Fast achromat- reduced aperture

[NGC 1502]

Ok, this is a well known procedure, but here’s what I’ve found myself….

I have a Skywatcher 120/600 achromat bought from an unexpected place. At my local recycling centre there’s a shop that sells stuff people have discarded.  This early model with a blue paint job was bought by myself at a bargain price in the summer of 2023. It looked like it had been in a dusty shed for many years but it cleaned up beautifully.

We all know that the primary use of such a scope is low to medium power wide field.  But over the last 15 months of ownership I found it wasn’t as dreadful as expected on bright objects like Jupiter. Surface details are there but the purple glow is unpleasant.  Lately I’ve experimented with cardboard objective masks to reduce the aperture to raise the f number from f5 to f7.5 using an 80mm diameter mask.

A few recent observing sessions have been productive.  Jupiter looks far nicer at f7.5 than f5. It’s easy to remove and replace the mask to see the difference.  F7.5 and 120x is nice, very obviously not Apo nice but unexpectedly ok.

Double stars too. Bright Gamma Andromeda at 120x and f5 doesn’t look nice, but at f7.5 the Airy disc with diffraction rings are there, and at 150x even better.

Tight triple Iota Cassiopeia cleaned up great. F5 and 150x a lovely sight.

Remove the mask and the double cluster is wonderful.  Using a borrowed StellaLyra 20mm 80 degree eyepiece knocked my socks off!  Thanks Alan for the loan, eyepiece reluctantly returned…..😊

I’m definitely not kidding myself, adding a cardboard mask doesn’t turn a cheap Achro into an expensive Apo……however it’s very satisfying to squeeze much more than expected out of my bargain buy from my local tip😊

Ed.

 

 

 

 

 

[Nik271]

You are in good company Ed: all the great refractors from 19th century had aperture blades and usually the planets were observed with reduced aperture. Mind you they were all at least  F/ 15 to start with.

[John]

I've tried that with some short achromats and its interesting to see the difference in CA correction as the focal ratio increases. It seems to my eye to work more or less along the lines of this well known chart:

What is harder to deal with is the spherical aberration (SA) that many of these short FL achromats seem to possess which seems to take the edge off their higher power performance even if the CA has been relatively well "tamed". This varies scope to scope though so if you get a good one (for SA) then the results of masking can be surprisingly good.

 

Edited October 18 by John