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Refractor vs. Mak?

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#1 cpsTN

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Posted 20 May 2011 - 03:41 PM

Almost all of my observing experience in the last 25 years has been with Newts. I have briefly owned a 90mm Mak and was wondering if you would choose between a long focus achro refractor or a Mak of the same aperture as far as Lunar and Planetary observing is concerned. What I have in mind is between getting a 90mm f/10-11 achro refractor or a 90mm f/13.9 Mak. Price, mounting, and photography are not concerns here, only quality of Lunar and Planetary views.

#2 BobinKy

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Posted 20 May 2011 - 04:22 PM

Charles...

Do you have any models in mind?

#3 cpsTN

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Posted 20 May 2011 - 04:57 PM

I was speaking generally but we can talk about Orion's 90mm f/10 achro and Orion's Apex 90mm f/13.9 Mak, or we can compare Sky Watchers or whatever. It is really just general question of similiar quality instruments.

#4 KWB

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Posted 20 May 2011 - 05:48 PM

For only planetary,Lunar and double star splitting,make mine the 90mm Mak. No CA to deal with but when using a 90mm F/10 acro,there is.

A handier setup for me than using a long focal length refractor. Take it outside and let it set for about an hour before using.

#5 Gene7

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Posted 20 May 2011 - 05:55 PM

My experience is that you get "junk" views from folded mirror scopes unless you kick in a few extra thousand for a premium brand. No so with other scopes. Gene

#6 MtnGoat

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Posted 20 May 2011 - 06:01 PM

Mak...simply an amazing scope usually. No CA, long FL, and a short package which is solid on any mount. I was shocked how crisp the Meade DSX90 i picked up on CList is, and I love that little thing. It's my favorite small scope.

Just let it cool down and it is simply amazing. I've run it side by side with F15 90mm achromats which are identical FL or close, and the central obstruction issue doesn't appear to have any visual impact and what does is the perfect color correction and razor sharp focus. Sure, it's smallish aperture at 90mm...but then so is the same sized refractor.

Now what I want is a bigger one! Someday I want one of the 7" F15 bad boys...2700mm of planetary power!

Here's what the lil mak90 can do on a cheapo mount with a cheapo DSI. The limb is pretty crisp for the large pixels of the DSI's sensor and there is no filter of any kind, and zero CA.

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#7 KWB

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Posted 20 May 2011 - 06:01 PM

My experience is that you get "junk" views from folded mirror scopes unless you kick in a few extra thousand for a premium brand. No so with other scopes. Gene

I've owned a couple 90mm Maks and even though they were aperture limited,when the scopes cooled down,the views were far from "junk". The Moon is a pretty bright target and the CO of these scopes is a non-factor. Jupiter is another good target for this compound design.

Whatever floats your boat.

#8 cpsTN

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Posted 20 May 2011 - 06:27 PM

Thanks everyone. For a short time, I owned an Orion Apex 90 but did not have the mount for it so I went away from it. I would like to get the 102mm f/12.7!!! I will be building a tabletop or similar mount when I get whichever it is. Aside from the good things about these scopes that MtnGoat mentioned, they use interchangable diagonals. I will be getting a correct-image diagonal for terristral viewing too. We'll see after I do some thinking.

#9 pubquiz

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Posted 20 May 2011 - 06:50 PM

The 'junk' and 'folded mirror' comment above is just plain daft!

You don't have to spend 'thousands' to get a good Mak.

I have had an ETX 90 a 105 and a 125 and all are great OTA's ..the mounts are a different story:)

Also my C8 'folded mirror' gives excellent planetary views.

The views of the ETX OTA's is very crisp and refractor like and I am sure there are even better Mak's out there for reasonable money

On bright objects the CA of an achro is a definate disadvantage compared to a Mak ...and an equivalent APO (and yes I love my C100) is dearer.

Tom

#10 Darren Drake

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Posted 20 May 2011 - 06:52 PM

There is a lot of variation in the quality of the achromats. I have the Meade 395 (which I am trying to sell) and my 80mm WO apo definately beats it on lunar planetary. I would think a 90mm mak would about match the WO and beat out my achromat. But the Meade does have some spherical aberration so there is room for improvement there As long as the maks are collimated and since they involve spherical optical elements they are more reliably well made. So I would recommend the mak as your best bet.

