reflector vs sct

[smenkhare]

which would be better for visual observing and imaging?

an 8" reflector or 8" SCT both on an EQ mount?

mainly for planets and dso's

[Terry B]

Originally Posted by smenkhare

which would be better for visual observing and imaging?

an 8" reflector or 8" SCT both on an EQ mount?

mainly for planets and dso's

What sort of reflector do you mean?
A SCT is also a reflector.
What do you wan't to look at or image more?
Galaxies (deep sky) or planets. The 2 are difficult to do with the same scope.
Imaging deep sky requires a different scope to visual (mostly)

[mental4astro (Alexander)]

Hi all,

As Terry says, a newtonian & a schmidt-cassegrain are different beasts, with respects to their 'traditional' focal ratios.

Typically, newtonians would have an upper focal ratio of around f/8. The lower ratio for an SCT would be f/6.3, there by implying an overlap.

Each optical system offers distinct pros and cons, as would be expected. I'll just give an overview of these technicalities and leave it to you to choose your weapon.

* The secondary obstruction of a newtonian is smaller percentage wise than that of an SCT, giving better contrast.

* The slower focal ratio of the SCT gives better views of the planets, and the prime focus image of the planets is likewise larger for the same apeture.

* The faster newt provides brighter images visually and reduced exposure times, both for DSO's.

* No spider in an SCT means no diffraction spikes around bright objects in images taken by an SCT.

If you are trying to decide on a particular set-up, you need to make a judgement call for yourself on the balance of optical properties. No single scope is a 'one-size-fits-all'. Off cause there are optical aids, like barlow lenses and focal reducers, but these add to the cost and put more glass between you and the universe.

I own scopes of both types. Though I don't do imaging, visually they each offer unique properties. Choice of which scope touches the night air depends on the observing plan for the night.

Physically they are also different, but that is not your question.

[Starkler (Geoff)]

8 inch scts dont offer the option of using 2 inch eyepiece do they?

I prefer a newtonian for its greater flexibility wrt usable magnifications, generally better optics and lesser thermal and dew issues. On the plus side for the sct is shorter physical length taxing the mount stability less.

Actually given that you want to use an eq mount, get the sct as spinning the whole newt tube in its rings with each target change becomes old very quickly.

[casstony]

Originally Posted by Starkler

8 inch scts dont offer the option of using 2 inch eyepiece do they?
.

You can use 2" diagonal and eyepieces, but the field stop on a 35mm Panoptic is as wide as you can go without vignetting from the hole in the rear of the 8" sct.

[smenkhare]

the short length is pretty much the whole reason i was looking at sct as opposed to the newtonian. i'm less interested in the moon and more interested in planets and nebulaes

[csb (Craig)]

Do you know about cooling down time, smenkhare?

Both types of telescopes need to be allowed to cool to equilibrium because thermal currents in the tube deteriorate the image (I think the actual mirror needs to reach same temp as outside air). Although you can still use the scopes as soon as you take it outside.

You need to set the scope outside for a while before getting a good image, especially deepsky.

Which cools quicker Newt or SCT?

[ngcles]

Hi smenkhare & All,

As others have already noted the biggest difference between the two is the native focal length of 2000mm for the standard commercial SCTs (Meade, Celestron) and the typical fl of your average commercial dob -- at f/6 is 1200mm.

They both gather almost exactly the same amount of light -- the SCT is a couple of percentage points behind becasue of the larger secondary -- but it's hardly worth mentioning.

When used visually at the same magnification, the image will be equally bright in both.

For visual use you will be able to achive a wider field more easily with the newtonian because of the shorter fl, and the flip-side is that with the SCT it will achieve higher magnifications with longer fl eyepieces which usually have wider eye-lenses and longer eye-relief that are more comfortable to use.

Because the SCT has a much larger central obstruction, it is not as visually contrasty as the newtonian but in practice, on 19/20 nights you won't see a difference when the seeing is either poor, average, good or even very good. On the 1/20 nights the seeing is excellent, an experienced observer will see more planetary detail more easily with the newtonian.

