Which scope?

[Decimus]

Hi, Everyone,
This is my first post as a relatively new member of iceinspace. This is great site and has been very helpful (especially the equipment discussions).

I would like a little advice from anyone as I am about to purchase a new telescope primarily for observational purposes. It has to be of very high quality and portability; otherwise, it'll sit there unused.

I have narrowed down my choices to:


The Takahashi TOA 130 NS triple apo refractor
The new Takahashi FSQ 130 (astrograph)
The Takahashi Mewlon 250 CRS
The Skywatcher 150 ED triple apo refractor
The Questar 7" Maksutov-Cassegarin LW 7

I have not really given thought to a suitable mount though the thought of spending some $6000 - $7000 on a good one seems excessive (rather spend it on a great scope). :)

The only astrophotography I will be doing is with a DSLR, not a CCD (I know nothing about CCD imaging, though the results are impressive, to say the least).

If anyone can spare the time to offer some advice, please do.

Cheers.

[barx1963]

Hi Richard and welcome!!

I will not attempt to discuss the scopes you list as I am not familiar with any of them enough. I will address the philosophy behind your post. I notice that at least one of the scopes (the FSQ 130) is an astrograph so I am assuming that imaging is going to be a significant part of your ambitions.
Mounts are the most critical part of an imaging setup. A fairly good scope on a good mount will produce pretty good images. The worlds best scope on a poor mount will produce poor images. To buy one of those scopes and attempt to image on a cheap mount will be a lesson in frustration.
If on the other hand visual is your thing, you may get away with a under rated mount.

Anyway good luck with your decision:thumbsup:

Malcolm

[Camelopardalis]

Welcome Richard :thumbsup:

I've seen several of those scopes in action and nothing about them or the mouths required to carry them could be regarded as easily portable :lol: but I guess that would depend on your fitness level, age, transport, etc.

Might be an idea to go to a star party to see some in action...

[bigjoe]

Hi Richard and welcome.

All of these scopes and mount required would be far too big to transport away from your house and setup regularly and so would remain largely unused; unless you are really keen!

These all really need a more permanent setup.

Something smaller is required eg 4" apo , 6" sct, otherwise you will lose all interest.

Cheers bigjoe.

[Atmos]

First the mount, you should definitely spend that bit extra on a mount that you'll be happy with and one that can easily hold the rig. If you do feel you want to go cheap though, the Alt/Az EQ6GT will hold most of those visually without any issues.

As for the scope, I personally would take the Tak FSQ130 off the list. As good of an astrohraph that it is going to be, as far as I know it is still yet to be released AND there is a chance that you may not be able to use a diagonal with it due to not enough back focus.

If all you want is visual and have I real plans of doing astrophotography, I would suggest getting the biggest that you can comfortably move. I would also suggest getting a doublet because it'll be lighter and cool down faster than a triplet.
I personally would keep away from a large Mak-Cass because it is likely to have a LONG cool down time although it may have fans and stuff, not sure. Having a telescope that cools and climatised quick will be your best friend.

[Decimus]

Thanks for your replies, Malcolm, Dunk, Joe and Colin. I was thinking 12 - 14Kg (OTA only) was around what I would call 'portable'. The Celestron CPC 925 is at what I call the upper limit of the portability range (I managed to get one for the college where I taught until retiring 3 weeks ago). It's optical quality that really matters to me; for example, I have seen reviews of the Chinese-made Skywatcher holding up extremely well against far more expensive refractors. (Cloudy Nights had an imaging shoot-out between the AstroPhysics 162 APO triplet refractor and the Sywatcher Esprit Ed 150; I couldn't pick any difference in the images)....Makes life tough, decision-wise...

[casstony]

If portability is a key issue I wouldn't go over about 10Kg for the optical tube.
Alternatively you could consider having two scopes, for example a very portable Tak FC100DF and a larger scope (C11 EdgeHD?) which requires a little more effort to use.

