Off-set Halos around Stars

Flattener Ritchey-Chrétien Acquisition Deep Sky
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Ron avatar
Ron avatar
Just recently purchased a TS field flattener for an 8" GSO Ritchey-Chretien telescope and I'm working on getting the spacing correct.  I've set the backspacing nominally at 109mm (the recommended backspace) and the stars appear pretty round, but the halos I see around brighter stars near the edges seem to not be centered.  They all seem to be consistently off-set away from the center of the image.

The question I have is whether this is normal even if the spacing of the flattener is correct or is it indicative of the spacing being a little too short?  I've attached the image for reference.

Thanks,
Ron

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Dave Erickson avatar
Dave Erickson avatar
Yes it is normal for halos to be de-centered with proportionately greater symmetric offsets about the optical axis for increasing field angle. This will occur when the optical system is not telecentric. A telecentric system is one where the chief rays for all fields are parallel or nearly parallel to the optical axis.  In a telecentric system the halos will be symmetric across the field.

The halos are caused by reflections from optical surfaces, in most cases from filters, whose reflection is out of focus at the image plane. It is the angle of the chief rays for various fields that causes field dependent halo offsets.
Regards,
Dave
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Ron avatar
Ron avatar
Thanks, Dave.  I appreciate your assessment.

I'm aware that the halos are from the filters.  Until I get a premium set, I'll just live with them.  They tend to be worst with narrow-band.

Anyway, I think this sums up what you said.  Essentially the flattener works at a give focal distance and since the image of the halos follow a longer path, they are not in focus.  Further, because they follow a longer path, they do not fall within the correct backfocus for correction.  Therefore, they show the uncorrected shape characteristic of the focuser being too close or under corrected.  But, their shape is not indicative that the stars themselves are improperly corrected, instead I should scrutinize the shape of the stars.

CS,
Ron
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Dave Erickson avatar
Dave Erickson avatar
Hi Ron,
Glad you found utility.

A bit closer look at the image shows what appears to be slight tilt top right to lower left. A small tilt adjustment might be all that is needed.

Not sure what degrees of freedom your adjusting mechanics have, but I have found it best to align on axis performance, removing lateral decenters, using either FWHM stars or interferometry, then balance the field at the corners with simple tilts and taking into account the decenter that occurs with those tilt corrections. It is rare to have mechanics that separate tilt and decenter there is almost always cross coupling with those adjustments.

Regards,
Dave
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Ron avatar
Ron avatar
Thanks again, Dave.

Optics is definitely not a forte of mine.  I appreciate your analysis.  Indeed, I have noticed the tilt and I'm slowly tweaking the primary of the RC to get it minimized.  I'm not convinced 100% that it's the scope and not the camera sensor that is tilted though, but it seems that altering the tilt of the primary is helping.  As you've noted, it's only going to take very small changes at this point.  At least I'm not going to be chasing my tail trying to get the reflections centered on the off-axis stars.  For that, thank you.

Best,
Ron
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Dave Erickson avatar
Dave Erickson avatar
Hi Ron,
Difficult to tell what the source of error is, but with the small correction needed it likely won't matter from a performance perspective. Glad there was utility in the discussion. Look forward to seeing more of your images.

Best regards,
Dave
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