The two other considerations are the price per inch -- the dobonian/newtonian will be much, much cheaper than the SCT.

On the other hand, the SCT is much more compact, easier to store and is more compact for transport. Both will require collimation from time to time but the newtonian more often. On the flip side, it is easier to collimate a newtonian than an SCT.

If you are contemplating imaging in the future, both can be good imaging 'scopes -- there are several designs better suited purely to imaging but really are specialist imaging optical designs.

If you are interested in imaging down the track, you will need a really good mount, but the SCT because it is more compact, a little lighter and where the camera position is places fewer stressors on the mount and you can get away with a slightly "lighter" mount. On the flip side, for deep sky imaging because it has so much fl, the SCT will be a much steeper and more difficult learning curve and require much longer exposures at f/10 with more chance of stuff-ups. You can buy a focal reducer to give f/6.3. However if you decide to do your planetary imaging with video, the long fl won't hurt so much and in terms of image scale will be a little easier to use -- for planetary imaging that is.

The newtonian at 1200mm and f/6 will be much shorter exposures for deep sky stuff and more forgiving for the learner and provide a wider field -- but you should use a coma-corrector. For video/planetary imaging you'll need a barlow or powermate (or several) to get good image scale on such a small target.

There is no right or wrong. There is only what suits you and your budget.


Best,

Les D

[smenkhare]

thanks for the detail reply.
this is the one i was looking at

http://yorkoptical.com.au/ProductMen...ee3%2cUnitCost

it is cheaper than a lot of smaller telescopes, is this something to be concerned about?

[csb (Craig)]

Andrews Communications have the same package for $2400

http://www.andrewscom.com.au/site-section-10.htm

[ngcles]

Hi smenkhare & All,

Originally Posted by smenkhare

thanks for the detail reply.
this is the one i was looking at

http://yorkoptical.com.au/ProductMen...ee3%2cUnitCost

it is cheaper than a lot of smaller telescopes, is this something to be concerned about?

No. Those smaller "more expensive" telescopes you are probably talking about are apochromatic refractors that are wonderful instruments but very expensive per cm of aperture.

It is a good idea to shop around to get the best combination of price and after sales service that suits your needs. I'm not particularly savvy when it comes to pricing of SCTs but as an alternative, you could buy a Synta EQ6 mount plus an 8" or even 10" Newtonian OTA for that mount for pretty much the same price as the one you are looking at. Neither (SCT or Newt) could be described as "high-end" imaging scopes but are both pretty decent all the same.

In the end as to which route you go down, it will depend largely on the factors I've already outlined as they touch on focal length. If you are going to do imaging as a beginner, you may well find the longer native fl of the SCT will make the learning curve very steep and difficult -- but you can also buy a focal reducer for about $2-300 that could help with that but also introduces other "difficulties".

Even at 1000-1200mm fl (like an 8" f/5 or f/6) it will be demanding. Many will advise beginning imagers to try a truly short fl to start with (like say and ED80 telescope on an EQ6 for example that has about 700mm-odd) to help you learn the ropes and get the basics nailed down before progressing to other 'scopes with narrower fields and longer focal lengths when the exposures will be longer and the "error bar" involved in taking a good image much, much smaller and "twitchier".

As for purely visual use, see my previous post.

Good luck with your choice. Before you outlay, talk to as many different people as possible in the hope of getting a good variety of opinion and then work out what is best for you.


Best,

Les D

[OneOfOne (Trevor)]

One point to keep in mind with either scope on an EQ mount is that with an SCT, the eyepiece will usually be in a fairly comfortable position, except when pointed close to the zenith (straight up). In these cases, they are litterally a pain in the neck. You can also use a diagonal to move the eyepiece to a better position.

The Newtonian however can have the eyepiece at very uncomfortable locations...you may have to lean right over the scope in some positions or sometimes maybe even get on your hands and knees. Other times it is in a very comfortable position. You can't use a diagonal to move the light into a better position.

[smenkhare]

hey everyone,
I've decided to get the SCT scope from york.
I know it's more expensive but they are quite literally next door to my work so much more convenient.