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LRGB filter advice please

Dave Smith

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Dave Smith

Dave Smith

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I have a DMK21 astro camera (not yet used) and am a bit confused by the range of filter sets that are available. Probably I will only need colour filters for planetary imaging and am unlikely to get a "proper" CCD camera for Deep Sky imaging as I am happy using a DSLR.

In no particular order the following filter sets seem to do the job.

Baader RGB with IR cut - £72

Baader LRGBC CCD filters - £278

Astronomik LRGB type II - £185

QHY LRGB - £69

For my needs are the cheaper filters good enough or would I later wish to upgrade? Any advice very welcome, I don't wish to waste money on something inadequate or of much better quality that I wouldn't notice.

I am considering getting the ScopeTeknix manual filter wheel - is this one OK?

Thanks for any input

Dave

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Psychobilly

Psychobilly

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Hi Dave,

I have got the same camera... The cheaper Baader Filters (Coloured glass ones) and the ScopeTeknix wheel. The 5 position wheel like the ScopeTeknix is the way to go... a 3 position one would be a waste of time a 4 would be ok RGB + clear...

The thicker "Baader" filters do fit but I opened up the wheel and removed one of the spacer washers and replaced it with a much thinner one and then adjusted the detent ball... there was slight drag of the filters on the inside of the wheel when I got it and i didnt want anything that could generate dust going on.

I have the IR Cut mounted to the camera noespiece. I have left one of the filter positons open and then mounted R,G,B filter and a UHC-S I happened to have lying around..

The only LRGB i have managed so far was indoors as the skies have been carp since i got the kit...

The filters don't appear to be parafocal - well at least at a distance of about 15 feet between the scope and the target they dont... hoping to try them on a proper target sometime soon...

I did post a thread with a few observations and questions but didn't get much response either...

Peter....

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brianb

brianb

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The 5 position wheel like the ScopeTeknix is the way to go... a 3 position one would be a waste of time a 4 would be ok RGB + clear...

A 5 position is ideal. I have the Astronomik 2c LRGB set in four positions of mine with a IR pass filter (also Astronomik, & parfocal) in the "spare" position.

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Dave Smith

Dave Smith

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Thanks Peter and Brian. Would I be right in thinking that being parfocal is important for the planets, in particular Jupiter, due to it's speed of rotation and not wanting to "waste" time refocussing for each colour?

If that is correct, it looks like the Astronomik filters are the way to go.

Dave

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FLO

FLO

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It is worth taking time when choosing your filters as the parfocal issue usually means that once you start you are generally committed to stay with that brand. We back the new Baader LRGBC and Baader narrowband CCD filters because we believe they lead the field. Astronomik also have a good reputation and I understand they have some new filters in the pipeline designed to offer similar anti-reflection/halo properties to the Baaders.

Check out IDAS too, we hear their light pollution filters are the very best.

HTH

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brianb

brianb

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Would I be right in thinking that being parfocal is important for the planets, in particular Jupiter, due to it's speed of rotation and not wanting to "waste" time refocussing for each colour?

That's the way I read it, though it may not make much difference if using an achromatic refractor as there will be some focus shift with colours anyway.

Focusing takes a hell of a lot longer than actually making the imaging runs & anything which saves having to redo the focusing constantly is helpful.

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Psychobilly

Psychobilly

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With the "cheap" Baader ones you have to stack the IR Cut with the RGB. I have done this by fitting the IR-Cut to the nosepiece of the DMK.

When your imaging L you only have the single filter (IR Cut + empty wheel position) and when your doing RGB you have 2 filters (IR Cut + R , G or :) in the optical train thats causing the non-parafocal issue?

Seeing as I am only "playing" with LRGB and the cheap set came up on buy and sell I picked them up to try them... as did the wheel soon after although its been reduced in price a lot so its only slightly more expensive "new" from ....you guessed it... one of the forums "favourite" astro suppliers which have a such a good reputation for service :cool: <- Note Icon.....

Peter....

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Dave Smith

Dave Smith

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Seeing as I am only "playing" with LRGB and the cheap set came up on buy and sell I picked them up to try them... as did the wheel soon after although its been reduced in price a lot so its only slightly more expensive "new" from ....you guessed it... one of the forums "favourite" astro suppliers which have a such a good reputation for service :cool: <- Note Icon.....

Peter....

Yes you just pipped me to the post on that one;)

If I am understanding you correctly, the filter wheel is also available at virtually the same price from our most reputable supplier. NB no sarcasm icon. i.e. FLO

Dave

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MrEd

MrEd

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Hi Dave,Peter and Brian

I use my setup mainly for mono/RGB imaging

of planets.

I use an AE filter wheel (not the best but its ok)

It has 5 slots and in them i have Astronomik

Type2 LRGB and 742nm IR pass.

I do not really have any advantage with parfocal performance.

I have to shift the focuse slightly when going from L-R.

The colours R+G are close and may not need refoucussing but i always have to alter focus for blue.

I think that seeing and the atmosphere has alot to do with focus

and no two nights behave the same.

When it comes to jupiter and Mars my advice is get the frames as quicky as you can to avoid rotaiton blurring and just use RGB not LRGB with a total exsposure of about 3 minutes on jupiter and up to 5 minutes on Mars.

I hope that helps

Ed Sampson

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Psychobilly

Psychobilly

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:cool: I took a punt on being able to mod it to take the Cheaper Baaders Dave as it was a close thing if they would fit... I was going to modify it it they hadn't but fortunately it was a really simple mod just altering the order of the spacing washers...

I wish it was available form FLO...

It's available from the Schizophrenic one.... been reduced to £80...

Peter...

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Dave Smith

Dave Smith

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Thanks for your input Peter, Brian, Steve and Ed. As my use of filters is mainly for Planets and the Moon it looks like the Astronomik filters from Bern and the ScopeTeknix manual filter wheel from FLO will satisfy my needs.

Now I must get back to my project write-up.

Dave

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brianb

brianb

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When your imaging L you only have the single filter (IR Cut + empty wheel position) and when your doing RGB you have 2 filters (IR Cut + R , G or :cool: in the optical train thats causing the non-parafocal issue?

No - the glass has to be the same thickness, or rather a very slightly different thickness because the refractive index varies with wavelength!

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FLO

FLO

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With the "cheap" Baader ones you have to stack the IR Cut with the RGB. I have done this by fitting the IR-Cut to the nosepiece of the DMK.

Regarding the cheaper of the two Baader LRGB filter sets, they are unusual in that they are absorptive filters, rather than dichroic. Most absorptive filters offered for astronomy are pants but the Baaders are actually well-made. Also, being absorptive (they absorb unwanted wavelengths of light whereas dichroic filters reflect them) they are inherently anti-reflective.

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Psychobilly

Psychobilly

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No - the glass has to be the same thickness, or rather a very slightly different thickness because the refractive index varies with wavelength!

Is that a NO its not the cause of the problem...?

Meaning theres no difference in going through one "planar" optical element rather than two?

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brianb

brianb

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Is that a NO its not the cause of the problem...?

Meaning theres no difference in going through one "planar" optical element rather than two?

Of course there is. I'm talking about the LRGB (or RGB) set.

But if the glass is say 1mm thick - and accurately controlled - there will still be a difference of about 0.005mm between the focus positions of the red & blue filters because the refractive index will be slightly greater for blue light, and you will have to refocus to compensate for this. A parfocal RGB set has to have slightly different thicknesses between the filters - or use glass with different optical properties - to allow for this.

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