Obsession 12" vs General 12" Dob view

[mbaddah]

Hello

I've been doing bit of research lately on the obsession telescope, and they claim that their mirrors is the equivalent of gaining 2" extra of aperature (i.e. 12" obsession is equivalent of 14" general dob).

Has anyone been able to verify if this is true or not? Do views in 12" Obsession excel those in standard 12" dob given same atmospheric conditions, eyepieces etc.. Thanks!

[stringscope]

Hi,

First up, I must say I am no expert in this area. However, I have had some exposure to mirror refurbishing issues and I hope the following comments may be of some assistance.

Hopefully Satchmo might hop in with some expert advice on mirror coatings.

While you have specificaly mentioned Obsession, I guess the core of your question relates to the practicality of the various "enhanced" coatings vs the more basic Al and protected Al.

In the past there has been some discussion on the web re the longevity of enhanced vs standard coatings. By longevity, I am refering to the ability of the coating to retain its specified reflectivity level over time. My limited understanding is that enhanced coatings tend to loose their higher reflectivity levels over time and in the longer term you end up with similar levels to basic protected Al.

Bearing in mind (as far as I am aware) there is no facility available in this country for renewing such coatings, you might might want to look at the balance between coating longevity vs benefit vs cost vs local recoating capability.

In addition I understand some types of "enhanced" coatings cannot be stripped in the standard manner using an etching liquid and the only way to remove the old coating is to re-polish the mirror.

Cheers,

[Starkler]

I have also read that 'enhanced coatings' scatter more light and there has been some debate about this over on CN, with planetary viewing purists preferring standard coatings.

[Omaroo]

That's a pretty big claim eh?

12" mirror vs. (virtual) 14"

For and extra 16% on diameter we'd get an extra 36% in area (454 sq. in vs. 615 sq. in for the 12 & 14 respectively).

Are they claiming a 36% increase in reflectivity for the same 454 sq. in area because of their special coatings? Oh come on....

[MrB]

It is a big claim.
I could have the wrong logic here (and the wrong numbers!) but I've read that standard ali coating is about 88% reflective.
a 14" mirror with 88% reflective coating is on a par with a mirror of 13.13" with an impossible 100% reflective coating!
So.. the 12" would need 120% reflective coating!!

Factoring in the secondary aswell, 0.88x0.88 = 77.44% reflective
A system with 100% reflective primary and secondary still needs 12.32" to match a 14" with standard coatings.

Did I get something wrong here?

[mbaddah]

Wow thats alot of info thanks for that guys... would there be other reasons why they would consider their 12" equivalent to a standard 14" mirror besides how reflective it is?

I have the pamphlet at home if you like i can scan it in (I didn't see it listed on their website). Cheers.

[gary]

Gary Kopff from Wildcard Innovations here.

First a disclaimer. Obsession Telescopes is a customer of Wildcard Innovations
and these days a very high percentage of their telescopes would ship with
one of our telescope computers.

Having had the opportunity to use many telescopes, I wanted to take the
opportunity to attempt to answer your question.

In the above statement, I used the expression "use" rather than "look
through". I have "looked through" more scopes than I have "used".

Please allow me to explain. The difference between a generic Dob and
a premium Dob, such as an Obsession, in using it is chalk and cheese.
Looking trough it is just one, albeit important part, of using it.

Let me draw a quick analogy. Perhaps you have been lucky enough to
own or drive a high performance sports car. Superficially, when comparing
the performance car to the commodity family sedan, both may have
four wheels and six cylinders, but when you get behind the wheel
and put the foot down on the accelerator or steer it or brake, the
similarity ends there. :)

Scopes such a the Obsession purposely come with thin mirrors so they
cool down quickly. These are then augmented with a well engineered
mirror cell and sling arrangement to keep the mirror from flexing.

Premium Dobs are designed with good engineering principles in mind
and in that regard, usually perform better that the generics. For example,
its low centre of gravity, buttery smooth movement and options such as
FeatherTouch focuser make the Obsession a joy to use, rather like
getting behind the wheel of a really nice car. On some makes
of scopes, if you push them and their movement is stiff, or if their mirrors
are so thick they take all night to cool down, then these add distractions
from observing.

Observing is not all about what is happening optically, The brain plays
an incredibly important part in the visual system. Distracted by a scope
that does not perform beacuse it overshoots the target when you nudge
it or it doesn't move at all, or still has not cooled down is as distracting
as what it would be like trying to race in a F1 Grand Prix when your car
is made in China and the gears stick. The generic sedan may get be good enough
to get you to work, or to the shops, or take you on holidays, but it will never
be an F1 winner. :)

My suggestion is to try and get to a start party such as the forthcoming
South Pacific Star Party and compare for yourself.

For example, there are many that suggest that a scope fitted with a ServoCAT
slew and track system is like adding a couple of inches of aperture.
I tend to agree. On those nights when the seeing is good and the power is
cranked up, observing the detail on Jupiter or the structure of a galaxy
is made all the easier when the brain is not distracted by having to
keep nudging the scope.

