why are the tiny APO's so good?

Yep, I understand. If I was just doing visual I would use a Mak. In fact I had a 5" Skywatcher Mak for a while & Saturn was just stunning through it. In fact, I absolutely love Maks.

But for photography there are a number of problems. The long focal length requires 4 times the exposure time to get the same amount of photons to the CCD so you need WAY better tracking. Also there is a loss of contrast due to the 25% (or so) central obstruction, placing it in between refractors and SCTs for contrast. The big issue for me though was that it was impossible to fit anything but the smallest DSOs on the CCD chip due to it's large focal length.

Beware sites that show Maks as having a 10-12% obstruction though, they are actually quoting area and not diameter.

Trev

Hi,

Very good quality refractors give excellent contrast in the images.
Have a look at my images of the 2006 Total Solar Eclipse taken through a 60mm APO. (at www.irishastrosoc.org/obs/ecl2006.htm ). Note the Mare on the dark side of the moon, this was extracted with minimal processing. These web versions are compressed and of lower resolution.

Many refractors advertised at Apochromatic are not really as such (not just technically but also in quality). They should be called semi-apochromats. However, most are still of very good quality. They are far better than achromats, all else being equal.

John

Its horses for courses I suppose.

Mind you, a 90mm Maksutov scope even with a massive 25% (of diameter) central obstruction has the contrast performance of a 67.5mm refractor.
Still superior to a 66mm APO refractor!
My wallet would definitely rule here!
Cape Verde looks nice!

Peter.

Many refractors advertised at Apochromatic are not really as such (not just technically but also in quality). They should be called semi-apochromats. However, most are still of very good quality. They are far better than achromats, all else being equal.

John

That's a good point John. Unfortunately, the exact performance definition of an apo is a little loose and it's not always clear how one apo is better than another. Not many companies publish their chromatic aberration graphs - this would help customers make direct comparisons.
Personally, if I were looking for an apo for astrophotography, I would look for a triplet lens and also try and get the chromatic aberration charts. William Optics for example are pretty good at publishing theirs. Some doublets are also very good of course.

Regards,

Its a case of the right tool for the right job.

If you want a widefield imaging scope, then a small refractor is a good choice, a Mak Cas is not suited to this as its small secondary will cause severe vignetting. Its also a case of size and manageability. A Mak Cas has a long focal length, good for high power planetary viewing.
i have a 4" Achromatic refractor, it has some false image, but not too severe, its what I used for the solar eclipse last year.
www.flickr.com/photos/61288385@N00/sets/72157594161261180/
In terms of bringing a scope abroad its just about the only one I could do it with. I'm bringing it to Russia for 2008, presuming I can get there.

As a general guide for scope types and size.
60mm - 150mm, then a refractor is manageable.
150-200mm, then a Mak Cass is the order of the day.
200- 400mm, get a SCT or Dob/Newtonian reflector
400mm+ get a Dob.

The long focal length requires 4 times the exposure time to get the same amount of photons to the CCD


Trev

i still have trouble understanding why this is so! i just accept it rather than understand it