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Reflectors and Catadioptrics


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I haven't spent a lot of time looking through catadioptic scopes, although I have recently bought a small (65mm) Mak, as much for terrestrial use as for astronomy. I notice that the FOV of the Mak seems to be pretty small, but I am wondering, as I think about upgrading over the next couple of years, whether I would be just as happy with an SCT as with a reflector (EQ or Dob) of the same size? my next scope should be 8 to 10".

Can anyone with more experience with these types of scopes give me a rundown of the advantages of one over the other?

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SCT's are generalllllly used for high magnification as they tend to be f/10. Hence the C8 has a 2000mm focal length. This is ideal for planetary observing as high magnifications are easily reached however for DSO work the larger DSO's can only be viewed by using very long focal length eyepieces, 32mm and higher sometimes. Alternativly a focal reducer can be used. Also and Cat scope has a larger central obstruction than an eqivilent newt. SCT's run around 35% CO. This means a great loss of contrast in the image. For DSO work this isn't always an issue, but for pin point stars and planetary work it's a right pain in the backside. I had a C8 and changed for a 4" APO and never looked back. Also dont forget that SCTs take a LONG time to cool, as much as 2 hours in the UK climate, and i dont think canada is much different.

The advantages of the cats though is there portability. They are small optical tubes. MUCH smaller than a newt of similar aperture. Even the C11 can be easily handled as long as your fit. The C14 can be a struggle for one person though. A 14" newt would be huge in comparison. A C8 is very easily moved around and about 5kg/10lb's ish. A small mount will take it happily. Another advantage of SCT's is both celestron and meade sell them on computerised fork mounts which are light and have goto. Not ideal for photography but for visual excellent platforms. Something like the 8i or the new 8SE is a scope for life for the visual observer. That is until you look through an APO.....

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Is noone going to mention Maks? :D

(Cut and pasted from an earlier post):

Advantages of a Maksutov Cassegrain are:

Incredibly sharp images that rival some of the best Refractors.

Convenient eyepiece position (thanks to the diagonal).

The slow focal ratio produces high magnification and allows the use of longer focal length eyepieces (better eye-relief).

The front correcting lens, small central obstruction and lack of spider vanes produce high contrast.

No need to re-collimate.

Sealed tube so mirror stays clean and bright.

Disadvantages:

Expensive (though considerably cheaper than an equivalent refractor)

Slow f-ratios mean longer exposures and narrow field of coverage

They tend to have longer cool-down times.

_______________________________________________________

Compared specifically to an SCT, the Mak has a smaller central obstruction and so greater contrast (assuming everything else is equal).

Now, for the unspoken truth:

The Schmidt Cassegrain corrector lens is a better design than the Maksutov. But, and its a big but, the Maksutov is much easier to manufacture, particularly at smaller apertures (look at them in cross-section and it will be clear why). Consequently, at apertures up to about 6" the Maksutov can be made to a higher quality (cost for cost). At over 6" the Schmidt's superiority, as a corrector lens, becomes apparent and affordable.

In summary:

Up to 6" - the Mak' should win.

Over 6" - buy a Schmidt, but expect less contrast.

Hope that helps,

Steve :D

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The other thing against "big Maks" is the size and weight of the front lens when you get over 7-8", they are massive (and expensive) pieces of glass.

Quite right, hence the long cool-down times.

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I can see the logic but....what do you reckon? :D

Interesting,

He says:

"...the corrector/meniscus has absolutely nothing to do with long cooldown time in a Mak-Cass. That's just a myth."

I'd have to disagree. The thicker the glass, the longer its stored heat will take to dissipate (a half-full cup of tea cools quicker than a full cup).

"If you think about it, a piece of clear glass at the objective end of the scope exposed to the night sky is similar to a refractor. My 6" refractor has thicker glass at the front than my 10" Mak-Cass yet it cools very quickly."

The refractors lens has thin edges.

"The thermal problems in Mak-Cass scopes lie in other areas, including the primary and tube."

I feel sure they all play a part...

After saying that, he is clearly an intelligent man; I am only offering an opinion and wouldn't wish to offend :D

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I am wondering, as I think about upgrading over the next couple of years, whether I would be just as happy with an SCT as with a reflector (EQ or Dob) of the same size? my next scope should be 8 to 10".

Can anyone with more experience with these types of scopes give me a rundown of the advantages of one over the other?

Sorry WH,

We seem to have hijacked the thread :?

I think Gordons response covered all the main points.

Convenience aside, I'd suggest the Newtonian for low/med power observing and the SCT for med/high power.