[scopey]

I own two TAKS tsa102, toa 130. you can't go wrong with a TAK. I would suggest a TSA 120 on az/eq6gt mount. If you ever want to upgrade you won't loose much money on the TAK, and the views are pin sharp when using the write eyepieces. I have the az/eq6 mount and have no trouble with the toa 130 for viewing. You can also photograph the planets-moon and auto guide for stars and DSO"S. I hope this helps
scopey

[Decimus]

Thanks, Tony and Les. I seem to have read somewhere on this site that the SW EQ6alt-Az mount was giving some trouble with maintaining belt tension? Maybe not a good choice for a Tak...Don't know. I do lean towards the Tak 130 but I couldn't believe my eyes at this comparison (I found the link on CN) between the AP160 and the SW 150 ED that I referred to earlier: http://www.cloudynights.com/topic/478771-ap-160-vs-sw-150-esprit/ (http://www.cloudynights.com/topic/478771-ap-160-vs-sw-150-esprit/) Food for thought.

Thanks again for your advice.

Cheers,
Richard

[brian nordstrom]

:):) Yes the SW 150 is the curve ball here and for the price and quality you wont do better , a 150mm scope will always show more than a 120/130 scope no matter how good it is .
And I beileve the lovely EQAZ6 would hold the SW just fine .

If it was my money I would grab the SW Esprit ( I too read that comparisim and was impressed as well ) , a lot of 6 inch APO sweetness for very little money :question: ,,, well compared to a 6inch Tak or AP , TEC , etc , etc .

Brian.

[Decimus]

Thanks, Brian. Yes, even though the TAK has the runs on the board, the SW 150 comes with star diagonal, tube brackets, finderscope, carry case, etc and performs extremely well. The TAK is just an OTA and would require an additional outlay of maybe $1500 just to match the SW outfit. And there is the old maxim, 'aperture is everything'! :)
Cheers,
Richard

[brian nordstrom]

:) Yep , aperture sure does rule , and in refractors it seems to be very evident in small jumps , like us , we have a small group of dedicated visual observers here and what comes to mind is a coulpe of Fluorite triplets of 110 and 115mm that are essentially perfect but worth thousands .

Now come's the curve ball , in the form of my 127mm f8 iStar achromat at the sum total of $630 and it performs so well that the APO owners perfer it over their scopes on many objects .

Side note ; one of these guys is seriously looking at the 150 Esprit ,,, cant wait personally for him to bite the bullet ;) .

Brian.

[Allan]

Hi Richard.

Do you have a sense of what you want to achieve when you start out? Purely visual, and what objects are you interested in, or purely imaging, or a mix of both.

The only reason I say this, is because looking at your list of 5 scopes, they are all over the place. Each one is quite varied in it's purpose.

If you can nail down a few specific areas that really interest you, I think you will get some telescope recommendations more tailored for the task.

Also, depending on your experience a "very high quality" telescope may be a waste of money to begin with, as you sort out your niche in this hobby. As beautiful as a 5" APO is, it's a poor choice if you find out you like chasing faint galaxy clusters.

[Wavytone]

Well... I have just bought an 8" f/8 mirror which is 1/21 wave (0.995 strehl) to make a lightweight planetary scope (dob) that should easily give any of these refractors up to 150mm aperture a run for their money, with a small secondary, curved vane spider and proper thought given to keeping stray light out. A project to build this winter in tie for spring/summer, I hope.

I also have had the opportunity some years ago to use both two Quester 3.5" and a Questar 7. While the small 3.5" Questers definitely are essentially perfect in every imaginable way and perform better than many would expect, I was somewhat underwhelmed by the Q7. Good, but if I was going to spend that amount on a scope I think I'd buy something else.

Incidentally if you ARE after a Mak for lunar & planetary I'd suggest instead you consider:

a) One of the scopes from APM in Germany assembled using russian optics; not only does APM offer the usual choices of aperture and focal ratio but you can also specify the quality of the optics (yes it costs more) or settle for the bog standard russian grade. And APM build a far better tube than Intes does.

b) A Maksutov-newtonian, there are several cheap commercial ones at 190mm aperture around f/5-6.

c) And if thats not enough, APM will assemble a custom Maksutov Newtonian to your specs.

