Takahashi FS152 VS. Starmaster 14.5″ Hybrid

[mbaddah (Mo)]

I came across the following review:

http://danielmounsey.com/

He claims the Takahashi outperforms the Dob due to thermal heat emanating from the viewers body and had the focuser been on the opposite side this would not have happened?

I'm a little confused and wonder how much truth there is to this statement. Would appreciate if anyone can shed some light on the matter, thanks.

[Gama]

You have to look at what magnification they were compared at as well.

But, just wait an hour or so, before popping outside to start your viewing with the Dob. It will be a totally one sided affair.
Just cant beat large "Good quality" Dobs.

Theo.

[gregbradley]

I owned an FS152 for years, it was my first expensive astro purchase.

It gave even to this day some of the finest visual views I have seen.

Fluorite doublet is a good formula for visual if not for imaging.

It would cooldown in no time and I was blown away by the initial views when I first got it. They were way brighter than I expected.

I haven't looked through a lot of Dobs but the guys point makes sense to me. Compound scopes are vulnerable to tube currents (micro seeing environment) that affect their ability. I had a long discussion with Anthony Wesley about this at Parkes and he has the same view on this. He of course is at the very top of his profession so his views are valuable.

6 inch refractors are also renowned for cutting through the seeing. Mike Sidonio and Marcus Davies both image with 6 inch refractors and they continually show how these high end APOs can cut through the seeing etc and it takes a far larger scope to outperform them. Even then that would be more affected by the seeing.

So much so that Roland Christen has said at times that the sweet spot for scopes is a 6 inch refractor.

The FS152 would be far less affected by the heat issue and that is the guys point.

Greg.

[mbaddah (Mo)]

Originally Posted by gregbradley

I owned an FS152 for years, it was my first expensive astro purchase.

It gave even to this day some of the finest visual views I have seen.

Fluorite doublet is a good formula for visual if not for imaging.

It would cooldown in no time and I was blown away by the initial views when I first got it. They were way brighter than I expected.

I haven't looked through a lot of Dobs but the guys point makes sense to me. Compound scopes are vulnerable to tube currents (micro seeing environment) that affect their ability. I had a long discussion with Anthony Wesley about this at Parkes and he has the same view on this. He of course is at the very top of his profession so his views are valuable.

6 inch refractors are also renowned for cutting through the seeing. Mike Sidonio and Marcus Davies both image with 6 inch refractors and they continually show how these high end APOs can cut through the seeing etc and it takes a far larger scope to outperform them. Even then that would be more affected by the seeing.

So much so that Roland Christen has said at times that the sweet spot for scopes is a 6 inch refractor.

The FS152 would be far less affected by the heat issue and that is the guys point.

Greg.

Very informative thanks for that. I think I know what my next purchase will be after my SDM arrives

[GrahamL]

So if the focuser is moved to the opposite side
( I don't get this bit at all )does then the 14.5 "newt kick some butt
on that dinky little finderscope

[rmcconachy]

I could be completely wrong (in which case I hope somebody corrects me!) but I believe the focuser issue Daniel is talking about relates to where the observer stands versus the direction from which the wind most often blows.

Let's say you live in the continental USA and the wind blows most commonly from the west. Most of your observing time will be spent looking at objects between the southeast, the southwest and the zenith. If, like most Dobs, your scope has its focuser placed on the right hand side as seen from behind the scope then you will spend the bulk of your observing time positioned on the western side of your telescope. While you observe the prevailing wind will tend to blow air warmed by your body across the light path of your scope (through the trusses and across the front). This warm air plume will decrease the local seeing within your scope and reduce the quality of the images you see through it. Of course, the whole scenario is reversed if you happen to be looking at something in the northern sky, e.g., Ursa Minor, and goes to pot if the wind is blowing from a different direction or not blowing at all.

In southern Australia I guess the wind also blows mostly from the west (more northwest in summer and southwest in winter?) but we are mostly looking between the northeast, the northwest and the zenith with our scopes. Therefore the common focuser position is good for this location.

