Filter Parfocal question.

kinetic (Steve) · #1 25-01-2009, 02:17 PM

Hi,

Recently I made my take on the standard '4 or 5 position manual
filter wheel'.

It works well, it's lightweight and it's as thin as I can feasibly get
it.

However, without doing a lot of homework on the actual typical
order and placement of the typical imaging filter arrangement, I went
with just what I thought should work.

In short, I have a rotating 4 way platter with Red, Green and Blue
1.25inch filters screwed into it.
(the 4th spot is empty, creating the 'straight through' aperture I need
for imaging luminance with my Mono DSI camera.
I have the Meade nosepeiece mounted at the front of the filterwheel
and the Baader IR filter screwed in there.

So for a typical image I always have the IR in place and select
NONE(straight through), Red(with IR), Green(with IR) or Blue(with IR).

I hope I'm being clear here, I'm probably over-describing what's obvious
to everyone...a look at the pics might just make it clear where the
filters sit.http://mywebsite.bigpond.com/astrost...ter_wheel.html
I assume this is a typical situation for filters.

My question....(finally!), is this:

When I image straight through (IR in nosepiece only) the focuser needs
to be at focus point A.
However when I do RGB (again with IR nosepiece in place) the focuser
needs to be at focus point B. (all colours, this spot is the same).

What is causing the focus point to shift? Is the Baader IR adding a slight
Focal reduction to the optical train and should it be closer physically
to the face of the colour filter?

I would like to have a situation where refocusing is not necessary or
at best minimal for all 4 shots (L,R,G,B)
Is this asking too much, or a typical situation?

Thanks in advance,

Steve B.

peter_4059 (Peter) · #2 25-01-2009, 04:19 PM

Hi Steve,

If i'm reading this correctly you are imaging through 2 filters for colour ie. R + UV/IR and through one (only UV/IR) for Lum. This will cause a different focus position for colour vs Lum. I have the IR filter that came with the LRGB filter set in the fourth filter wheel position so the Lum image is through a UV/IR mounted on the front of the DSI and an IR in the wheel. This way the whole lot remains parfocal.

Are you using the Baader LRGB set? If so then what you need is a second UV/IR filter. Hope this helps,

edit - looks like Meade filters in the picture - the Meade filter set that comes with the DSI pro normally has IR, R, G, B. In this case put the four Meade filters in the wheel and the Baader UV/IR on the nosepiece.

Peter

rally · #3 25-01-2009, 04:19 PM

Steve,

The light is travelling a different path because there is no "lens" element in the luminance position.
Try a clear/luminance filter in this position of the same thickness as the RGB and the problem should clearup !

This is the standard setup for LRGB imaging - the Luminance filter should have all the same sort of anti reflective coatings etc as the other three.
LRGB filters are usually sold in parfocal sets.

Rally

kinetic (Steve) · #4 25-01-2009, 05:32 PM

Quote:

Originally Posted by rally

Steve,

The light is travelling a different path because there is no "lens" element in the luminance position.
Try a clear/luminance filter in this position of the same thickness as the RGB and the problem should clearup !

This is the standard setup for LRGB imaging - the Luminance filter should have all the same sort of anti reflective coatings etc as the other three.
LRGB filters are usually sold in parfocal sets.

Rally

Pete, Rally, thanks for the help,

Apart from the reason to equal the lens effect of the nosepiece IR
in the Luminance position.....wouldn't it be 'doubling up' on the
IR side of things?

Two filters doing the same job...? This is how it's really done?

As for my filters, I have only the cheap Meade RGB/IR set that came with
the DSI Pro II......and an extra Baader IR cut fulter.

I use the Meade RGB in the platter and the Baader up front in the nosepiece.

I will try the spare Meade IR in the 4th platter position, hopefully it doesn't decrease the results in my luminance channel too much.

thanks guys,

Steve

peter_4059 (Peter) · #5 25-01-2009, 06:01 PM

Thats exactly what I have - ie the Meade LRGB set in the wheel and the Baader UV/IR on the front (of the camera). I'm no expert but was lead to believe that the Meade RGB filters don't have IR blocking so should be used in conjucntion with a UV/IR filter - not sure if this is true or not.

In terms of doubling up on the Lum I guess it's going to reduce the light transmission slightly but is better than having to re-focus. The other advantage is it keeps dust out of the filter wheel.

I think some of the more expensive filter sets are both parfocal and have the IR blocking.

http://www.astrovid.com/prod_details.php?pid=216

http://www.astrodon.com/products/filters/e-series/

peter_4059 (Peter) · #6 25-01-2009, 06:06 PM

Here's what one of the Meade forum gurus had to say about it...

http://meade4m.com/forum/index.php/topic,755.0.html

kinetic (Steve) · #7 25-01-2009, 06:57 PM

Quote:

Originally Posted by peter_4059

Here's what one of the Meade forum gurus had to say about it...

http://meade4m.com/forum/index.php/topic,755.0.html

Thanks so much Pete, that pretty much says it all eh....
Next night out, that's the plan!

Cheers.