[brian nordstrom]

:thumbsup: You are right there Wavy , in NZ I have a 10inch f10 , yes its a 2.5 metre focal length :D but with its 20mm , yes !!! 20mm secondary the detail seen on the moon and planets easily equaled our clubs excellent C14 and easily beat an AP 155 super planetary , it's just BIG !!! note the steps in the background .
Thats her the long alloy tube on the Dob mount .

A good long focal length Newt can rumble with the best 'for sure'.

Look at the gold scope , its a 5inch f13.7 also with 20mm secondary , its a lunar killer esaily taking 400x plus on a good night and with the EQ pipe mount it is easy to do.
Both Newt's have rotating tubes .

Brian.

[Decimus]

Thanks for all the advice, guys. I guess it all boils down a trade-off between the detail that aperture can bring from large reflectors versus the high-contrast and sharpness that can be found in a high-end refractor. I am still inclined towards a large refractor (although a Tak Mewlon is tempting). I love deep sky objects (M42 and Eta Carinae looked spectacular through the Celestron CPC 9.25 SCT I used with students at the college where I worked) but I also recall the spectacular view of the Moon through a 5" Vixen refractor last year and that was 'persuasive'! Back to the drawing board for now....
By the way, for anyone who missed the AP 160 refractor vs the SW 150 Esprit, here's the link again: http://www.cloudynights.com/topic/478771-ap-160-vs-sw-150-esprit/ (http://www.cloudynights.com/topic/478771-ap-160-vs-sw-150-esprit/)

Cheers,
Richard

[Eden]

You may be interested to know that the Esprit 150 VS AP 160 comparison will be coming back for another round next month. Jerry will be testing both scopes again only this time he will be using CCDs instead of DSLRs. Should be interesting given that the performance of these two scopes is a lot closer than the price delta that separates them!

[Decimus]

Thanks, Brett. I am sure looking forward to the new review.

Meanwhile, I still cannot make up my mind about whether to buy a TAK 130NS APO or the Skywatcher Esprit 150 APO. The extra inch of aperture in the latter might outweigh any superficial differences in image quality, maybe....

[Eden]

I believe there is a Tak VS Esprit comparison somewhere on CloudyNights, if I'm not mistaken.

You mentioned that you were primarily looking for something to do visual work with. I've not looked through either of these scopes but I don't think anyone will dispute the fact that the Takahashi has superior optics.

A Takahashi will also retain it's value much better than a scope which is mass-produced in China, so that might be something else to consider.

[Camelopardalis]

The Esprit 150 is far from a mass-produced scope...as I understand it, they are made to order.

I'd like to see a head-to-head as I'd put any claim of superior optics down to diminishing returns ;) there is a huge difference in price...

The extra inch going from the 130 to the 150 results in a 33% larger light gathering area...not insignificant if the goal is visual observing.

[casstony]

The allure of the shiny Esprit 150 may wear off when one realizes that this heavy but modest aperture performs poorly on deep sky compared to a 12" truss Dob. The Dob would also be more portable than the Esprit and heavy mount needed to support it.

I think you'd be better off with a 5" refractor then later add a larger scope for deep sky.

[Atmos]

After reading the Esprit vs Tak thread, it did get me wondering if some of the improvement of the Tak over the Esprit was an artefact of the Tak having a longer focal length. With a longer focal length (smaller image scale) you will get better looking stars on a OSC.
Of course, having good filters and a CCD will give a very good indication of perceived colour correction and the like.

As nice as the Esprit 150 is, it is a bit heavy for my liking. Although having never looked through a larger refractor than my 130mm, that alone is 10 kg and I do agree that getting a Dob to complement a more usable refractor is the way to go.

[Decimus]

Thanks again, guys. Trouble is, I agree with all of your observations :( I think Dunk's observation about the extra 33% light-gathering power is persuasive. I was just looking at APM's website in Germany and was gob-smacked at the cost of their APM-LZOS 152mm triplet APO - over 10 500 euros...Puts things in perspective as that is nearly 17 000 AUD :eyepop: for an OTA only (the likely cost of a new TAK FSQ130 (F5) APO quintuplet astrograph....This is sure big bucks wheen compared to SCT, Mak and Newtonian reflectors and Dobs in general...