How big a difference the above makes in the real world I don't know. Sticking your warm hand in a scope's entrance shows very obvious heat plumes so that part seems sound, I'm mostly not sure about how consistent the wind direction is, especially in Melbourne! Also, a shroud may help deflect warmed air over and under (rather than through) the scope but it does nothing for any warmed air passing in front of the scope.

[ausastronomer (John Bambury)]

I am yet to see a 6" refractor of any quality that can remotely come close to equalling the performance of a "good quality" 14" newtonian, in any aspect of visual observing, on a consistent basis.

Truth is, a 6" refactor will not outperform a 10" newt, let alone a 14".

Cheers,
John B

[gregbradley]

I am not that experienced looking through Dobs just at star parties a few times and an observatory and I would have to agree with you there John from my limited experience.

I liked the views from my Celestron 11 GPS. The FS152 was about as bright but much better contrast and sharpness than a Meade LX90 so if that helps position it better. Better views than a Meade LX90 not as bright as a
Celestron 11 but of course sharper and therefore more pleasing but more detail on planets from the 11 inch SCT.

The FS152 was better visually than a Tak BRC250 or a 12.5 inch RCOS but then the large secondary made them somewhat unpleasant as visual instruments giving a dull look to objects (they are designed to be efficient imagers).

Refractors have a lovely look to them visually that isn't quite captured with compound scopes. But of course aperture rules. APO's contrast and sharpness puts them higher than their aperture would indicate. But no good for dimmer galaxies but then is any amateur scope really?


Greg.

[rmcconachy]

I agree with your message John, a collimated, properly cooled, "good quality" 10" or 14" newtonian will visually beat anything that any 6" scope can do. The refractor may have a `cleaner' aesthetic to the image (which I admit to liking) but the resolution will be less and it will be much dimmer compared to the bigger scope. My post above was trying to explain the `focuser on the wrong side' theory, not suggest that the 6" would show more.

[Paul Haese]

In an open air environment a compound scope will perform better overall only if the optics are at ambient. Often people talk about refractors being able to cut through the seeing, however one can never really know what the seeing is unless the optics are at or very near to ambient temperature. This is of course where the problem lies. No scope larger than 10 inches will be at ambient inside 4 hours without active cooling. Even with fans the mirror gives off heat for hours.

I have never seen a scope affected by the body heat of a person in an open air environment. It can happen in a dome observatory. The position of the focusor is irrelevant and the real issue at hand is the heat of the mirror.

[ausastronomer (John Bambury)]

Originally Posted by Paul Haese

No scope larger than 10 inches will be at ambient inside 4 hours without active cooling. Even with fans the mirror gives off heat for hours.

That is plain and simply not correct and to be honest it's "old school thinking". It certainly holds true for "full thickness mirrors" but things have moved on a long way from there.

I have an 18"/F4.5 Obsession with a 2" thick OMI mirror and a 14"/F4.5 SDM with a 1.3" thick Zambuto mirror. Both scopes are fitted with mirror cooling and laminar fans. The 18" scope because of it's thicker mirror can invariably take forever to cool and during summer nights at elevation with rapidly falling temperatures the mirror never catches up to the ambient temperature. After a few years experience with the 18" scope and it's cooling properties (or lack of them) I set about setting the specs for the 14" mirror and scope, to address this issue. The cooling properties of the 14" scope, with it's thinner mirror, are world's apart by comparison. The mirror cools so well in fact that I quite often don't bother to run the fans for more than about 30 minutes. Similarly, some of the larger conical mirrors produced by Mark Suchting and Bob Royce in the US address the cooling issue very well.

There were many opinions these thinner mirrors, which are becoming common place today, were astigmatic, particularly when viewing near the horizon. The truth of the matter is this is a non issue if the mirror cell is properly designed and built with a stainless steel cable sling. Further, when I do observe objects low in the sky which is very rare because the air is crud down there, the seeing and atmospheric effects clearly outweigh any astigmatism that might be introduced.

