Page 1 of 1

how good are Acromat scopes

Posted: Sun Dec 10, 2017 4:25 pm
by skyhawk
But just how good are Acromat scopes

I don't want an APO

thanks

http://www.opticstar.com/Run/Astronomy/ ... 10_1_1_245

Re: how good are Acromat scopes

Posted: Sun Dec 10, 2017 4:29 pm
by David Frydman
They are excellent if long focal ratio.
Preferably f/15 but f/10 also.
But they must be good quality.

I have excellent 120mm f/8.3 and smaller f/14 or f/15.

Skywatcher may be O.K.

The Russian Tal 100mm f/10 is supposedly good.

The Pentax 100mm f/12 is unbelievably good.
135mm f/16 Merz excellent.
Cooke 150mm f/15 excellent.
Harvard 9.5 inches f/12 also good.
Northumberland 12 inches f/18? also excellent. But the new objective probably special glass.

Horace Dall ~108mm f/30 camera obscura objective maybe the world's best.

What size do you want?

Regards,
David

Re: how good are Acromat scopes

Posted: Sun Dec 10, 2017 5:11 pm
by Brian
Hi Skyhawk. There seems to be a problem with the link you gave. Did you mean this one:

http://www.opticstar.com/Run/Astronomy/ ... 10_1_1_245

regards,

Re: how good are Acromat scopes

Posted: Sun Dec 10, 2017 10:11 pm
by David Frydman
Thanks Brian,
I couldn't get it to work.

Regarding chromatic aberration.
A 3 inch f/9 doublet refractor.
4 inch f/12
and 5 inch f/15
are reckoned to be CA free.

However, using the stricter Conrady limits it would be.
3 inch f/15
4 inch f/20
and 5 inch f/25.

However, in the real world a Tal 1000mm f/10 is reckoned to be very good.

I have no idea what size is intended.

Weight, portability, fixed mount or not are all important especially as we get older.

Regards,
David

Re: how good are Acromat scopes

Posted: Mon Dec 11, 2017 10:48 am
by brian livesey
APO refractors are expensive, but no mention has been made of the cheaper semi-apos.
I have a semi-apo CELESTRON 80mm/f.7.3 refractor and it gives superbly sharp results. This particular model isn't produced now, but there are equivalents out there.

Re: how good are Acromat scopes

Posted: Tue Dec 12, 2017 10:08 am
by skyhawk
Thanks all