[Eden]

Colin, Tony and Dunk have all raised some good points that are worth factoring in. It sounds as though you have a reasonably large budget in mind. In that case, what Tony said is especially valid -- even a large, quality refractor will hold you back on visual compared to something like a Dob and with the money you're looking to spend on a refractor, you could get your hands on a good quality 16" or 18" Dobsonian. Sure, there are larger refractors available from Germany (200mm+) but aside from being both expensive and a heavy, I doubt they would come close to a 18" Dobsonian on visual.

[Camelopardalis]

Certainly you can't beat aperture for visual.

The EQ6 class mount required to carry the 150 would double up as a suitable AP solution :D I've seen that combination in action so need no convincing that it works well. Doubtful that it's significantly heavier than a 12" Dob...

[Eden]

I was pleasantly surprised when I saw that a properly configured EQ6-class will happily guide a fully loaded Esprit 150.

Not sure if weight is even an issue with a Dob, Dr. Erhard Hänssgen's 42" Dob weighs just over 350kg and can be carted around, setup and operated by one person.

http://www.cruxis.com/scope/scope1070.htm

What I would give to have an eyeball through that!

[UniPol]

SkyWatcher lists the weight of the Esprit 150ED OTA at 11.5kg which I think is not too bad. My Tak TOA-130NFB weighs in at 11.4kg. As matter of interest a Tak TOA-150 weighs in at 14.6kg (plus 4.8kg tube balance weight cradle). Speaking of prices, the FSQ-130ED comes in at around $16,000 and I think the TOA-150 was about a $1,000 less. These prices were around July last year.

[casstony]

11.5kg seems way too light. The Esprit 150 weighs closer to 15kg with rings according to a few sites I looked at.

[Eden]

I was wondering about this, because Skywatcher Australia have it listed at 11.5kg but the US site has it down at 14kg. Having said that, the US site describes the 150 as a "Super APO". Are they the same product or are there two versions of the Esprit?

[Camelopardalis]

It's not physically that heavy...it's just fairly large. IIRC weighs about the same as a C11.

[Decimus]

Thanks for the additional remarks and advice on portability, guys. I actually think 15kg is not bad at all. The CPC 9.25 I used was almost 40kg (including the tripod, fork mount plus scope). The thing that bugs me about reflectors is the constant need for adjusting collimation (time taken away from observing) and that secondary mirror obstruction which, in some models, is substantial (50% or so in the Officina Stellare Riccardi-Honders astrographs. With that sort of 'light loss' where is the benefit in greater aperture?). Oh well, back to the drawing board, as they say.

[raymo]

The light loss caused by the secondary in most reflectors is negligible.
My 8" Newt has an approx. 2" secondary which is obviously 25% of the
diameter, but only causes a 6.25% light loss, leaving the light grasp of a 7.75"scope. Amazingly, until it is thought about more deeply, even the 50% you mention, with a 12" scope for example, leaves you with the light grasp of a 10.4"; still much larger aperture than almost all refractors.
Regardless of how big the obstruction is, the scope retains it's
resolution limit.
Collimation is quick and easy after you've done it a few times, just a few minutes, a minute fraction of the duration of a typical imaging session.
Some people obsess about collimation, demanding perfection, but if you
do an experiment you'll find your collimation can be further out than you
would think before you can see any effect at all on your images.
The only slight negative is the very slight loss of contrast caused by the
obstruction.
raymo

[Fox]

Hi Richard

As those have said already, portability is a key consideration here, and you've already expressed awareness i.e. you don't want it to sit around unused. The Espirit 150 is without doubt one glorious scope, but at a price which not just dollars - the OTA alone is close to 15kg. To me, it would become quite the chore/disincentive in having to set up each time, although you seem to feel differently.

Then there are others on your list, e.g. the 7 inch Questar Maksutov - don't those things have a focal ratio near f/15 or so? The range and cost of eyepieces needed if low power takes your fancy, starts to emerge.

There is a lot to weigh up here, especially when comparing such different types of scopes. Have you had a scope before, long term? If so, may we ask what is was/is?