Thats all really :D

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WH. Depends how many scopes you want! I have a fork mounted 8" SCT and an ED80 on a super polaris and there is just no competition betweem the 2. Not wanting to get stamped on but IMHO the SCT is a massively superior scope in just about every respect. I can lug it out into the garden with it attached to the tripod. Go in to get my tea and it is cooled when I come out again. Fork mounted scopes are a pleasure to use compared to GEMs. There is no ignoring aperture for picking out detail. DSO views are vastly superior through the SCT than the ED80. It also out performs the 100mm apos I have looked through on DSOs in terms of visible detail.

There is some loss of contrast on planets but nothing like as much as you might be led to expect. The FOV is small, however an F10 scope works very well with budget EPs and the Moonfish 32mm EWA EP works superbly well. You can also buy an F6.6 F/R which I haven't tried.

Fork mounted scopes in alt-az aren't ideal for astroimaging because of field rotation - unless using fastar (most SCTs aren't compatible with that).

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Sounds like you got a good C8 Martin, but the QA isn't that consistant and some C8's are dire. The C9.25 is a much more consistantly good performer. If you can get a good C8 (and that may mean swapping a few times) then i agree, it's an excellent scope. And fork mounted is easy to handle if you arn't into astrophtotography in a big way. The new 8SE's are probably very good, and i've got my eye on them.

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Sounds like you got a good C8 Martin,

:D :D The nextar GPS range are becoming classics and it does seem they were made to a higher standard than the subsequent CPCs. Kevlar tube, better gears and ?optical quality/consistency. Just ask Roger!

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usually true, but the new celestron one arm forks have dovetails. I'm sure other fork mounts will follow and you'll see meade forks with dovetails soon enough etc.

Actually the SE one arm fork looks a nice mount. Stronger than the SLT and nearly as portable. Just a shame you have to buy it with a OTA and the cheapest it comes in at is nearly a grand.

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I may settle on an 8" or 9.25" SCT on some sort of goto mount. I am thinking of storage space, and my diminishing ability to carry heavy weights. If I were to get a scope on a HEQ5, I would be able to switch OTAs if I wanted to, but I may settle on one scope, finally. I am trying to talk myself back into the idea of getting an SCT. I don't mind about the fork mount, if that's what I end up with, as I am not into astrophotography in any serious sense.

Actually, I hurt my back carrying my EQ3-2 mount a while ago, so a HEQ5 might be a problem. :scratch: We'll see. I appreciate the time you all took to comment on this. It's been a great help.

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An 8" SCT is a great all rounder WH if you can get a good one. Take a look at this:

http://www.celestron.com/c2/product.php?CatID=13&ProdID=416

The SE mounts have a dovetail so if you want to change your scope you can can and keep the mount. Something like an 8" SCT and a little 80mm APO would be an ideal combination perhaps. The 80mm APO for quick peeks and the 8" SCT for when you have the time to allow it to cool properly. Both use the same GOTO mount and it's quite light, the mount is 9lb's and the whole shebang with the 8" SCT is only 24lbs.

I think that when i get around to having ANOTHER change around in scopes and mounts i'll go the same route. Although i know from experience that a 4" APO gives better views than an 8" SCT, however the SCT gives more resolution being a larger aperture albeit with a contrast destroying 35% central obstruction.

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As we are discussing SCTs v Newtonians and, we are expressing opinions:

I have been very pleased with my combination of a 10" Dobsonian for low/medium power, wide-field views of DSOs and a 4" catadioptric for high power views of solar/lunar objects. Throw in a short-tube refractor for Grab & Go views and .... well, it works for me :D

If I had spare cash, I would upgrade the 10" Dob to a 16" truss Dob, the 4" cat to a 5 or 6" (Celestron's new 6" SCT is whispering seductively!) and the semi-APO refractor to a full APO... I can dream!

One more thing: I have enjoyed wonderful views from a number of Meade 8" SCTs. The OTA has appeared on several mounts over the years; it has to be one of Meade's most popular and successful SCTs. Odd that it doesn't get more recognition here...

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i'm sorry but that is just not true for scopes manufactured nowadays and has not been true for many years. I can honestly say i cannot remember the last time a C8 was returned for any optical QC issues. Independant magazine reviews back up Celestron consistent optics.

I can second that,

Over the last two years, I have never had an SCT/Mak returned with an optical fault.

Mount/GOTO - yes!

Optical - No :D

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Well, i guess for new scopes that might be true, but I for one buy most of my kit second hand to try and keep the costs down. I've never owned a brand new C8 and am not likely to so can't comment. I bow to your superior experience(s). I can only go on what i've experienced with the scopes i've owned and the experiences i've read others comment about.

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