Focus Issue with OIII filter on edgeHD 8 - Forums

Topic starter

5 days ago

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

5 days ago

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.

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Damien Galanaud

5 days ago

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)

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Amit

Topic starter

5 days ago

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

5 days ago

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

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bigCatAstro

5 days ago

Amit:
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.

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