SCT Design: Meade vs Celestron

[casstony]

Not sure if it's cool to pinch whole posts from other forums, but this info from Roland Christen is very informative:

There is a real difference between these two optical systems. Although both are SCTs, the Meade has an aplanic coma-free design, whereas the Celestron is an all-spherical mirror design similar in performance to a Dall-Kirkham along with characteristic off-axis coma.

What does this mean to you the average telescope user? Well, having a coma-free design puts other restraints on the optics. The corrector of the Meade ACF is twice as strong as the Celestron, resulting in more sphero-chromatism on axis. The result is lower performance on-axis but better off-axis images for the Meade. The Celestron all-spherical design theoretically would have better on-axis performance and worse off-axis.

Can you see the difference normally? Visually there is little benefit to the off-axis coma-correction of the Meade. Only when using it for CCD imaging with big chips would you see the benefit to the wider corrected field of the ACF. Stars are sharp over a wide CCD field, whereas the celestron shows quite comatic stars not too far off-axis. One can tame these somewhat with certain telecompressor accessories, but not fully supress them.

There are other considerations besides this if you are primarily a visual user. An 11" scope has more light grasp than a 10". Mechanically there are differences also. Mirror shift, collimation shift, focus shift as you move the scope around the sky can put a real damper on your observing pleasure (these are real problems and all these scopes have them to some degree - minor in some, major in others). The Celestron design certainly is fussy with collimation, but the ACF design with its hyperbolic secondary is many times more sensitive to miscollimation. You should check these scopes out before you buy.

[astropolak]

Thought I should post here as I absolutely love SCT's.
In my opinion Meade has a clear advantage with the ACF and I can confirm that my 10" LX200R works way better than standard SCT, this is why I have upgraded from LX90. The Meade also has a mirror lock missing in Celestron design.
I would disagree that SCT coma is only an issue with astro photography, I find it really annoying in visual observing, the LX200R does address this problem very well indeed.
The collimation of my LX200R is no more tricky than standard SCT and it holds collimation very well. The collimation is actually easier to perform on Meade ACF as star testing can be performed off centre - standard SCT would introduce coma off axis.
My 8" LX90 was better in terms of mirror slump - it had none (not noticeable in visual observing), LX200R does have some, this is probably due to the increased weight of 10" mirror as opposed to the 8".

One has to also think about local support for the products and from that standpoint I am definitely staying with Meade.

Joe

[g__day]

I like Celestron's Carbon Fibre tubes - means focus once set almost never has to be reset due to temperature variations.

[Gama]

From last comparison i read, the case is carbon fibre only, but everything else inside (Chasis) that holds ALL your components are metal, and thus does nothing to stop temp from changing focus.

I managed to find also from ATSCOPE, were they sell the RCOS scopes, a line about the same thing :

"Note: some SCT manufacturers are now using carbon fibre outer tubes, but still fully mount and move their mirrors along an aluminium baffle tube, totally negating any benefits of a carbon composite tube assembly."

Theo.

[casstony]

Thanks for your post Joe. Reconciling your experience with the first post I'm thinking it's no more difficult to collimate the ACF design, but miscollimation will do more damage to the image.

Theo, I doubt that statement by ATSCOPE. The primary is near the rear end of the tube and connected to the rear end by the focusing rod, which doesn't negate the fact that the structure separating the mirrors is carbon fibre. It's just a shame Celestron didn't design in a couple of filtered vents to let the heat out.

[Gama]

Its not just ATSCOPE, its been mentioned numerous times on other forums.
This is why the RCX was classified as the first one with no more re focusing needed for the mass produced SCT's.

Theres more info on it on the web, just need to dig to find it.

Theo.

[g__day]

Theo,

With a Carbon fibre tube body the distance between the front corrector plate and rear mirror cell is effectively temperature invariant. Given expansion / contraction of a material is proportional to its length, and that this it the longest part of the OTA - a carbon fibre body addresses the crux of the problem.

The focusing rod holds the mirror - I presume some 10mm - 150mm from the rear of the tube so its only expansion / contraction of that (and the read end of what is hanging off the OTA) that can affect focus.

As I mentioned 4" vs 24" of material that can change size is alot smaller problem to deal with. Using CCD Inspector to check focus barely chnages over the weeks!

[gregbradley]

I agree.

The idea that the carbon fibre tube is wasted on an SCT is false data.

I had a Celestron Nexstar 11GPS with carbon fibre tube. It was a wonderful scope with fantastic visual views.

Greg.

[coldspace]

I have been using my 12 inch LX200R for the past 12 months and love it. The images are sweet and for visual use they are crisp and sharp all the way out to the edge. Looking at Fred's ( Bassnut) latest work in the deep sky section makes me feel I chose the perfect scope for the money to use as an all rounder.

Matt.

[AlexN]

Good info in here for a prospective SCT buyer... seems both designs have their merits...

[rally]

Slightly OT but not totally

So how does a true Dall Kirkham compare against these two ?
I am assuming slightly easier to collimate.
Better for a large CCD ? . . . .

Rally

[Bassnut]

The Celestron has a carbon tube, less weight and less focus drift, but the stupid mirror flop. Meade LX, better optics and no mirror flop with the focus lock. If your mount can handle the extra weight, and you refocus now and then, Meade is the go ("RC" optics, and isnt an equivalent Celestron more expensive?). BTW, I dont bother with regular refocusing and I image without meridian flip (without an ME, you have to be joking). As Roland states, coma on the ACF is better, the rest is carefull attention (eg collimation, not that ive checked mine recently)

Megadata my friend, thats the key, regardless of OTA.

[gregbradley]

The corrected Dall Kirkhams seem all the rage now with several on the market.

There's Planewave Instruments who make a 12 inch (F9), 17 inch (F6.8) and 20 inch at low prices (relatively that is).

There's Orion Optics UK who make corrected Dall Kirkhams up to 16 inch I think but not in the hands of users yet but probably a nice scope (F6.8).

Then there's the Ceravolo Astrograph which 300mm and both F4.9 or F9 but you need a corrector for each F ratio and they are expensive.

A and M Astrotech also make a beautiful tubed one as well as a gorgeous Mak Cass with Carbon Fibre tube. They are somewhat expensive.

Mewlons are really more visual instruments and I don't think they have a real large corrected circle for imaging but there are some OK images around from them.

There are also some RC scopes coming out that are cheap like the DSI 10 inch and the Astrotech 6, 8 and 10 inch.

There is also an RCOS 10 inch Astrograph at F7.0.

As Fred says it boils down to megadata as in long hours of exposure and I would add fast F ratio (though that tends to make you more widefield unless you use a small chipped camera with high QE).

Greg.