What is so fascinating about 80mm APO refractors? In recent years there has been a strong trend for buying 80mm refractors of varying apochromatic-ness. Personally I think it is getting a bit boring...

Just about every single telescope manufacturer seems to produce an 80mm APO Refractor. It seems that everybody wants to own one. I sometimes sit and laugh watching people constantly buying a refractor, selling it, buying another one.. always the same type of scope. For about 350 quid you can get yourself an 80mm instrument, usually between about F6 and F7. On the other hand, I can go to TMB optical and buy a 80mm F6 apo refractor for about 1500. Methinks it is the high quality of the premium product coupled with the inherent "sexiness" and attractiveness of a small telescope that seduces people into buying an inexpensive medium-slow 80mm APO telescope.

If course, they don't go much faster than F6. Getting good colour correction and a flat field faster than F6 is tricky. So usually you need a very expensive reducer to make the scope work from an astrophotography point of view.

Of course, the device you are buying into is 400-500mm of astrophotography instrument. Which brings me onto my favourite grudge against these instruments. They are inherently easier to image with because, at the pathetic image scale your using, telescope tracking and seeing becomes much less of an issue. OK, so they are going for beginners you might say, but it seems that EVERYBODY is getting seduced into medium wide field photography. Most people will tell you that a lot of deep sky targets ARE quite big and do need a large field of view. True, but there are many more higher resolution images to take.

How many 80mm APO images of the North American Nebula do we have to sit through every summer? And they all look the same!! They are BORING scopes. Everybody seems to have one, producing the same images all the bloody time! When high resolution images get posted to internet forums, it is the exception, not the norm.

And is it really that good for a beginner? A very popular scope everyone talks about is the ED80. The ED80 is a whopping F7.5, why would anybody, especially a beginner, want to do DSO imaging with an F7.5 instrument? As you should know (although telescope salesmen sometimes like us to forget) the higher the f-ratio, the less light per unit area reaches the image plane of the system. An F7.5 ED80 requires DOUBLE the exposure time of the same aperture at F5. So you have to get quite some exposure time to get a nice glassy smooth image. This is part of the popularity of things like the Hyperstar (putting a camera in place of a secondary on an SCT) and some of the ultra-fast astro-graphs appearing these days. You hear about people working at F 2 or F3 - that is the way to image! Mind you, such scopes have other issues. There is no perfect scope.

The final thing that winds me up about cheap refractor images are the stars. Most of the images I see from cheap 80mm refractors seem to have very bloated bright stars to my eye. Most unsightly.

Some imagers do amazing work with small refractors - but they are prepared to do a proper job with long integrations and taking care to remove gradients and so forth to produce classy images, however, if I see another North American Nebula image I AM GOING TO SCREAM

Just for the record, I don't have an 80mm APO, I do have a ST80 for guiding, which I don't need so much now with the OAG unit. I did nearly get seduced into a medium-expensive 80mm APO myself, but I've resisted so far. :-)