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difference in telescopes.....

Started by smithsee2000, 10/26/2009 05:20PM
Posted 10/26/2009 05:20PM Opening Post
Other than length of the OTA, what is the difference between an 8" mirror (newtonian) and an 8" SCT. I understand the physical difference in the scopes themselves, but what is the visual or photographic advantage to one over the other? Can anyone help me with this? Is one better for photography? or for visual observing?
Posted 10/26/2009 05:59PM #1
Todd Smith said:

Other than length of the OTA, what is the difference between an 8" mirror (newtonian) and an 8" SCT. I understand the physical difference in the scopes themselves, but what is the visual or photographic advantage to one over the other? Can anyone help me with this? Is one better for photography? or for visual observing?

They all work and they all have their quirks.

Most Newtonians are dobsonians and better for visual than photography. They are typically f6 and have a central obstruction of about 20% (of diameter). All else being equal, this should give the Newtonian a wider maximum field of view, and higher "planetary contrast", a real but subtle characteristic.

SCT, being shorter, fit on a GEM more easily and are therefore more suitable for imaging (in this way). they are typically f10 (almost any eyepiece will work well) and are more compact for transportation. Their central obstruction runs about 35%.

For simple visual use the dob 6 to about 15 inch has a lot of "bang for the buck," to use Eisenhower's classic phrase.

Taste has a lot to do with it, and star parties are a great way to discover one's own taste.
Posted 10/26/2009 06:36PM #2
In addition to Lorne's answer, (and leaving aside the physical description because you say you are familiar with them) SCT's have an advantage over many newts in imaging because:

They are physically smaller for inch of aperture. This means they are easier to mount, and a mount is very important in imaging.

Their focusing system allows a wide range of focusing positions. (You have lots of infocus and outfocus.) This means you can hang all sorts of fancy equipment off the back end (like filter wheels, adaptive optics, special focusers, and whatever) in addition to a camera!

Their construction generally means you can balance everything a lot easier. This again allows easier mounting.

They do not leave stars with diffraction spikes, which many imagers find distracting.

They have some disadvantages over newts:

They are generally more expensive.

Their contrast is not inherently as high as the typical Newt. (Although this can be debated, and can change depending on the size of the secondary.)

For higher magnification, the SCT's will offer more focal length. Of course, you are trading off field of view for this.

If you are going strictly visual, a newt is probably a more popular choice. You can get a lot more aperture for a lot less money. You do not need fancy electronics, etc.

If you are going photographic, the most important choice is the mount. For a tube, consider a refractor as probably the best way to start (and you will always want a handy sized refractor around), and then an SCT with a F6.3 corrector. The newt will come in third in the long run. (Unless you have a very good mount, in which case the Newt can do you proud.)

Whatever you do, don't buy anything until you have hung around for a while.

Alex