Which scope?

[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 (Colin)]

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 (Richard)]

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 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 (Brett)]

Originally Posted by 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 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...

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 (Dunk)]

Originally Posted by 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.

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 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 (Brett)]

Originally Posted by 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 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...

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]

Originally Posted by 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 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...

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]

Originally Posted by UniPol

SkyWatcher lists the weight of the Esprit 150ED OTA at 11.5kg .

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

[Eden (Brett)]

Originally Posted by 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.

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 (Dunk)]

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

[Decimus (Richard)]

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 (Richard)]

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 (Dunk)]

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 (Mirko)]

Originally Posted by raymo

We've come full circle here, back to Dunk's pointing out that the 150 has
33% more light grasp than the 130.

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 (Dunk)]

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

[N1 (Mirko)]

Originally Posted by 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

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.

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 (Dunk)]

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?