Which scope?

[N1 (Mirko)]

Originally Posted by 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?

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 (Dunk)]

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

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 (Mirko)]

Originally Posted by Camelopardalis

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

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...

OK.

[Decimus (Richard)]

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]

Originally Posted by Decimus

how much brighter will the image be in the FSQ than its 'optically slower' brother, the TAK 130NS?

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 (Dunk)]

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

[Decimus (Richard)]

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! 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 (Dunk)]

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

[ausastronomer (John Bambury)]

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.

Originally Posted by Decimus

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.
Cheers,
Richard

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.

Originally Posted by Decimus

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.

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 (Richard)]

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 (Mirko)]

Originally Posted by Decimus

I have narrowed down my choices to:

[...]
3. The Takahashi Mewlon 250 CRS
[...]

Originally Posted by 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 ....
[...]
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.

What a difference 2.5" can make

[ausastronomer (John Bambury)]

Originally Posted by N1

What a difference 2.5" can make

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 (Glen)]

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

[Rodstar (Rod)]

Originally Posted by glend

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

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

[Decimus (Richard)]

Thanks for the insightful (and sometimes even 'inciteful' comments guys ), 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