Focus Issue with OIII filter on edgeHD 8

Celestron EdgeHD 8" Auto-Focusing OIII
5 replies•146 views
Ancient.Photon avatar
Ancient.Photon avatar
Hello, I recently acquired a 533 mono camera and have been collecting some test shots weather permitting. I use Zwo Eaf for focusing. With the Luminance filter the focus point was very close to my previous configuration (osc camera + cls filter). Last night I polar aligned without any filter and when I put the oiii filter the focus point shifted which is expected but the stars still have a donut. EAF said focus successful with a star size of around ~6 but the stars still still have donut. Attached a snapshot of a 5min sub. I tried focusing manually but the star always had a donut and going to the other side it went out of focus.

Is this a backfocus / spacing issue? Please advise.
Chris White- Overcast Observatory avatar
Chris White- Overcast Observatory avatar

Looks like its out of focus.

When focusing with narrowband you will need longer exposures so that enough stars are recognized. I’d try again with longer exposures.

Helpful Concise
Damien Galanaud avatar
Damien Galanaud avatar
Do you use it at native focal length or with the reducer ? I agree with Chris, you do need a longer exposition time when using narrowband filters, especially with slow scopes like the Edge HD8.
By the way, if you bought your filters through the same manufacturer at around the same time, check if it guarantees them as being parafocal. If it is the case, you should be able to perform your focusing with the luminance filter, and then switch to the others without loosing focus. That’s the case for my 2 batches of ZWO filters (31 mm LRGB & 36 mm LRGBSHO)
Helpful Concise
Ancient.Photon avatar
Ancient.Photon avatar
Damien Galanaud:
Do you use it at native focal length or with the reducer ? I agree with Chris, you do need a longer exposition time when using narrowband filters, especially with slow scopes like the Edge HD8.
By the way, if you bought your filters through the same manufacturer at around the same time, check if it guarantees them as being parafocal. If it is the case, you should be able to perform your focusing with the luminance filter, and then switch to the others without loosing focus. That’s the case for my 2 batches of ZWO filters (31 mm LRGB & 36 mm LRGBSHO)



thanks Chris and Damien for suggestions. The OIII filter is from svbony and the uv-ir luminance is zwo. I will check regarding the parafocal. But I did use longer exposure up to 10sec. I also moved to brighter star and then 1 sec was cery bright and good but the results from the zwo eaf was still a donut star. I will repeat this again with a refractor and again with edgehd 8 . Also I am using it with the 0.7 reducer.
Jan Erik Vallestad avatar
Jan Erik Vallestad avatar

The ZWO and Svbony filters are not parfocal. I think they have about 0.2mm difference in filter thickness.

Well Written Concise
bigCatAstro avatar
bigCatAstro avatar
Ancient.Photon:
Damien Galanaud:
Do you use it at native focal length or with the reducer ? I agree with Chris, you do need a longer exposition time when using narrowband filters, especially with slow scopes like the Edge HD8.
By the way, if you bought your filters through the same manufacturer at around the same time, check if it guarantees them as being parafocal. If it is the case, you should be able to perform your focusing with the luminance filter, and then switch to the others without loosing focus. That’s the case for my 2 batches of ZWO filters (31 mm LRGB & 36 mm LRGBSHO)



thanks Chris and Damien for suggestions. The OIII filter is from svbony and the uv-ir luminance is zwo. I will check regarding the parafocal. But I did use longer exposure up to 10sec. I also moved to brighter star and then 1 sec was cery bright and good but the results from the zwo eaf was still a donut star. I will repeat this again with a refractor and again with edgehd 8 . Also I am using it with the 0.7 reducer.

With my SVBony dual narrowband filter (Ha/OIII) I use up to 25 second exposures for my autofocus run in NINA.  I get much better results when in the 20-25 second range.
Well Written Concise
Related discussions
Askar FMA180 Pro focusing issues and coma problems
Hello everyone, I am currently building a wide-angle setup, but I am facing serious issues with my Askar FMA180 Pro. Firstly, there is a problem with focusing: I am unable to reach focus at the manufacturer’s specified 55 mm backfocus distance. Inste...
Focusing issues with mono camera and filter-related optical problems similar.
Feb 21, 2026
Focus issue or collimation issue?
I am setting up an AG Optical 12.5" iHW (f/3.9 Harmer Wynne design at roughly 1250mm focal length) and I am shooting with a QHY600 along with Chroma filters. I noticed that some nights I get these nice rings around stars on the RGB filters. Some...
Focus issue with filters; author experiences variable ring artifacts across filter types.
Aug 26, 2024
EAF + telelens issue
Hello everybody, I recently bought the ZWO EAF and a support to use it on my canon 70-200 f/2.8 L. I have problems in reaching the perfect focus. First of all I manually set it, then I run the focus feature on my Asiair Plus but I never get a V-shape...
Focus Issue with OIII filter on edgeHD 8 — uses ZWO EAF for focusing troubleshooting.
Jan 13, 2024
Asi585//PDS150
Good morning,I have an issue with focusing and I am really struggling with it.i have checked the backfocus,I have tested the camera with the extra spacers but always my focus is a bit off. also I have collimate my telescope and the laser seems to be ...
Focus issue with filter; author uses coma corrector and tests focus like your setup.
Jun 30, 2024
Focus cube running to zero with out of focus camera issue
I’m using N.I.N.A./Skywatcher Esprit 120 ED/Pegasus Focus Cube and a ASI533 mm camera. I’ve used this setup for a long time with good success. Last night when trying to focus I’m running to 0 and the focus program shows an error. If I disconnect the ...
Similar camera model experiencing focus issues with external focuser.
Sep 12, 2025