Field Flattener / Coma Corrector

[yuklop]

Dear All

I have a Skywatcher 150mm f5 achro. I have recently read a review about a Chromacor which is supposed bring an achro towards an apo in terms of optical quality.

It appears that a Chromacor is now very rare. What is around instead are field flatteners. Does anyone know the difference between these two?

Would a field flattener make a marked improvement on a large fast achro or is it a bit of a waste of cash as they are not so cheap?

I have looked around but remain confused so any help would be greatly appreciated.

cheers

Dan

[John]

I've owned an used a couple of Chromacor's. They are rare though - I've only seen 3 for sale in the past few years and I bought two of them !

They are designed to work specifically with the 150mm F/8 and 120mm F/8.3 achromatic refractors produced by Synta / Skywatcher. The also work with other achromats of similar specifications.

A Chromacor needs to be carefully matched to the refractor objective lenses for best results. When the match is good the results are quite impressive with a significant reduction in chromatic abberation (much more than filters can achieve) and the correction of spherical aberration which means that higher magnifications can be used more effectively.

There was a Chromacor II which was reputed to be even better than the Chromacor I that I used but I've never seen one for sale in the UK. When marketed their price was close to $1,000.

There is reputed to be another such device in the pipeline from both the original manufacturer, Aries and another source but so far these have not emerged.

The only alternative that I'm aware of for reducing CA is filtration. I'm not sure how successful that will be with a 150mm F/5 as that design does produce a lot of CA.

The joy with the Chromacor's is that they corrected the light, ie: bought it to focus closer to the right point, rather than removing it as filters do.

Personally I wonder if it's better to accept the F/5 achromats as good low to medium power instruments and get something else for high power viewing.

I believe a field flattener, as it's name implies, reduces / removes field curvature which is a different issue and more of interest to imagers than visual observers. Coma correctors reduce / remove coma but are principally designed for use in fast newtonian reflectors for both visual and imaging use.

[dmahon]

They look excellent if you can get one:

Chromacor II - Review

[yuklop]

Indeed, I saw that review too, which got me thinking...

They also look pretty expensive. Probably cheaper to simply get ED refractor these days. However as is often stated aperture is king and a 150mm apo would be pretty great! However I can't realistically afford a 'proper' 150mm apo. Sadly.

Also, thanks John, I have been thinking about one of the Schmidt or Maksutov Cassegrain eg a Skywatcher Skymax or C60 / C80 for higher powers. But not yet. Still haven't got close to the limits of my big frac.

I do take some images with it as I live in a city and its a nice way of 'seeing' stuff! What are people thoughts on a field flattener... worth it with such an instrument or not?

cheers again

Dan