Look forward to seeing what you decide,
Cheers
Fox

[Decimus]

Hi Raymo and Fox,
Thanks for your comments. The dimunition of light caused by the large central obstruction in some reflector telescopes is much less than I had believed, Raymo, so thanks for pointing that out.

I think the Tak 130 is still the way I'll go, Fox, but the Esprit 150 is a close runner up. The weight differential between the 3 scopes is one or two kg, I think. However, the Tak 130FSQ has me mesmerised, if only because it's fast and would be a superb scope for visual observing rather than just astrophotography. But big dollars. :(

[raymo]

We've come full circle here, back to Dunk's pointing out that the 150 has
33% more light grasp than the 130. It also has about 11% better
resolution, [for a lot less money]. For visual, aperture is king, I would
go with the 150 every time, but obviously I'm not you. Enjoy whatever
you end up with.
raymo

[Camelopardalis]

FSQ is wasted glass and weight over the TOA/triplet purely for visual.

You'd get the same FOV with an 8" f/4 newt for a lot less money and weight...and light grasp.

Fast scopes come with their compromises...most visual scopes are around f/7 to f/10 for a reason...and a good f/7 doublet can be a mighty effective wide field visual scope without the need for the extra weight and mount.

[N1]

Correct me if I'm wrong, but I'm not sure that the whole "light grasp" thing is actually very helpful in evaluating the difference between 2 instruments. Sure, the percentage sounds a lot, but unfortunately, a lot of extra light is needed for even a modest increase in image scale. The 150 will show the same DSO 150/130=1.15 times bigger than the 130, at the same brightness. To make an image just twice the size, a telescope would need 300% more light grasp!

[Camelopardalis]

Light gathering is an excellent way of comparing two instruments since the number of photons collected by a cylindrical telescope tube increases by the square of the tube radius :shrug:

[N1]

I'm not sure a value is an excellent tool just because it increases quickly with aperture. Because, what does that translate to for the observer? For the purposes of visual observation from a regular observing site on Earth, I suggest the 150 when compared to the 130 provides an image that is

less than 33% larger (it's about 15% larger);
less than 33% brighter as perceived by the observer (linear increases in perceived brighness require logarithmic increases in actual brightness);
less than 33% more detailed (using either scope at its Dawes limit requires sub-arcsecond seeing, so the seeing will limit both more often than not)

Diminishing returns I guess.

So, in practice, at the eyepiece, what does improve by 33% for the observer (apart from pride of ownership perhaps)? I'd like to know please. There's got to be something that does, because otherwise "light gathering power" would be pretty meaningless in the field, would you not agree? If that were so, the only difference between it and random marketing garble would be the fact that it's based on a formula (http://www.amazon.com/BARSKA-Power-Starwatcher-Refractor-Telescope/dp/B000LJMOYK).

There are many good reasons to prefer a slightly larger scope over a slightly smaller one, but in my view, light grasp isn't one of them.
BTW, I totally agree on the "wasted glass" bit regarding the FSQ if visual is the main purpose. A TSA 120 might be worth considering, despite the 130 FSQ having 17% more light grasp ;)

[Camelopardalis]

So let me ask you this...if incremental increases in aperture don't bring anything to the table, then why do many visual observers end up with large reflectors?

[N1]

Dunk, the answer to your question is in the first part of your question. However, I wouldn't say they dont bring anything to the table (hey, they can make the difference between splitting a double and not splitting it), but let me quote from my earlier post:

To make an image just twice the size, a telescope would need 300% more light grasp!

That means doubling the aperture (as you might expect if you want to double the image size, resolution etc), not times-one-point-one-fiving it (or whatever little difference an extra few mm achieve). You're in big tube territory pretty quickly. It's the single most important cause of aperture fever. People go up in small increments thinking they are achieving lots, but after a few nights of viewing find that they have not.

If I was to increase aperture purely for the extra light grasp, I'd consider anything less than 50% increase in diameter, e.g. 8" > 12" (or in ligh grasp terms, +125%), a waste of time, and would seriously consider simply doubling it. YMMV, my personal view, needless to say.