#11 andromeda59

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Posted 20 May 2011 - 07:21 PM

My 125mm ETX puts up razor sharp images. No junk here...

#12 Gene7

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Posted 20 May 2011 - 09:39 PM

Yes, I know folks, there ARE good Cats and Maks, and I read the reviews and checked the posted Lunar photos before I bought my 127Omni and my AT6RC, it is just that I do not have any good ones. I am advising anyone who gets one to check it out well before they decide to keep it. I am not happy to read good reviews of an item and not be able to get an acceptable unit. Gene

#13 j3ffr0

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Posted 20 May 2011 - 11:14 PM

You could also consider a 127mm Mak which puts up very nice planetary views. That's getting a little bit big for a table top mount, but the resolution is better and it's worth considering.

In the 90mm range I would rather have the long focus achromat assuming it was a good sample. Longer than f10 would be better, but I'd still take the F10 over the 90mm Mak.

#14 KWB

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Posted 20 May 2011 - 11:27 PM

In the 90mm range I would rather have the long focus achromat assuming it was a good sample. Longer than f10 would be better, but I'd still take the F10 over the 90mm Mak.

Hello

I'm curious as to why you recommend the long acro over the Mak for mainly planetary and lunar viewing? I still own a 90mm F/10 acro. It gets used almost never.

#15 Jon Isaacs

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Posted 21 May 2011 - 07:47 AM

Hello

I'm curious as to why you recommend the long acro over the Mak for mainly planetary and lunar viewing? I still own a 90mm F/10 acro. It gets used almost never.



The advantages of the long focal length achromat are it's lack of a central obstruction and thermal stability. There is no getting around the effect of the CO and the need to set the scope out an hour or so early to get the good views make it less practical than the refractor. The refractor's brighter views are also helpful in these small apertures.

The ideal small achromat would be closer to F/15 than F/10 which makes for a large tube to deal with.

Given the cost of used 80mm apo's, I think they make the most sense. Relatively compact, thermally stable, no CO and no color fringing. I see used ED-80s on Astromart for under $300.

Jon

#16 94bamf

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Posted 21 May 2011 - 08:06 AM

Hello

I'm curious as to why you recommend the long acro over the Mak for mainly planetary and lunar viewing? I still own a 90mm F/10 acro. It gets used almost never.




Given the cost of used 80mm apo's, I think they make the most sense. Relatively compact, thermally stable, no CO and no color fringing. I see used ED-80s on Astromart for under $300.

Jon


I agree completely. Suprised it took so long for somebody to say it in this thread! :grin:

Ken

#17 Midnight Dan

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Posted 21 May 2011 - 08:28 AM

In the 90mm range I would rather have the long focus achromat assuming it was a good sample. Longer than f10 would be better, but I'd still take the F10 over the 90mm Mak.


I'd have to disagree here. For the OP's stated goal of lunar and planetary viewing, hi magnification is in order. The chromatic aberration seen in achromats will just look worse at higher mags.

Now, as Jon says, an 80mm APO or ED-semiapo would be a good contender if the price was right. But between a Mak 90 or an achromat for planetary, I'd go with the Mak.

-Dan

#18 j3ffr0

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Posted 21 May 2011 - 09:41 AM

Kenny,
Exactly what John said. The slower the scope the less CA - f12 would be better than f10. I think a decent sample of a 90mm Achro refractor will have better contrast than a 90mm Mak.

I'm glad someone mentioned the 80mm ED. Let's think of a few other possibilities. If I was going to spend the neighborhood $500 or less on a planetary scope, here's how I'd rank the choices (ymmv):

1) Used Orion/Skywatcher 100ED at F9 (These can be had for around $450 and really can't be beat for the price)
2) 127mm Mak (although it will take a long time to cool)
3) 80ED
4) Tie 90mm Achro or 102mm Mak
5) 90mm Mak

#19 helpwanted

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Posted 21 May 2011 - 12:46 PM

i'm glad to see someone mention the 100ED from Orion (j3ffr0 = Jeffro?).
this scope is not reviewed very ofter, but when it is, it gets great reviews. it must be a sleeper!