In short, if you design and build the scope properly mirror cooling becomes a much less significant issue. In addition, scope cooling can be further assisted by intelligent storage, transport and scope setup practices. The 14" SDM scope is "easily" the best planetary scope I have used. My planetary scope usage range covers refractors (apo and achro) from 50mm to 378mm, reflectors from 76mm to 915mm and catadioptric and compound scopes from 102mm to 400mm. The 14" SDM with thin Zambuto mirror and 18% central obstruction wins in a canter.

Cheers,
John B

[gb_astro]

Having read the Daniel Mounsey article it is pretty obvious
that he is not saying that an excellent 6" refractor is, over all, a better
scope than an excellent 14.5 reflector.

In the second paragraph he says:
"When it comes to resolving fine details in globular clusters,
galaxies, planetary nebula, selected nebula and certain open clusters,
the 14.5″ Starmaster wins hands down."

His last paragraph begins with:
"Reflectors are still king for planets and still my favourite choice..."

What he is complaining about is his state of the art Starmaster 14.5, (Zambuto optics, wire mirror sling etc.),
being compromised by body generated thermal currents.

This seems to me to be a valid concern for anyone with an open truss type scope.
Must admit that I have never noticed it but then again I have never looked for it either.
Also of concern is his claim that thermals pass right through current light shroud materials.

Should be easy to investigate:
Orient scope at right angles to wind with focuser upwind, look for thermals through focuser,
rotate scope 180deg with focuser downwind, look for thermals through focuser.

Interesting to get some user reports on this one.


gb.

[Peter Ward]

There was a very informative piece written in S&T about this very issue, ....well....over a decade ago...I think it was something like "ten telescope myths" probably worth google-ing.

The short answer is aperture rules.

But there are a lot of caveats.

It is a good deal harder...and more costly... to make an exquisite 14" scope compared to say a 6". Apart from the optical figure, mechanical and thermal properties become a good deal more problematic.

And how do you mount a 14-20" scope really well?? Dob? Equatorial? The latter means big money.

That said, a large aperture will always collect more flux and show more detail...the Pro's use metre class optics not because 6 APO are wanting, it's simply because the physics says aperture will put you in different league.

As to whether the observer recognises this is another matter...as the aesthetics, rather than real information, of an image can often give the illusion of a "better" image.

[Paul Haese]

John you seem to know more about this issue than even Anthony or I. Active cooling is the only way to get a mirrror near ambient. Using the fans you use will get the mirror to about 2 degrees but never to ambient. Have you use a sensor on several areas of the mirror? Laminair fans are not really dealing with the problem and this in your words is just Old School. Some graphed results may change my mind, but there are only a few of us that have done this and I don't recall your name on that list.

Even thin or conical mirrors can take quite a while to cool, Anthony's graphs show that a 14 inch mirror releases heat for many hours even with active cooling. This is less of a problem in coastal areas but in reality what you are saying is well inconsistent to what we have observed and tested.

[ausastronomer (John Bambury)]

Paul,

Anthony takes things to the nth degree for "high resolution planetary imaging". This thread is discussing the virtues of 2 different telescopes for visual use only. What is needed for high resolution planetary imaging and visual observing are in fact worlds apart. I know a lot of people who use large newtonians for visual planetary observations and NONE of them have active cooling fitted to their telescopes.

John you seem to know more about this issue than even Anthony or I

Paul you aren't remotely qualified to put yourself in Anthony's class. He has been researching active cooling for years and his recent real world achievements speak for themselves. I consider you still have a bit to learn. It is only recently you spent (some would say wasted) a large amount of money on an absolutely superb telescope only to find out after you bought it that it was totally unsuited for the purpose you intended to use it. I could have told you that before you bought it. SDM 14 I believe. An outstanding visual instrument which was never going to be worth a cold frankfurt as an imaging telescope, which was the purpose you bought it for. I believe when you figured out you had made a mistake you subsequently sold it.

http://www.sdmtelescopes.com.au/SDM014.html

Cheers,
John B

[Satchmo]

Originally Posted by gb_astro

Should be easy to investigate:
Orient scope at right angles to wind with focuser upwind, look for thermals through focuser,
rotate scope 180deg with focuser downwind, look for thermals through focuser.

