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MN56 or M6511 or 4 1/2" refractor?

Started by Dragonophile, 06/25/2010 05:13AM
Posted 06/25/2010 05:13AM Opening Post
I need a second telescope to complement my TMB 92L - a 92mm refractor. I have never owned any refractor over 4" or spent more than a couple minutes looking through a Dob or SCT. I have never looked through a Mak-Cass or Mak-Newt. I need some advice on choosing between these MCTs, MNTs and a larger refractor. I am posting here because I am a rank newbie on Maks and I didn't want to post in the refractor or cat forums because that would probably engender a partisan response.

First, I am visual only. Second, I would not consider any telescope over about 13 pounds. I don't want a Dob or SCT, so no suggestions there, please. My primary viewing is lunar, planetary & double stars though I wouldn't mind tackling some brighter Messiers though my suburban environment is light polluted.

Here are the choices I am considering. In refractors, an AT111EDT or Skywatcher 120ED. An excellent quality 127mm Mak-Newt MN56 (1/8 wave or better). A 165mm Mak-Cass with 1/8 wave and 165mm aperture and 32% CO (MN6511).

If anyone has used a 4.5" or so apo/ED refractor and one or both of these Maks, I would love to hear your pros & cons.

At this point, I am leaning strongly towards the refractors because I know them the best. But, again, I am pretty ignorant of what a MNT or MCT can do except from what little I have read about them. I know they may need collimation periodically & they may need more cool down, especially with large temperature drops. MNT have the reputation of being "apo-like" but the MN56 has mostly been compared to a good 4" apo and that wouldn't be enough of an improvement over my 92mm apo. The MCT has more aperture but a larger CO and more cool down. Unfortunately, I don't know anyone who owns either - as I know that looking for oneself is the best test.
Posted 06/25/2010 08:40AM #1
I once did some side-by-side comparisons of a 4" APO and a 6" F/12
Mak-Cass on planets. The views were pretty similar, except the
Mak-Cass took a very long time to cool-down and equal the APO.
So given your interests and choices, my vote would be for something
like the 120 ED.

Between a 120 ED refractor and MN56, my vote would again be for
the refractor. The MN56 central obstruction will decrease planetary
resolution slightly, and mirrors (typical coatings) will give
slightly dimmer images. And again there are some cool-down
issues with the MN. If you were looking at a 6" or 7" MN
then I might reconsider.
Posted 06/25/2010 10:58AM #2
A few thoughts:

- When considering the manageability of a telescope, the weight of the telescope is important but it's physical size is also important because it determines the requirements for the mount. The 120mm refractors and the 165mm MAKs are relatively big scopes and what is going to require the effort in setting up and using is not the telescope itself but the mount. In my experience, any of the scopes with the possible exception of the AT 111EDT are going to require a CG-5 class mount.

Looking at the Celestron page, the CG-5 ASGT weighs about 42 lbs. This would probably be with a single 11 lb weight. The Orion SkyView Pro with 11.5 lbs of counterweights weighs 36.5 lbs, the sort of lightest reasonable breakdown would be about 25lbs.

- My experience is that any refractor much over 4 inches and maybe 26 inches in length becomes a serious telescope that is no longer "grab and go." So I just have no interest in a refractor much over 4 inches because the difference between a good 4 inch and a decent 5 inch, particularly when it comes to double stars, is really pretty minimal.

To really make a serious difference, one needs more than an incremental aperture increase. The 165mm MAK will provide a moderate increase in aperture over your 92mm TMB at the expense of the larger CO and the relatively long cool down times.

So, the main things, the mount is at least half the equation, the scopes your are considering require a considerably beefier mount than your current 92mm.

You should not expect to be overwhelmed when moving from a 92mm to a 120mm. The differences between my 80mm APO and my 101mm APO are more the same than they are different... If I want a different class of view, a whole new world, that requires something of at least 8 inches.

Jon Isaacs

Posted 06/26/2010 02:08AM #3
Since you mention the Skywatcher 120ED I would like to pass along my experience. I too wanted something a little larger than my Orion ED80 that I would still be able to use as a "grab-and-go" setup. I snapped up a SW 120ED that came up for sale here on AM.

Optically and mechanically speaking the 120ED is on par with the stock ED80 - everything I expected it might be with one disappointment - the focuser. I could not adjust the stock two-speed focuser such that it would hold focus with heavier eyepieces (31mm Nagler) - especially when pointing towards zenith. After performing all the standard mods to make the typical Synta Crayford work I still could not get the focuser to function properly. I finally replaced it with a Moonlite. This was a great upgrade and I'm now very pleased with the OTA setup.

I've used it with a Televue Telepod on a heavy-duty Bogen-Manfrotto tripod with great success, though I'd say this f/7.5 scope, as light as it is, is about at the limit of this particular mounting arrangement.

    There is only one constant in our infinite universe; it is change.
    D&G 8” f/15, AP178 f/9, C11, C8, ED80, TV-85, TV101&102, SW120ED, 6"Cave f/8, Solarmax 40, ES127MC,
    DM-6, Losmandy Titan 50 GEQ, G11, AP800, ES Twilight-1, UA Unistar Deluxe, WO EZ-Touch, 
    Denk BVs, 7x50 Fujinons, 10x50 Nikons, 15x70 Astrophysics, 16x70 Fujinons, 22x100 Oberwerks, 
    Sim Picheloup CPT, Canon 40D, 60Da, Vixen Polarie, SXV-H9, DMK 41AU02, DBK 21AU618, 
    Shelyak Lhires Lite Spectroscope, Rainbow Optics Spectroscope, Field Tested Systems RSPEC,
    Vice Chair, Southwest Region of the Astronomical League (SWRAL)
    President Emeritus, Historian, AL Awards Coordinator, Texas Astronomical Society of Dallas (TAS)
    Member/Volunteer, 3RF Comanche Springs Astronomy Campus (CSAC)
    ALCOR, Advisor to the Board, Texas Star Party (TSP)
    NASA-JPL Solar System Ambassador Volunteer (SSA)
Posted 06/29/2010 02:17AM #4
I'll just shoot my mouth off here! Personally, I believe that optical differences between types of telescopes are often exaggerated (covnienence, weight, personal preference are a different matter).

I did a fairly careful side-by-side comparison of 3 five-inch scopes as follows. My intes-micro MN 56, my friend's D&G f12 achromat, the school's Takahashi 130 reflector. Right off the Tak got killed on contrast. This was not subtle. Then, we took a piece of black construction paper and made a dew shield for the Tak, and differences in contrast became subtle, very subtle, possibly observable. Over time we reached a consensus that the D&G was definitely brighter, and the MN 56 definitely dimmer (Ed Ting made a similar observation in a similar side by side comparison that I came across later). We also reached a consensus that execution of the design is the more important factor.

Flame on!