufacturing technique is perfect.

I hope this helps.

 Derek

[SnakeyJ]

Thanks. A stepper focuser sounds like a great idea. I've gyoit the standard Meade Microfocusser but may have to upgrade at some point.

It's well recommended Bob - I followed the instructions from Tekkydave in this thread http://stargazerslounge.com/topic/218975-arduino-ascom-focuser-mark2/  he's written an ascom driver for it and a windows focus utility.    All in the bits cost around £ 20, with the motor, nano and most of the parts from Ebay China and the MXL drive wheel and belt from Motionco.co.uk - about 1/10th the cost of a commercial product and it works very well indeed!

Your Baader LRGB CCD filters will be "parfocal" because they are the same glass type and the same thickness, the other one will likely be on a different glass and a different thickness.

The apparent thickness is dependant of the refractive index and the wavelength so at the wavelength scale even the set of Baaders are not parfocal but you cannot tell and cannot adjust the system finely enough.

What I suspect you would want to do is find out the glass thickness and the glass type used on the Baader IR Pass.

Ronin is spot on here - the LRGB CCD filters are very close to parfocal, certainly within the limits of seeing on my F4.8 newt under UK skies.   Though with a refractor the difference may be more discernible due to differential refraction at different wavelengths, particularly with achromats and less well corrected APOs.   If I ever get a night of really good seeing, I will try and measure the difference and then I can add offsets for all my filters in my capture programs.

[bobmoss]

Thanks Jake. I'll have a look at the DIY focusser thread, I do electronics as a hobby so it should be a good project to have a go at.

[SnakeyJ]

Thanks Jake. I'll have a look at the DIY focusser thread, I do electronics as a hobby so it should be a good project to have a go at.

So simple even I could do it with my dodgy soldering skills   Definitely recommend a 12v stepper if you are using filter wheel and barlows as the image train gets quite heavy!

[Earl]

No filters are parfocal disspite anyones claims, each filter should be focused independandtly.