#20 KWB

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Posted 21 May 2011 - 06:11 PM

In the 90mm range I would rather have the long focus achromat assuming it was a good sample. Longer than f10 would be better, but I'd still take the F10 over the 90mm Mak.


I'd have to disagree here. For the OP's stated goal of lunar and planetary viewing, hi magnification is in order. The chromatic aberration seen in achromats will just look worse at higher mags.

Now, as Jon says, an 80mm APO or ED-semiapo would be a good contender if the price was right. But between a Mak 90 or an achromat for planetary, I'd go with the Mak.

-Dan

We are on the same sheet of music,Dan. My opinion was based on just those 2 scopes. No doubt in my mind about the ED80 being the superior alternative.

#21 payner

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Posted 21 May 2011 - 08:29 PM

If I was considering an MCT or f/12+ achromat of the same aperture I'd go with the former. No central obstruction, and better thermal stability (unless actively/passively cooled in most climates). The clear aperture of the refractor will be the better telescope on planets and double stars.

Best,
Randy

#22 KWB

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Posted 22 May 2011 - 01:37 AM

If I was considering an MCT or f/12+ achromat of the same aperture I'd go with the former. No central obstruction, and better thermal stability (unless actively/passively cooled in most climates). The clear aperture of the refractor will be the better telescope on planets and double stars.

Best,
Randy

Hello

Your post puzzles me. The "former" being the Mak?

A 90mm F/12 acro is a different animal,though I know of no such setup offhand that is presently for sale. A 90mm F/10 acro is the subject at hand. Much has been made of the central obstruction to Maksutov in this thread but IME it isn't a deal breaker when viewing Luna or Jupiter. For me the deal buster is chromatic abberation which the 90mm F/10 acro has in spades at 150X and above.

I'm really no 90mm Maksutov advocate but would choose it over this one that I own as to the thread title question. Even with all its warts as to cooldown and lack of overall versatility for my purposes. See the Synta ED80 for what I would recommend,but maybe that isn't what Charles wants to hear. My response was what I though the lesser of 2 evils.

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#23 payner

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Posted 22 May 2011 - 04:12 PM

Obviously I meant the latter. I stated a f/12 or slower due to CA. The comments on clear aperture remains, especially in the size of telescopes being discussed.
Best,
Randy

#24 Starman1

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Posted 22 May 2011 - 05:55 PM

Inch for inch, at long f/ratios the refractor wins the resolution and contrast battle.
But, add aperture or shorten the f/ratio and the Maksutov inches away.
An f/13 4" refractor will beat a 4" Mak at f/13, but barely.
An f/13 5" refractor will *barely* beat a 5" f/13 Mak, and good luck finding an f/13 5" refractor!
And f/13 6" refractor will be beaten by the f/13 6" Mak and you won't find any such refractor currently available.
It isn't fair to compare triplet refractors unless you compare them with comparably-priced Maks, and then the portability and mount requirements of the Mak still win out.

90mm is too small an aperture for much other than Moon, planets, and double stars, but I've looked through quite a few of these 90mm Maksutovs with upgraded star diagonals, and the images have been generally quite good--sharp, with no chromatic aberration.

I own one of the Synta 127mm versions, and the images, under good seeing, hold up to well past 60X/inch, which is a sign of good optics. It's worth the expensive eyepieces I use in it...and the upgraded star diagonal.

#25 KWB

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Posted 22 May 2011 - 09:44 PM

90mm is too small an aperture for much other than Moon, planets, and double stars, but I've looked through quite a few of these 90mm Maksutovs with upgraded star diagonals, and the images have been generally quite good--sharp, with no chromatic aberration.

Exactly to my point as to why I prefer the little Maksutov for these three class targets. With dielectric diagonal,quality RDF and Nagler eyepieces,this compact setup when out my front door far more often than my 90mm F/10 acro did after doing a couple of initial side by side comparisons. At 100X I also prefer the 4 pure white star images of Epsilon Lyra seen in the Mak.

Once again,YMMV.

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