Steve

Merlin66 (Ken) · #8 26-01-2009, 03:58 AM

Using the two UV-IR filters in tandem will give you about 10% less light, but this can easy be compensated in the exposure.
Has anyone seen the transmission curves for the Meade filters?? Can seem to find them on the net - would be interesting to compare them with Baader/ Tru-Tek/ AstroDon......

renormalised (Carl) · #9 26-01-2009, 11:36 AM

The transmission curves for the Meade filters aren't on the Meade site either. You'd think they would be.

Merlin66 (Ken) · #10 26-01-2009, 06:35 PM

Must be the ONLY commercial set of LRGB filters without transmission curves?????? Makes you wonder???
I'll try and get the spectroscope set-up and compare the transmission with Baader/ TruTek ( the others I have)

peter_4059 (Peter) · #11 26-01-2009, 06:58 PM

Try this link

http://meade4m.com/forum/index.php/t...38.html#msg538

Merlin66 (Ken) · #12 26-01-2009, 07:44 PM

Good catch!
I'll do a plot based on Peter's written description; pity he didn't give the graphs!
BTW the secondary link to Astrodon seems to be dead?
I've emailed Mark to see if he can help.

peter_4059 (Peter) · #13 26-01-2009, 08:01 PM

Not sure if this is the link he refers to but I found this..
http://www.astrodon.com/articles_faq...ase:391,355,49

Quite an interesting read.

Merlin66 (Ken) · #14 27-01-2009, 03:17 AM

Mark was kind enough to send me his spreadsheet with all the data and transmission curves.
To big to attach; I'll try to extract as a graphic the curves only.
Other than that PM me with an email address and I'll send a copy.
[Edit] The graph is now attached as a doc file

kinetic (Steve) · #15 27-01-2009, 09:33 PM

Thanks Merlin!

So much better looking at a graph rather than a description of a graph.
First noticeable thing is the secondary unwanted spike of all colours
up in the IR.

That's obviously screaming out....'must be used in conjunction with
an IR block filter'

Also, just as I have read somewhere ages ago, the Meade IR lets in
slightly more of the IR than the Baader as seen on your graph.

Thanks again, PM sent re email,

regards,

Steve B.

Merlin66 (Ken) · #16 27-01-2009, 11:03 PM

That's OK.
We're all travelling the same road.

kinetic (Steve) · #17 27-02-2009, 06:42 AM

Hi,

Just for the record, the arrangement suggested in the posts below
works fine after testing ...refocus is almost not needed now
between R,G,B or just Lum shots.

This was after I put the Meade IR in filter position 4.
R is pos 1
G is pos 2
B is pos 3

and the Baader IR is in the nosepiece.

HOWEVER!!!!!, the Meade IR filter cuts off the IR far too early
compared to the Baader IR and so, when I am imaging something like
the Horsehead Neb, it's a struggle to pick up the shape of the
horse with the Meade IR in place.

Previous attempts at the HH with only the Baader IR , (in combination
with and without RGB), the shape was easily seen in 5sec subs.

I don't know if this loss of the Ha is due to the combination of
using both IR filters combined...or just purely the premature
cutoff characteristic of the Meade IR.

Looks like I should ditch the Meade IR and get another Baader IR...
or just put up with refocusing between L and RGB.

Steve

Merlin66 (Ken) · #18 27-02-2009, 07:09 AM

The Baader IR cuts at 700nm; not sure what the Meade does, but I'd be REALLY surprised if it cut-in close to the Ha.
Something else must be happening.............
I'll try to find a Meade Ir cut and check it with the spectroscope.

gregbradley · #19 28-02-2009, 01:06 PM

Hi Steve,

For imaging the Horsehead or dust type objects you are better off with either no filter at all or a clear filter with no UV/IR cutoff. It is already very dim and it is robbing you of valuable light for the image. Your stars will bloat slightly but you will be better off.

Greg.

Quote:

Originally Posted by kinetic

Hi,

Just for the record, the arrangement suggested in the posts below
works fine after testing ...refocus is almost not needed now
between R,G,B or just Lum shots.

This was after I put the Meade IR in filter position 4.
R is pos 1
G is pos 2
B is pos 3

and the Baader IR is in the nosepiece.

HOWEVER!!!!!, the Meade IR filter cuts off the IR far too early
compared to the Baader IR and so, when I am imaging something like
the Horsehead Neb, it's a struggle to pick up the shape of the
horse with the Meade IR in place.

Previous attempts at the HH with only the Baader IR , (in combination
with and without RGB), the shape was easily seen in 5sec subs.

I don't know if this loss of the Ha is due to the combination of
using both IR filters combined...or just purely the premature
cutoff characteristic of the Meade IR.

Looks like I should ditch the Meade IR and get another Baader IR...
or just put up with refocusing between L and RGB.

Steve

TheDecepticon · #20 28-02-2009, 05:26 PM

If you are using Meade filters, you will need to image through a colour filter and the IR filter at the same time due to their spectral response. If you want to be parfocal, put all your Meade filters in the wheel and the Baader filter in the nose piece, effectively imaging through 2 IR filters or 1 colour and IR at the same time as stated before. Also, with the DSI Pro, you will need at least an IR filter, you cant use them too well with out. A clear filter is not recommended due to very quick pixel saturation by bright stars in your image field. I run mine this way for the same reasons you ask. Have had no problems.