Could somebody please tell me, what is so great about a F10 SCT telescope? I think large schmidt cassegrain telescopes, known as SCT telescopes, are a poor choice for the beginnner in astrophotography. Examples include the meade LX200 and the Cesestron C11 Here are the reasons behind my thinking...

One of the most popular varieties of telescope in the modern era is the F10 SCT. These are commonly found in 8, 10 and 12 inch variations. Some are even 14" or 16".

It is still a mystery to me why people buy these scopes.

I think it all started with the adverts in the leading Astronomy magazines. In our youth many of us poured over the glossy adverts for a Meade LX200. How great would it be to visit 50,000 different celestial objects in one night?! If only we had the 2999 required to buy one.

Then we all went to university, met women, got jobs and generally lived off beans on toast for half a dozen years. Coming out the other side we suddenly find we have credit cards and disposable income.. and what better way to spend it then on the machine of our dreams from the back page of a ten year old copy of Sky and Telescope?

This might not be exactly the case for everybody... but at the same time I am hoping some people spot the grain of truth in that. Once a newcomer to a hobby sets his or her heart on a particular purchase, it is difficult to deflect them with sound advice.

"It's a great visual instrument"

This is the usual argument. Now, I don't do much visual. In my mind a "great visual instrument" is a 20 inch dob. Preferably one with a with motors to keep the thing pointing in the same direction. Why a 14inch SCT should be so much better than a f4.5 14inch dob beats me.

The F10 SCT is responsible for more beginners dropping out of astrophotography than anything else.

Why?

The reason is quite simple. Any astrophotographic system normally requires two different bits of optics. It needs a primary optic... usually a big mirror or a big APO lens, and it needs a corrector, flattener or reducer type thingy which goes up the camera end at tries to correct for the drawbacks of the chosen primary optic. A refractor normally needs some kind of focal reduction and field flattening, especially for big chips. A Newtonian requires a coma corrector and a thumping great SCT needs a thumping big focal reducer. There are few scopes that take great photos without the bit of glass up the camera end to sort everything out.

Now, when you are new to astrophotography, you generally don't know about the second bit of glass. You assume that the optics will take great photos out of the box. It works fine with an eyepiece, so it should work fine with a camera? Right?

Most of these people who rushed out and purchased the giant SCT of their dreams eventually decide to try some astrophotography. Normally this starts with either the family DSLR or a small chipped entry level camera like the ATIK IC or DSI. At this point the first major drawback of the SCT become apparent. Your ATIK IC running on a F10 10inch SCT with a focal length of 2500mm yields a field of view of 4 arc minutes by 3 arc minutes. Not a lot. And then they wonder why the entire Orion Nebula doesn't show.

This is quickly followed by the second problem. F10. At F10 you gather light on the imaging plane FOUR TIMES more slowly than with an F5 instrument. Ah ha you think - I'll get a reducer! So you spend more money on a reducer and get the bugger down to F6 or so. And then you find the problems with cheap reducers. Focal reducers are very difficult to make. The are expensive. They are very difficult to use as well, and the camera to reducer spacings must be correct.

There is a need for slower focal ratio scopes - but only when your pixel sizes increase. Most entry level camera are in the 6um pixel size range and therefore not suited to imaging at F10.

A F10 10incher running at F6.3 is still a medium speed scope and your ATIK IC is still only going to give 10 arc minutes field of view. Trying to get that sucker to track (or dare we venture autoguide) at that image scale is going to be difficult for the beginner who doesn't know anything about tracking or autoguiding or periodic error. Many of us have forgotten, but most newcomers to astrophotography don't know anything about the difficulty of tracking stars. They just think you slap the camera on the telescope, and it will just work... I remember that I did. I was most disgruntled to learn of the existance of periodic error.

SCTs have these huge plates of glass at the front of the tube corrector plates. Dew delights in settling on this large plate. The longer you are out at night, huffing and puffing around your telescope, the more misted up the corrector plate becomes. The corrector plate is pointing up at the sky, and is doing so at least a foot above your head. You don't notice the slow buildup of water on the front of the SCT corrector plate. How many frustrated beginners out there have got frustrated to the point of packing everything away, only to learn that the front of the scope is completely opaque with condensation? And have you seen how much kendrick charge for their anti-dew systems for telescopes?

And the last one: Wedges. Equatorial Wedges. The delightful SCT usually comes on a jolly fork mount. A jolly ALT-AZIMUTH fork mount. In short, a crap mount for astrophotography. To take long exposure photos with any degree of success your telescope needs to be equatorially mounted. Another set of problems for the beginner to learn about and spend more money trying to solve. Which leads us up another alley - most commercially available wedges of SCT scope are at best poorly constructed and overprice, and at worst downright dangerous. They are comprehensively unsuited to astrophotography unless you spend a great deal of money, or find some of the more niche manufactures.

"It is a great planetary imaging scope"

This is the other one you hear all the time. Long focal length telescopes are fantastic for planetary imaging. The great Damian Peach uses one of the ultimate large SCT scopes: The vast Celestron C14.

There is a problem with planetary imaging. Of the eight planets in your solar system, one of them we stand on. Of the other seven, only 3 of them show very much in the way of surface markings. Of those 3, Mars is a once-every-two-years event, and jupiter and Saturn are either rapidly heading into, or lurking around in some of the least favourable parts of the sky for UK imaging. Most of the upcoming Mars apparitions are fairly poor.

It is a bad time for planetary imaging in the UK. Gone are the heady days of the early part of this decade when Saturn and Jupiter were riding high in the wintertime skies and Mars was making headline rattling appearances. We'd all just learn how to use registax and a toucam. The golden years of planetary imaging. These times will not return to the UK for at least 6 years. It is a bad time to buy a dedicated planetary scope.

So there we have it. A huge focal length and its attendant tracking problems and a focal reducer of dubious quality and operating at medium to slow focal ratios. Add a misted up corrector plate and a equatorial wedge made from rubber metal and it all adds up to a difficult imaging proposition for a beginner.

The F10 SCT is responsible for more people dropping out of astrophotography than anything else.

When is such an instrument good for astrophotography? Take the new RCX things from Meade. put one of these on a great mount and put one of the modern breed of large format, large pixel cameras on the back and put an experienced imager in charge, and you'll be getting some cracking images. Assuming you can afford the 50mm filters.

So there we have my two main scope dislikes. SCTs because they are too slow and have too much focal length and shouldn't be sold to beginners in astrohpotography. 80mm apos because everybody and their dog has one and they are getting a bit tedious.