Interesting to get some user reports on this one.
gb.

I don't understand why the owner doesn't just put a shroud around his truss and get on with life

[Peter Ward]

Originally Posted by Paul Haese

.... Active cooling is the only way to get a mirrror near ambient. Using the fans you use will get the mirror to about 2 degrees but never to ambient. ....

My truss 14.25" RCOS typically has a 0.6 degree split.... running the fans at 80% power most of the night.

I also have an extractor fan on the side of the dome so ambient air is constantly being drawn in through the dome aperture. It all helps.

[strongmanmike (Michael)]

Back in 06 when I still lived at Mt Campbell Observatory I and a couple of other astronomers had the oportunity to directly compare views through a Meade LX90 a Synta 6" achromat, a 6" AP Starfire EDF and Meade 12" LX200GPS under rather good seeing.

Result:

The images through the 12" SCT were significantly brighter, faint objects were more easily revealed and under the good seeing the resolution on Jupiter was about equal to the Starfire but the brightness gave the impression that it was easier to see these details, at least this was the consensus (may be wrong of course).

The Starfire images were only marginally better than the 6" Synta, with very slightly more contrast and less colour fringing on brighter things (all expected).

The views of Jupiter through the Starfire were deffinitely crisper than through the larger SCT but dimmer. We could see red spot junior and detail inside the great red spot through both the Starfire and SCT but again the slightly better crispness was there in the Starfire.

Although the star images were not as perfect, we were able to split a 0.6" double star through the 12" SCT but could only manage 0.75" through the Starfire but they were two beautifull white dots with a single broken faint ring around each = optical perfection.

The views through the Starfire, as far as brightness of faint extended objects goes, were about on a par with the LX90 with perhaps a very slight edge to the LX90 when going back and forth between the scopes. EDIT: actually I just went back and had a look at my observing log, of the three observers present, two thought the exrtended objects were marginally brighter through the LX90 the other thought the Starfire (not me) - ie it was that close. Aparently the double star splitting was better through the Starfire (I hadn't recorded what stars we looked at doh!) and all three thought the contrast was better through the Starfire. Worth keeping your observing logs, huh?

In other words a high quality (the best?) 6" APO is at least as good as an 8" SCT but provides crisper more contrasty views.

I imaged through both the SCT and Starfire and I have to agree with Greg that the overall results through the Starfire are superior to the 12" SCT even with less Focal Length.

These are just observations and far from diffinitive but they are good indicators of how the range of scopes compares generally from an observers perspective.

Mike

[marki]

Originally Posted by ausastronomer

Paul,

Paul you aren't remotely qualified to put yourself in Anthony's class.
Cheers,
John B

Forgive me if I am wrong but Paul aren't you a qualified engineer? I thought I read that somewhere.

Mark

[Bassnut (Fred)]

A bit off topic from visual differences..... seems to me the importance of thermal issues are totally different and incomparable to whilst imaging. My 10" (tube) RC has 3 temp sensors and full PID fan control. It gets to less than 1 deg delta between ambient/primary within 20-30mins and the fans then go from full to coasting the rest of the night.

There are so many constantly changing variables in imaging say over a 6 hr period with dozens of subs. eg guiding, seeing, focus, that the thermal current effects of a degree or 2 diff on the primary to ambient is a relatively minor problem compared to all the other factors.

Optical differences aside (ie not counting the tighter images just due to the optics), my 12" non-fanned SCT produced similar results over long imaging runs, albiet with focusing more often due to temp changes.