Meade LX90/200's

[Alpha Prime]

Whassup people? I'm new on this board, but definitely not new to astronomy. I'm here to look for your advice on 2 things:

1: I currently own a 12" Sky-Watcher Dobsonian, but am finding it useless since I can't get the collimation right, not even with a laser-collimation tool. Is collimating mirrors always a problem with dobsonian/newtonian build? Is it possible to get a perfectly sharp view with these telescopes? I'm talking planetary detail, here, not nebulae.

2: In case I still find my dobsonian useless after your advice, I'm considering buying a Meade LX90, 12". However, I'm concerned with the following: I own a Meade ETX90, and it has proven to be faulty in the long term: The secondary mirror is glued to the back of the primary lense. Over the years, gravity has taken its' toll and dragged the mirror into misalignment. Is something like this likely to happen with SC or RC configuration?

I will be grateful for your advice!

[Alpha Prime]

So, how do I interpret the lack of responses? That people on this telescope forum don't own any Schmidt-Cassegrains/Ritchey-Cretiens or that they don't like to help?

[kniclander]

hmmm - maybe people didn't respond cos they felt sorry for your poor dob being dissed like that. There are many very happy dob users on this forum and sure we all had our collimation learning curve but most of us have managed (to an extent) to get to grips with it.

The laser collimation tools are very easy to use but inherently problemmatic at the same time as if there is any focusser slop at all or if there is any error at all in centering the dot in the primary, then it won't work properly. Also, sometimes the laser is not itself collimated. TBH however, I don't think you'll ever get a "perfectly sharp view", not unless you leave the planet;).

I've been happy with both my dobs but my new 3" refractor kicks the a**e of the 10" dob when it comes to splitting tight doubles. Is it cos my collimation's not quite right or is it just something about the scopes? I don't know...

ps - what sort of mags are you using - if the view isn't sharp, maybe is too high?

I expect others will be along to give you better answers in due course.

[John]

Well I've owned 4 dob's from 8 to 12 inches in aperture and I've had nice crisp planetary images with all of them. I don't think a 12" SCT or RC will provide better views than a decently collimated 12" dobsonian / newtonian because of the larger % secondary obstruction.

My advice would be to read, or re-read as much advice as you can on collimation and then give your dob another chance to shine.

John

[GazOC]

Theres only really Saturn up there at the moment, other than the rings and a couple of bands theres not really that much to see. What planetary detail do you think the scope is "missing"?

An f5 Newt will need collimating, it come with the territory but after the first time its really nothing to worry about.

[barkis]

How true is your laser collimater? I believe they need collimating too, so if yours is out, it will not do a proper job for you.

Ron.

[canuck]

I'm concerned with the following: I own a Meade ETX90, and it has proven to be faulty in the long term: The secondary mirror is glued to the back of the primary lense. Over the years, gravity has taken its' toll and dragged the mirror into misalignment. Is something like this likely to happen with SC or RC configuration?

The mirror on the rear of the corrector is simply an aluminized spot, and I can't see how it's possible to go out of alignment...are you seeing optical alignment faults? Having said that, the baffle for the secondary on some of the early ETX's used glue that eventually let go, which caused the secondary to slip down. Is this what you're seeing? This can simply be re-glued into its' proper place.

Check Weasner's ETX site...there's likely to be pages & pages on how to fix it. It should be straightforward.

Richard