[Camelopardalis]

Regardless of how you quote or emphasise your text, it doesn't mean you'll beat me into submission :P

While I agree the effect from 130->150mm will not be night and day, it will still be significant...who doesn't want a bigger scope?!?

Btw, surface brightness - by definition - is dependent on the surface area of the objective, not the diameter of the aperture. As a result, the increase in light is not 150/130 = 1.15, but (150/130)^2 = 1.33...

[N1]

OK. :hi:

[Decimus]

Interesting comments, guys. I have to agree that the F5 Tak FSQ 130 is not going to offer significant visual observing benefits over the F7.7 Tak 130, and it costs virtually twice as much. What I would like to know is, as the aperture is the same, how much brighter will the image be in the FSQ than its 'optically slower' brother, the TAK 130NS? And for visual purposes, will this translate to say, the equivalent in photographic terms, of 1 f-stop? Judging by what you have said, this will not be the case as one stop represents twice as much brightness, photographically speaking (eg F8 is twice the brightness of F11 and requires half the shutter speed for the equivalent exposure value at F11). Basically, this is confusing...To add to this enlightening (;)) conversation, has anyone used or heard about the APM - LZOS Telescope Triplet Super ED APO Refractor 152/1200 CNC LW II ? Sounds like a beast but at 11000 euros, a lot of dough. I saw that it had a strehl of .997 at the short wavelength end of the spectrum - totally competitive with the Taks and the Astro-Physics scopes, if the data can be believed on the German website....Thoughts, anyone? Thanks again for all of your interesting and educational observations.
Cheers,
Richard

[casstony]

Visually, no difference at all. You just use different eyepieces to get the desired magnification or exit pupil.

Anyway, enough jibber jabber from the crowd :) just buy 3 or 4 telescopes and keep the one you like best (make 2 of the 4 a TSA120 and C11 Edge ;) ).

[Camelopardalis]

Richard, LZOS have a reputation for making top notch optics...

[Decimus]

Thanks, Tony and Dunk. I guess there comes a time when you can over-think all of this and become bamboozled with all of the options! :confused2:I think two scopes, a frac now and a reflector (later?), seem to be the go. I live in West Hobart and I have cross-city views (clear horizon to the east only) so seeing is pretty restricted. I have to journey to the AST observing site (part of the UTAS radio telescope facility), a nice spot, but with TWO scopes? Part of the fun, I guess!

Cheers,
Richard

[Camelopardalis]

Yeah it's never easy, and no single scope excels at everything...best thing I can suggest is to get your feet wet...many folk have more than one scope, different horses for courses...and opinions of course :D

[ausastronomer]

Hi Richard,

While some of the telescopes you are considering are very high quality, they are all very small instruments by todays standards. Their small aperture limits their light gathering power and their angular resolution, which is the ability to resolve fine detail. This is the reason over 90% of the worlds best visual observers use medium to large aperture newtonians. They may own some smaller telescopes but the majority of their observing is done with a newtonian.



There's no trade off, a modern properly constructed high quality truss dob in the 12" to 14" class will easily outperform any of the telescopes on your list, as a visual instrument in all performance criteria. It will be just as portable and even with Argo Navis and Servocat drives fitted, will cost about 1/2 of what you plan to spend on a small telescope and mount.

A 12.5"/F5 Teeter Classic with a Zambuto mirror, Argo Navis and Servocat would cost about $US 9,000 plus shipping

http://www.teeterstelescopes.com/#!classic/c1221

This telescope would blow anything on your list into the weeds as a visual instrument.



This newtonian is a specialised astrograph designed for imaging only. Newtonians properly designed for visual astronomy will have a central obstruction less than 20% which does not affect the light gathering ability and essentially gives the same Modulation Transfer Function (MTF) curves as an unobstructed telescope.

There is no constant need to collimate a high quality newtonian. You do it once when you set the telescope up, which takes less than 2 minutes and you observe for the rest of the night and invariably don't touch the collimation again for the entire night.

The difference in visual aesthetics viewing showpiece targets like Eta Carina,
M42, The Tarantula (NGC2070) and the big Globular Clusters like Omega Centauri, 47 Tuc, NGC 6752 etc etc in a 6" telescope compared to a 12.5" telescope is huge.

