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Chroma LRGB Filters not homofocal with Narrowbands

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#1 120decibel


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Posted 23 September 2021 - 01:48 PM

Hi folks,


I had a 36mm LRGBHOS (3nm) Chroma filterset to play with a couple days ago. I usualy focus using the L-Filter with and the Autofocus with NINA and recheck with a Batinov just in case.

Once focused the RGB filters where perfectly in focus but the Ha filter showed donuts on my f/4 Newtonian, forcing me to refocus.


Did anybody else experience this with faster scopes, I can't find any informations on the Chroma webpage as to if they guarantee homofocal filters.?


clear skies


#2 SilverLitz


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Posted 23 September 2021 - 02:23 PM

Telescope optics, excluding the filters, can have significant focus differences across the different wavelengths.  Higher quality scopes will have smaller differences, but the biggest deviations are generally in the extremes, e.g. violet side of blue and IR side of Red.


Reflectors should have less problems that refractors, but reflectors will require glass lens elements to correct for coma, and these will introduce CA.


Upshot is to always refocus on filter changes, and focus each filter individually or use filter offsets.


The image below compares the focus differences of two highly regarded Vixen refractors.  The lines are the focus deviation for differing wavelengths, with hugging the zero axis being perfect.  The y-axis is the distance from the center of the FoV.  This will also be a design ideal, assuming perfect alignment, and is the telescope only.


Vixen AX103S SA.png

Edited by SilverLitz, 23 September 2021 - 02:24 PM.

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#3 120decibel


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Posted 23 September 2021 - 03:43 PM

I''m well aware of chromatic aberration and its nonexistance on mirroring telescopes... even the CA induced through the corrector is nonissue on telescopes above f/4 since the shift of the focalpoint is smaller then the focal plane...

You must have overread that I statet that once focused with the Luminance Filter the images taken with the RGB filters Red - to Blue are perfectly in focus. Thus the whole wavelength spectrum ist covered and chromatic apperation (as stated above) is a non issue here.

The focus shift only occures when switching from LRGB to Narrowband filters and only on the Chromas. My Astrodons don't show this behaviour at all.


Thus the question is solely meant for useres of Chroma filters and their experience if focus shift when switching from LRGB to Narrowband.




#4 jimwww


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Posted 24 September 2021 - 03:03 PM

I see some difference with my Chroma 2" 3nm Narrowband filters vs. the LRGB filters.  All of the LRGB filters all focus within 20 microns of each other on my Planewave 12.5" CDK.  The narrowband filters are also within about 20 microns of each other, but have about a 130 micron offset relative to the LRGB filters.  It is not enough to require a refocus of my OAG camera, so the offset is of little concern to me, as it is handled by filter offsets in my imaging program.  A faster system might be a different story regarding defocusing the guide camera when using an OAG.






#5 Morefield


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Posted 24 September 2021 - 06:05 PM

Yes, my NB Chromas are about 130 steps off of my LRGB.  And I believe the step size is 1 micron so that's substantial and well outside the CFZ of my F5 FQH106.

#6 jrmagill


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Posted 24 September 2021 - 07:08 PM

I recently computed focus offsets for my Chroma filters, both SHO and LRGB.    With L as the reference, the maximum offset was -2 steps of the focus motor, which translates to 3.6 micron movement of the drawtube.   This maximum offset was for the Red filter.   All other filters (SHO, GB) had an offset of -1 steps, which translates to 1.8 micron movement of the drawtube.   I was delighted with these results.    All filters are 36mm diameter.   SHO are 5nm bandwidth.


These measurements were made on a 61mm refractor nominal f/5.9;  I was using a flattener/reducer with 0.8x so the effective f ratio was 4.7.


My filter offsets were computed using a Pegasus FocusCube v2 electronic focuser and NINA computing the optimum focus.   Focus backlash was compensated with Overshoot turned on in NINA.   Using overshoot for backlash compensation was the key to getting such low filter offsets.   I had used Absolute mode with fairly poor results (average offset of 31 motor steps and max offset of 84 motor steps).    So beware,  exactly how you compute filter offsets is very important.


FYI, I think the standard term of equal focus distance is parfocal.

Edited by jrmagill, 24 September 2021 - 07:19 PM.

#7 SeymoreStars



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Posted 24 September 2021 - 10:49 PM

I am not familiar with the term "homofocal". Is this the same as parfocal?

#8 120decibel


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Posted 25 September 2021 - 04:31 AM

@SeymoreStars - yes you are right in German the term is "homofokal" apparently the right translation ist parfocal that's probably the reason why I didn't find any informations when googling it...


@Jim thank you, this is the exact behaviour I'm observing and I find it somewhat annoying, especially since my Astrodons don't show this behaviour.

I like to rotate Filters ever 2 or three images to gather RGB and Narrowband during the night and refocusing is time consuming and filter offsets are always prone to backlash variations an fast systems.


@jrmagil your results give hope, as the filters I have here right now where purchased last years. They might have upgraded ther supstrate. Will test  overshoot in NINA thanks for the hint!


Thanks everybody for your very helpful replies.

Edited by 120decibel, 25 September 2021 - 04:33 AM.

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#9 andysea


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Posted 25 September 2021 - 10:14 AM

I can't remember now, but does Chroma say down to what f ratio they are par focal? I remember Astrodon having that information on their website but that was from before the company got sold. 

I haven't looked a filters in a long time but it may be useful to do that research. I use a mix of Chroma SHO and Astrodon LRGB so I can't comment on the actual offsets.

#10 SeymoreStars



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Posted 25 September 2021 - 10:17 AM

A call to Dick Stewart will answer the question. If someone calls please share the info here.




Dick Stewart

Chroma Technology Corporation

an employee owned company

10 Imtec Lane

Bellows Falls, VT 05101

800-824-7662 (usa only)

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#11 gordoabc


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Posted 25 September 2021 - 09:17 PM

I found my Chroma LRGB filters all have the same focus plus or minus 40 microns or so which given the shape of the v curve is pretty much not resolvable.  The Ha filter was about 200 microns off from where the LRGB minima were clustered.  It was still inside the bottom of the focus parabolic minimum but it was outside the error of the focusing routine.  The 40 micron or so variations I saw with LRGB were really more about the ekos linear focusing algorithm than a measurable offset in terms of star hfr. The Ha shift was still pretty small in terms of stars hfr but it was outside the focus algorithm variability, easily taken care of with an offset in the filter wheel settings of ekos.  This is with an f3.9 refractor.

Edited by gordoabc, 25 September 2021 - 09:26 PM.

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