Cardboard Coma Corrector

[bobro]

650mm fl /130mm aperture  reflector scopes are great for imaging with the fast focal ratio of 5, but they do suffer from coma aberration due to this.

My scope is no exception and seems to have especially bad coma. As coma should reduce with increasing focal ratio, as an experiment I cut a 108mm circlular hole in a piece of card and placed it centrally across the front of the OTA, effectively changing the focal ratio to 6 with the reduction of the aperture from 130 to 108mm.

Below are two single subs : one without the card(f5 at 120secs) and one with the card (f6 at 180secs). I was surprised by the great reduction in coma in the f6 image. (Almost full Moon so rather bright subs.)

Of course the reduction in aperture means an approximate 50% increase in exposure time, but it could be useful for brighter targets.

[emyliano2000]

Hi.

Can I ask how did you come up with the 108mm hole? Is there some formulae that tell you this?

Emil

[RichLD]

This is generally referred to as an aperture mask - I and a few others on here use step-down rings in front of camera lenses to reduce the aperture, which in turn increases the f/stop value, making depth of field greater and reducing the coma/astigmatism that tends to occur on optics using non-aspherical elements at low f/ratio.

I'm sure you'll be familiar with the capped hole in the dustcaps that are fitted to Newts. That hole is an aperture mask for high brightness (e.g. lunar) observing/imaging.

Top result for a bit of cardboard! ?

[bobro]

7 hours ago, emyliano2000 said:

Can I ask how did you come up with the 108mm hole? Is there some formulae that tell you this?

I wanted the f ratio to be 6 as there shouldn't be much coma at this f ratio, so just divided the folcal length of 650mm by 6. The f ratio is the focal length divided by aperture.