LRGB filters - which ones?

Yikes, lots of replies. Thanks for the info guys.

Yep, the LRGB ones that most of the stores have are just dyed filters. I wonder if when they refer to narrowband if they mean the HA, OIII & HB?
Those are supposed to be the bees knees alright, in fact I just ordered the 7nm HA one from Bernard at ModernAstronomy along with the QHY. I did hear that they have real lrgb ones coming though - don't know if it's true.

I know I saw a discussion on CloudyNights before but can't find it. But here's one from UKAI ukastroimaging.co.uk/forums/index.php?topic=35682.0

Pretty sure I saw a mail from Bern saying the same thing so I think I'll try for the Astronomiks or Custom Scientific.

Saw some 2" Astrodons second hand for over 500 quid!

Aha, found it - it looks like they are plain glass
www.modernastronomy.com/filtersBaader.htm#bLrgb

dmcdona wrote:
no filters are precisely parfocal

Dave,

This is something that has come up a number of times on the SBIG Yahoo forum and it is my understanding that filters are parfocal for telescopes with a focal length around 1200mm ... ex. AP160. Mr. Green

oh didn't know that at all & based on all the info from ye guys it looks like I've got some reading to do again. Gotta find my pdfs of Wodaski's book methinks

Thanks again.

no filters are precisely parfocal

Dave,

This is something that has come up a number of times on the SBIG Yahoo forum and it is my understanding that filters are parfocal for telescopes with a focal length around 1200mm ... ex. AP160. :mrgreen:

I can certainly vouch for this claim (ie. 1200mm) since it is something I have observed repeatedly with my AP160+SBIG filters.

I guess for true perfectionists (like myself ) with sensitive focusers (like the PDF - steps of 0.004mm) parfocality (is that a new word?) is harldly attainable... Still, if they are close enough for offsets to be just a few steps out from each other, then you could call them parfocal...

I *do* know that were some unhappy bunnies when it came to the Astrodon L(c)RGB set being quoted as parfocal since they were clearly not...

Anyhow - lucky you with your AP160 and soon to be Mak-Cass (?)....

I guess for true perfectionists (like myself ) with sensitive focusers (like the PDF - steps of 0.004mm) parfocality (is that a new word?) is harldly attainable...

I found that at that kind of resolution (or higher) erations of focusing produce very slightly different results due to atmospherics. I found if you average the HFD over a couple of focusing sessions you find a set of figures which shows that parfocality (to quote your good self) is in fact acheived.

I hear ya. I've been doing focus runs on all my filters to find the offsets. Surprisingly different results (albeit at the 0.004mm level) each time. Still, averaging them all out should do the trick.

Of course, I was doing all this manually until Bob Denny pointed out that ACP has a built-in script... Doh!

Anyhow - lucky you with your AP160 and soon to be Mak-Cass (?)....

Fingers crossed ... :mrgreen: :mrgreen: :mrgreen: