Jump to content

Telescope Types - THE BIG DEBATE


Recommended Posts

There are many different designs of telescope that exist. Whilst I understand the optical layout of the main designs, I would like to know the pro's and con's of each type from an astronomers perspective and the types of objects that each design would excel at or struggle with.

The main designs that I would like to focus discussion on are: Refractor, Newtonian, schmidt cassegrain, Maksutov, Maksutov Cassegrain and Maksutov Newtonian.

Again, I understand the optical configurations of each design. I'm more interested in the strengths and weaknesses of each design from your experiences.

Link to comment
Share on other sites

  • Replies 27
  • Created
  • Last Reply

Wow - thats a big ask :)

I've owned all the designs you list and they all have their strengths and weaknesses but I'd need pages to go into the details :)

This link is a reasonable summary I think:

Telescope, designs and their optical layout

Sorry if that's a cop out :)

Edit: I realise that you have an understanding of the optical designs already but the above site also sumnarises some of the pros and cons of the designs as well.

Link to comment
Share on other sites

Oooh! Controversial topic!

I've only owned refractors and reflectors, although I've looked through some SCTs. My 2 cents is that what is right depends on your budget, interests, lifestyle, commitment level, observing location, etc... It's more than just which scope is good for which target.

For me, a major consideration is focal length. Some of those designs (e.g. Maks) tend to have long focal lengths for the aperture. So you're getting a relatively narrow field of view, even with a 6" scope. Personally, if I'm going to use small apertures then I want to trade that for a wide FOV. Right now I'm happy with my Dobs. Big apertures and fairly generous fields of view. Really stunning planetary views. Only problem is the larger objects don't fit in the FOV.

Link to comment
Share on other sites

Its Easy, have one of each!...My main scope is a 10" sct, great even in alt az as you can swing round the sky and star hop. For more difficult objects the goto is great, also the fine movement when powered up is excellent.

For grab & go a short tube 120mm refractor, a great compromise and allows me to view whan there is a quick gap in the clouds at home or when on hols. Sure has its limitations a small amount of colour, but if it gets dropped or a collects a bit of sand, dog slobber etc its not a killer. Also it gives good dso views. As it is 2" just swap eps from the main scope.

Just sold my 102mm mak as that was my grab & go. excellent planetary telescope.

Also sold my 8" newt to fund the LX, if I am taking a big scope out I want it goto to maximise time when viewing is poor.. like er now!.

So I do not have one preference. Over the years it is the scope you use the most rather than having a 'trophy' scope that does not get used or you are afraid to take it out in the cold.

Link to comment
Share on other sites

So I do not have one preference. Over the years it is the scope you use the most rather than having a 'trophy' scope that does not get used or you are afraid to take it out in the cold.

Spot on :). I've been lucky enough to try most designs with the exception of a SCT and they've all got strengths and weaknesses.

In my experience, apochromatic refractors give the best view for a given aperture and they're the easiest to image with, but they're not affordable over 120mm so if you want aperture you need to go elsewhere.

Mak-Newts give the next best view, and they're a lot cheaper aperture-wise but again, they're not afforable bigger than 8" so it's either reflectors ot SCT's if you want the aperture needed for DSO viewing.

If you want something small, compact and something with a long focal length, then it's hard to see past a Mak-Cass as the Chinese made models tend to be made of focal ratios 12-15 (some of the Russian made models are made at f10, same as SCT's), so it's easier to get the necessary magnification for detail on the Moon and planets.

I'm slightly generalising here, as for example you can buy long focal length newts that excel at lunar/planetary work or the astrograph Mak-Newts which probably aren't as good for viewing but in general, I think that's you'll find with off the shelf models.

Tony..

Link to comment
Share on other sites

I'll volunteer to discus one type, refractors! Apo refractors are small, sharp and expensive. They have low maintenance, cool down fast and need moderate sized to small mounts. With field flatteners or quadruplet designs they are popular amongst imagers who just need to get on with it and waste no time. Once they get to about 150mm they become impossibly expensive and their focal ratios become too slow for imagers. Visually they give (uh-oh... I've started so I'll finish!!!) the best quality view of all per unit aperture but they are usually small so cannot see deep in the way reflector systems can.

Great for grab and go and deep sky imaging.

Olly

Edit, sorry Tony, we crossed, but we seem to agree. Surely the Tak 6 inch is perfectly affordable at thirteen grand???? (For Bill Gates.)

Link to comment
Share on other sites

big ones are best. and they should be white. :)
And preferably in orbit :)

So far as the pro's and cons of the various types of telescope are concerned. I'd suggest that a lot of them derive more from the quality of manufacture from individual suppliers, and the support they get, than from the intrinsic properties of the optical design.

So a good implementation of type <X> could be better than a mediocre model of type <Y>, even if in theory <Y>s have the potential to be better, optically, than <X>s

Link to comment
Share on other sites

Wow! Thanks everyone for your comments so far. I find it very interesting to hear your different views and comments on each type of telescope.

Rather than the usual 'which telescope is best for me' thread of which there are many on this forum which are usually started by people on the market for a new telescope, this thread is mainly to improve my (and hopefully other beginners) understanding of the different designs and what objects they are best for.

So if anyone reads this, this tread is not about which telescope I should buy, its for improving knowlage on the subject of telescopes ...

Keep your thoughts comming :-)

Link to comment
Share on other sites

So when people say that certain scopes are better for planetary than DSO's, this is mostly due to some designs such as the Maksutov having such a long focal length limiting it's ability to provide a low power view with the usual low power EP's?

And vice versa for 'good DSO scopes' with a short focal length that would need to have small focal length EP's and possibly a barlow to be good on planets.

Other than that is there any reason why a telescope would be a 'planetary' or a 'DSO' telescope?

Link to comment
Share on other sites

So when people say that certain scopes are better for planetary than DSO's, this is mostly due to some designs such as the Maksutov having such a long focal length limiting it's ability to provide a low power view with the usual low power EP's?

Sort of. The long focal length of a Mak-Cass makes it easier to get high magnification, but Mak-Newts (in particular the Intes Micro models) are excellent planetary performers due to their small secondary mirror and quality optics, yet it still has the same focal length as a standard newt with the same geometry.

I think there's a bit of perception issue with Mak-Casses as they're pretty much the same as a SCT in terms of design, yet the SCT is seen more of an all-rounder. I guess that's because the bigger manufacturers have more SCT's than Mak-Casses in their lineups and have widely available accessories for it. It's perfectly possible to do the same things with a Mak-Cass, you just have to look a little harder for the kit to do it.

And vice versa for 'good DSO scopes' with a short focal length that would need to have small focal length EP's and possibly a barlow to be good on planets.

IMO, a good DSO scope needs aperture and decent optics. Even a fast scope with a large aperture has by default a long-ish focal length.

Not simple is it? IMO, you can do pretty much any type of stargazing or imaging with any scope design, it's wether it's easier/affordable/quicker/better to do it with another. And that, is why people have sometimes long, heated debates about it!

Tony..

Link to comment
Share on other sites

I never understand why the SCT is singled out as being an all-rounder. Why is this not equally the case for other designs? Newtonians particularly. In fact, Newtonians will give SCTs a run for their money on most counts, particularly in apertures over about 10".

"And vice versa for 'good DSO scopes' with a short focal length that would need to have small focal length EP's and possibly a barlow to be good on planets. "

You don't need a long focal length to have a good planetary scope. Any scope with good optics and a central obstruction under 20% is a good planetary scope. The best planetary views I've had to date are, by a long shot, through my 18" f/4. Frankly, I wasn't expecting that. Oddly, an 18" f/4 is pretty good on DSOs too :)

Link to comment
Share on other sites

As I have recently bought a refractor (it's blue rather than white) I'm happy to be able to use my 40mm Omni EP without suffering from secondary shadow, which always makes it self known when using long focal length EPs in my Newtonian and Mak scopes. As I am by far a visual observer with a preference for the wider field views, the refractor has already had more use than my Mak, plus it now has the clock drive fitted so I can hop from the 10"SNT to the 90mm frac, thus enabling me to observe two objects simultaneously (almost).

Link to comment
Share on other sites

So when people say that certain scopes are better for planetary than DSO's, this is mostly due to...

Entirely so, I'd say! - but it's very misleading in the case of Maksutov-Cassegrain scopes.

For example, the number of big, sprawling objects that DON'T fit into the field-of-view of a Mak is really very small - just a handful really - and the VAST majority of DSOs fit into FOV of SCTs and Maks with plenty of room to spare; furthermore, I'm sure my Mak 180 gives any type of telescope a serious run-for-the-money when it comes to resolving globular clusters! :)

The focal length / field-of-view argument is a little bit spurious - especially when you consider that a small Mak - surprisingly - have the same (or similar) focal length to a Dobsonian anyway - in fact they have an even larger field of view than a big dob once you add a focal reducer into the equation.

The issue is more about the power-per-inch you can get out of the telescope, which broadly falls into four categories, each of which has it's strengths on a particular target type:

1) 4-8x pi - for open clusters and general sky-sweeping

2) 8-16x pi - for galaxies, nebulae and globular clusters

3) 16-32x pi - for lunar and planetary observing

4) 32-64x pi - for double stars

The lowest power on a Mak generally starts about halfway into category (2) above - and that can be a problem: It means that for generic sweeping about the sky, appreciating the beauty of the heavens, a Mak isn't the tool of choice.

However, for examining a specific object in the view, such as a galaxy or medium-to-small nebulae, then a Mak is as good as anything else really...

Link to comment
Share on other sites

I am going to give a plug for our club's web page, and Moderators, if this is not appropriate, please feel free to delete this post.

TSA Reasources

On this page, there are several links to "beginners guides" to picking out and using your first scope. It will give you a good handle on what you need to know to make a fairly knowlegible decision for a first scope.

Hope it helps!

Jim S.

Link to comment
Share on other sites

I currently own 4 scopes so I thought I'd contribute some quick notes on each one, as I see it. I've tried to relate the comments to the scope type / spec rather than the particular brand. Scopes are a pretty personal thing though so I'm sure others will have other views on theirs. I have owned schmidt-cassegrains and maksutov-cassegrains in the past but these notes just cover my current "fleet":

Vixen 102mm F/6.5 ED refractor, Japanese made

Pros: very light and compact, easy to mount, fast, so wide fields of view possible, chromatic aberration (CA) well controlled for it’s focal ratio, very quick cool down. Crisp star images, no collimation issues.

Cons: limited aperture, needs short FL eyepieces / barlows to achieve high powers. Quite expensive for the amount of aperture.

Skywatcher 120mm F/7.5 ED refractor, Chinese made

Pros: reasonably light and portable for a 120mm refractor. Allows 200x plus to be used quite easily so good for planets, lunar, binary stars. Very good suppression of CA. Really tight star images and no collimation worries. Short cool down time.

Cons: Still limited aperture for deep sky objects, expensive per inch of aperture. Needs a tall, sturdy mount.

Intes 152mm F/5.9 maksutov-newtonian, Russian made

Pros: Excellent for planetary, lunar and binary stars and enough aperture for decent deep sky observing too. Fast focal ratio allows wide views. Holds collimation very well. Delivers “refractor “ type images and contrast.

Cons: Needs 30-60 minutes to fully cool, heavy scope, needs sturdy mount. Small secondary increases image quality but won’t fully illuminate field of eyepieces with large diameter field stops. Collimation can be tricky.

Orion Optics 250mm F/4.8 newtonian, UK made

Pros: Really useful aperture for deep sky objects. Not a light scope per se but light for its aperture. For it’s aperture, offers wide views but can also deliver respectable planetary / lunar views. Largest scope that I can comfortably move around easily.

Cons: Some 30-40 mins cool down needed. Needs quality eyepieces because of fast focal ratio. Collimation needs “tuning” at each use.

NB: Please note that I don't do imaging - I'm "visual only" :)

Link to comment
Share on other sites

I have only used refractors and newtonians (dobsonians). without doubt my favourite scopes are dobs. I love modding things and my scopes have had this treatment. I have no fear of changing things if I feel it will improve the view or operation of the scope. I am a visual only observer.

my key criteria for scopes are as follows:

aperture - I really like a decent aperture (12"+). at this size you really start to see detail and depth in objects; background stars pop out etc. my current big scope is a 16" dob.

comfort - I find the dobsonian observing position much more comfortable than others. my head is broadly vertical when viewing at whatever position and sitting to observe is much easier. with refractors I find that I am bending my neck a lot more even from a seated position. dobs to me are also smoother in motion (without tracking) so higher power viewing is easier.

size - it needs to be small enough to move and fit in my car for those rare occasions when I can get out with it.

for me, field of view is not critical - one degree is fine - I have a small frac and finder scope for really big things like M44 etc.

I also have a 6" f11 dob which is superb on planets and double stars but to be honest the 16" with an aperture mask is very very close if not better than the 6". I have a small 90mm f5.5 frac and this is lovely for wide field viewing.

my opinion is that there's no such thing as an all round scope for all targets and I am lucky to have what I consider to be all three bases covered (faint stuff, planets and doubles/moon and wide field) but if I could only keep one there's no doubt it would be the 16". and yes, it's big and white!!

not sure if this directly answers your question but it's my slant anyway.

Link to comment
Share on other sites

hi Jeremy

I should have possibly qualified this with 'big dobs'. I find the motion of a smooth dob superbly smooth and easy. I can move my 16" dob with one finger. The key is that the pressure point/'lever' is at the end of a long tube, generally well away from the balance point. with fracs (and I imagine with SCTs too) the 'lever' is much shorter and therefore the movement takes more effort.

I really do like my Giro alt az mount but (with my light frac) possibly moves a little less smoothly than if there were more weight on it, especially as the 'lever' is only a few inches long and my dob moves with less force, even the 16".

I hope I am making myself clear and this is not gobbledegook.

Link to comment
Share on other sites

1) 4-8x pi - for open clusters and general sky-sweeping

2) 8-16x pi - for galaxies, nebulae and globular clusters

3) 16-32x pi - for lunar and planetary observing

4) 32-64x pi - for double stars

The lowest power on a Mak generally starts about halfway into category (2) above - and that can be a problem: It means that for generic sweeping about the sky, appreciating the beauty of the heavens, a Mak isn't the tool of choice.

.

So if i was to geet a 10" dob what number would this fit into.

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.