Cheers,
John B

[Decimus]

Hi, John. Thanks for the insight. I know I will probably eventually get something big, but transportability, set-up time and storage are problematic for me. Where are Zambuto mirrors made? (I remember reading about them somewhere, but I can't recall where - supposed to be excellent).Do they have their own telescopes too? I love the high contrast of refractors. The CPC Celestron 9.25" (a telescope I have used with students and which will get an airing this Friday) delivers delightful views of M42, Eta Carinae, etc, but it's like looking at a very dark blue sky, not a black one...

I guess all of this shows why so many amateur astronomers need both types of scope. Ah well. I feel so confused about it all, I am going to delay my decision for maybe a few months (maybe we'll have clear skies by then). Thanks again.

Cheers,
Richard

[N1]

What a difference 2.5" can make :P

[ausastronomer]

Well as a matter of fact it makes a huge difference. The difference is 2.7" not 2.5" BTW.

I have explained why this is so on many previous occasions, but for Richard's benefit I will spell it all out again.

Note the Secondary Mirror sizes of the 2 scopes.

250mm TAK Mewlon (9.8" clear aperture)= 72mm (2.83") Secondary.

317.5mm/F5 Zambuto/Teeter (12.5" clear aperture) = 53.3mm (2.1") Secondary

So the 250 Mewlon has a 72mm secondary for a 28.9% Central Obstruction and the 12.5" Teeter has a 53.3mm secondary for a 16.8% Central Obstruction (CO)

When you deduct the Central Obstruction and calculate the effective clear aperture of each telescope the 250 Mewlon has an effective clear aperture of 9.38" and the 12.5" Teeter has an effective clear aperture of 12.32". This equates to an effective increase in the diameter of the telescope of 31.3% and an effective increase in light collecting surface area of 72.5%. This is significant. The human eye can detect a 5% change in light intensity / brightness. The 12.5" Teeter will throw up a significantly brighter image. In addition the 12.5% will have a theoretical resolution (seeing limited) of about .45". The 250 Mewlon will have a theoretical resolution of about .6".

That's a 72.5% increase in light gathering power and a 25% increase in resolving power.

Lets introduce some additional factors. The 28.9% Central Obstruction of the 250 Mewlon will have a noticeable effect on the Modulation Transfer Function Curves (MTF). If you look at the two graphs below you will see that a telescope with a CO of 20% or less has a similar MTF curve to a perfect unobstructed telescope. Whereas the telescope with a 32.5% CO has a similar MTF curve to a clear aperture telescope with a 1/4 wave of spherical aberration. Consequently, even though a 250 Mewlon may have almost perfect optics, the best it can hope for is to perform like a 1/4 wave telescope, due to its large central obstruction.

How does this affect image quality? Here are some links to some simulations of the effect on image quality of different aberrations and aperture by world renowned photographer Damien Peach. When you look at these simulations keep in mind that we are not comparing a 25cm telescope against a 30cm telescope but against a 31.7cm telescope.

http://www.damianpeach.com/simulation.htm

Lets introduce some more factors. The 250mm Mewlon has 2 additional air to glass surfaces compared to the 12.5" Teeter. The corrector and the star diagonal, which further reduces light gathering ability and introduces additional diffraction effects.

It's a no brainer as a visual instrument. If you want to take pictures, that's an entirely different argument.

If you don't believe me just ask your wife if she would notice something which increased by 31.3% in diameter.

Cheers,
John B

[glend]

Assuming seeing conditions allow you to enjoy the difference. Often that's the equaliser.

[Rodstar]

Building from Ausastronomer's final remark, I think even in poor seeing conditions (lights off) your wife would notice a considerable difference. :lol:

[Decimus]

Thanks for the insightful (and sometimes even 'inciteful' comments guys :D), and for the humour. Jokes aside, however, the telescope will reside in the lounge room, not the garage, so conflict with the wife is inevitable should a C11 or equivalent occupy the space. Come to think of it, a Tak, Tak Mewlon or SW Esprit 150 on an iOptron CEM60 would be equally space-hungry...It's a no-win. Hell, I may end up in the garage...

Cheers,
Richard