Anomaly with moon shots

[Jon_Manchester]

I have a mead 10" sct, I've been trying some close up moon shots, using a Canon R5  and a 2x barlow and a 3x barlow.  A 90-98% moon. I've been getting a doughnut shape in the centre of the field. much more noticeable if the moon is off centre or it is less than full phase. If the moon is almost full, you can see a hot spot where this anomaly occurs.  is this a known issue with an sct.

[RT65CB-SWL]

Hi @Jon_Manchester and welcome to SGL. 

Yes! is the simple answer.

The ‘doughnut’ is the reflection of the primary mirror when imaging/photographing bright objects. It even happens with photography catadioptric lenses.

Edited January 28 by RT65CB-SWL

[Jon_Manchester]

Is there any fix for this,  I did suspect something like this because of the small secondary mirror etc. Would a ND filter over the corrector help, or is the moon off the menu?

 

It works fine on a 5" celestron,  but a big issue on the 10", obviously much greater light gathering,  it is the f6 version.

[RT65CB-SWL]

Are you using a single ND filter or variable polarising filter?

The variable may help in reducing some of it!

[Jon_Manchester]

I did use an nd filter, but surely that just darkens the whole image, and this light has already picked up the erroneous light. Sort of after the fact if it's something in the tube or because of the design.  

[Louis D]

I'm guessing it's due to the plane flat and highly reflective imaging sensor reflecting light like a mirror back up the tube.  I recall that some vintage macro lenses designed for film have this hot spotting issue when used with digital sensors.  The Tamron 90mm f2.5 Adaptall comes to mind because it has a plane flat rear element reflecting that light back to the sensor as a hot spot.  This wasn't an issue with film because it always had a bit of curvature to it despite the film rails trying to hold it flat.

I suppose you could try tipping your sensor a bit if you have a tilt adjuster in the optical train.  That would send the reflected image off to the side instead of right back at the secondary mirror.

Edited January 29 by Louis D

[Jon_Manchester]

No option to a alter the angle of the sensor, using a Canon R5 camera. I may try projection rather than a barlow.  Just to see if it helps. 

Is there any fix for this,  I did suspect something like this because of the small secondary mirror etc. Would a ND filter over the corrector help, or is the moon off the menu?

 

It works fine on a 5" celestron,  but a big issue on the 10", obviously much greater light gathering,  it is the f6 version.