Achros and APO's (at HA wavelengths)

[Macavity]

In the course of wondering which scope may function
best as a donor HA Frankenscope, I have been trying
to find some sort of *quantitative* way of judging the
performance of the commonly available Achromats at
H-Alpha wavelengths. It's not that easy to find?! But:

http://interferometrie.blogspot.co.uk/
(I am NOT an notable expert on this!) 

But it seems possible to get a feeling e.g. for the
(relative) merits of Short Tube Achromats... How
these compare with better ED APOs... for HA etc.
(Such might be of interest to Quark owners too!)

For the purpose of my Fankentests, I will still only
be using (probably) a "stopped-down" ST120. Not
brave/foolish enough to start removing baffles or
taking an angle grinder to *any* f/7 ED APO!  

[ronin]

Looking at the test and results further down you need one of these: 127/660 TeleVueNPis

No point mucking about with achro's etc and hoping, start with a goodie.

Nice information - liked the bit about half way down that shows errors if the O ring is too small, somone had this problm about a week ago, just need to find the post and work out how to point them to the section as there are no real "pages".

[Peter Drew]

The first requisite for a Ha modification is that the donor telescope, as closely as possible, matches the F ratio of the original Ha unit. Achromats and APO's are both suitable candidates however the focal ratio of F10 for the PST mod should offer better potential performance than the F7 required for a Lunt mod. Although Ha is monochromatic such that CA is not such a consideration as it is for other purposes, the SA residual in most of the popular short focus achromats will have an effect. The big plus for APO's is that they are generally figured to a higher standard, delivering the best images.  

[Merlin66]

Peter has said it all.

An apo is definitely not mandatory for solar Ha.

[Macavity]

Heh. Maybe I am hoping an APO is not [too] mandatory?
These are (obviously) no more than my personal thought 
experiments? Disclaimers, where not explicit, are implicit!

Naturally I would agree with Pete! I do also find "working
through" reasons for (and against) stuff is useful. Maybe 
share with a few kindred headbangers enthusiasts here?

I think I will still buy an ST120... Stick on my "Sky Tee"...
Do some simple visual astronomy. It's been a long time!