Refractor / Mak Cassegrain Comparrison

[Dick]

Hi Guys,

I'm absolutely new to all of this but have been trying to read up on as much as possible before I purchase a scope (which has just confused me more with information overload).

I am just wondering though what is the viewing comparrison between a similar sized short tube refractor and a maksutov cassegrain (ie. are you going to see as much with a 90mm short tube refractor that you would with a 90mm maksutov cassegrain??).

Thanks. Dick.

[Steffen]

I'd say you "see more" in an X mm refractor than in an X mm Cassegrain, simply because the latter has a smaller effective aperture (central obstruction). In addition, the Cassegrain has two reflective surfaces on top of the corrector lens with its two air-glass boundaries. This would affect the overall throughput as well.

Keep in mind that a short tube X mm refractor will have a much shorter focal length than an X mm Cassegrain and a much faster focal ratio, with everything that entails.

I believe what you may have heard is that an X mm Maksutov Cassegrain will rival a Y mm refractor, where X is somewhat larger than Y (as in 6" vs 4"). This stems from the fact that Maksutov designs are inherently well corrected for coma (and field curvature?) and good implementations have very good contrast giving a refractor-like view.

Cheers
Steffen.

[casstony]

Unless there is a particular reason why you don't want one, an 8" Dobsonian is by far the best option as a first scope. It's cost effective and you'll be able to observe many more objects than with a 3" scope. Since you live in Adelaide it should be pretty easy to get to a public observing night to try out different scopes.

Welcome to IIS .

[Dick]

Thanks for the quick replies.

Having been looking over this website for the last few weeks I quickly gathered that the 8in Dob is the overall recommended scope for newies getting into this hobby - however I just don't have the storage for it and would need something I can put up in the wardrobe (safe away from the kids aswell!!!).

As for the comparrison, as mentioned I am still trying to get my head around all this new information. For info though, Andrews Comm (again without this site I wouldn't have known they existed) have me half sold on a 114 reflector (Nexstar 114 SLT reflector 114mm x 1,000mm with Go-To).

Dick.

[casstony]

The 8" Dob tube could go on top of the wardrobe (~20 pounds) and the kids would have a ball spinning each other in circles on the base - double the value.

I don't have any experience with the 114 SLT but I'm sure someone else will chime in.

[astro744]

You will be much happier with an 8" Dob over a 4.5" electronic GOTO scope simply for the 3x extra light gathering capacity especially for the same money.

The electronic GOTO can fail in both drive mechanism and hand controller leaving the telescope with no manual override and expensive replacement once the warranty has expired. (I know someone in just that boat with a similar telescope). I'm not saying they'll all fail but anything electronic is prone to more things going wrong than a completely manual telescope. You can always add a solid EQ mount to your 8" later if you want tracking or even GOTO.

The 8" Dob will be more sturdy too and you can learn the sky with printed sky charts available (many free) online. GOTO is like a GPS in the car; it will get you to where you want but you really have no idea how you got there. I find learning your way around the sky with printed charts is much more rewarding; and yes I still use a Gregory's road map!

[dannat (Daniel)]

short tube refractors aren't meant to be good visual instruments - most people use them for guiding. a 4" mak will be ok for visual - because of the longer length you can push the magnification out a little more easily without having to squint in the eyepiece. andrews do have a 5" celestron mak i think which would gather a little more light - they fit on a sturdy camera tripod
if it was me i would go for the mak..but in a light polluted area dso's will be dim

[big_dav_2001 (Davin)]

half sold on a 114 reflector (Nexstar 114 SLT reflector 114mm x 1,000mm with Go-To).

I see the tepmtation to go with the 114mm reflector, and especially the go-to mount... i fell victim to the same type of scope...

The problem im faced with when using my simmilar 114mm Meade reflector and #494 GOTO is that due to the small appeture, ive now got a telescope with a goto mount which can point out more than 4000 objects that are too dim to see through it!!!...

Having said that, there ARE a lot of brighter DSO's which can be found and tracked well through it, but once you spend some time viewing these, its only a matter of time before you want to see more... (aka, the ever-popular appeture fever...)....

My advice (like most) would be to go for a small to moderate sized dob, a 6 or 8" GSO Dob would provide good views and should satisfy your appeture fever for a while...

Hope it helps
Davin

[mikemasey (Mike)]

Hey there,
lots of good ideas,
I have an 8" dob that I use with a 25mm wide and a 2xbarlow and its just fantastic for what I want.
I also have a 90mm Orion desk top(on a tripod) Mac-cas and that is also great.
As a test I used the trapeziem in Orion and can split all four stars.
Obviously the image or picture is far better on the Dob but of coarse the Mac wins hands down for storage and portability.
It must be remembered that Magnification isnt every thing, ie: I can look at the same object with both scope and the image will be the same size(if you have the same mag in each) but the picture is just that much more clearer in the dob.Its to do with the amount of like gathered or something(all maths!).
good luck

[mikemasey (Mike)]

Hey there,
lots of good ideas,
I have an 8" dob that I use with a 25mm wide and a 2xbarlow and its just fantastic for what I want.
I also have a 90mm Orion desk top(on a tripod) Mac-cas and that is also great.
As a test I used the trapeziem in Orion and can split all four stars.
Obviously the image or picture is far better on the Dob but of coarse the Mac wins hands down for storage and portability.
It must be remembered that Magnification isnt every thing, ie: I can look at the same object with both scope and the image will be the same size(if you have the same mag in each) but the picture is just that much more clearer in the dob.Its to do with the amount of like gathered or something(all maths!).
good luck