Highly recommend Dave Kriege's book, the Dobsonian Telescope.
One quickly sees that a good scope is something that is engineered.

Like a high performance car, you get what you pay for.,

In the blockbuster movie, Apocalypse Now, Willard replies
to Kurtz with regard his methods, "I don't see any method at all".
Sometimes I look at the design of some generic scopes out there and those
words pop into my head. :)


Hope the above helps.

Best Regards

Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au

[Omaroo]

It still sounds as though the claim is, in simple terms: That imparts, to me, that the optics perform as though they are 36% larger, not that it's "nicer" to use as such. They haven't said it's a "36% better user experience".

mbaddah - could you scan that brochure?

[Starkler]

Whilst Gary has given a good description of the advantages of choosing a premium made scope over budget models, I would just like to comment on the statement above.

The obsession website states 2" thick mirrors for the 15 and 18 inch models, and "full thickness" for the 12.5" model. The marketing blurb appears to spin thick mirrors as an advantage, but modern telescope design is now more about thin mirrors which cool more quickly.

When I asked Mark Suchting to make for me a 15" diameter 2" thick mirror, he strongly suggested that I take a 1.6" thick one would cool much more quickly (40% quicker i found out from online data) and be lighter in weight. The reduced weight does however necessitate taller rocker and mirror boxes to accommodate higher mounting of the altitude bearings to achieve balance.

[Satchmo]

That applies only if you are dropping a thinner mirror into a scope allready designed for heavier , thicker mirrors. The best solution to balnce in using a thinner than 2" mirror is to make the top-end a fraction lighter without compromising stiffness. You can also just add a little weight to the mirror cell though I am not in favor off adding mass to a telescope.

There is typically a 5:1 moment arm leverage on a modern Dob. You don't need to shave off much weight off the top end to make up the difference in balance between a 1.6" and 2" mirror.

[ausastronomer]

Hi,

Lets forget the marketing blurb and look at the practical realities of the situation when comparing a premium truss dob to a generic dob. I have an Obsession and a generic dob and have used lots of other scopes from both generic and premium categories.

The Obsession has either an OMI mirror or Galaxy mirror with enhanced coatings which offers two advantages over a generic dob. A premium grade optic and enhanced coatings. You will read negative publicity about enhanced coatings and increased scatter. I think most of this stems from when enhanced coatings were in their infancy and the processes being developed and improved. I can't fault my enhanced coatings and will order them each and every time if offered by the mirror supplier. What do they gain you? Its hard to say and quantify. I can say my 18" with enhanced coatings equals or betters Andrew Murrells 20" which has older standard coatings for target brightness. The premium optic gives improved contrast over a generic optic which gives a perceived aperture increase. Again the magnitude of this is difficult to quantify.

Thirdly having an Obsession with smoother movements than a generic dob or better still motorised tracking via servocat, again gives a very real perception of increased aperture.

All of these things add up to a very real gain in the observers eyes of increased aperture and more importantly slightly improved image quality. That proportionate perceived increase in aperture generally costs proportionately a lot more than the performance increase over the generic scope however.

In simple summation. You definately see more in the Obsession, or any premium scope, than you do in the generic scope because of a combination of the 3 factors I have mentioned. How much, enough to be noticeable. Is it worth the extra cost? Only you can answer that.

Cheers.
John B

[Satchmo]

John

There must really be some issues with the coatings on Andrews 20" secondary and primary. I've never seen a good 18" equal a good 20" regardless of coatings. I've personally never had any luck with longevity on enhanced coatings, and theres no affordable source of enhanced recoating in Australia.

[ausastronomer]

Hi Mark,

There aren't any "real issues" with the coatings. They are about 12 years old and it's probably about time the mirror was re coated. There is nothing wrong with them. If you didn't have another reference scope beside it, you wouldn't pick the coatings for older ones.

It's a combination of both factors. The enhanced coatings offering a slight brightness increase over standard and Andrew's older standard (non damaged) coatings being down a fraction on what a freshly coated 20" would deliver.

Cheers,
John B

[mbaddah]

Again thanks Gary and the rest of you guys for the advice. Indeed when I watched the obsession video I was amazed with how easy they looked like they were moving their dobs compared to my standard 6"! I know all too well about been comfortable when observing and how important it is (definitely worth the extra money), hence the reason I'm saving for one.

Perhaps your words Gary (and ausastronomer) should be mentioned in the brochure i received from Obsession. I have attached the part I mentioned about gaining 2" of aperature due to it been more reflective. To a beginner like me I would not have known the article meant what you said Gary unless they explicitly mention it.

Have I misunderstood the ad? Cheers all.

[Satchmo]

Most scopes have enhanced secondaries these days and for example high end mirror with a correctly applied quartz overcoat ( about 90% reflectance ) gives a total relative aperture drop at 12.5" aperture of 0.37" compared to the Obsession 94% transmission example ( eg 12.12" compared to 11.75" ). No coating coating `gains' aperture. A mirror with an idealised perfect coating can only approach its nominal aperture in transmission not exceed it.
I think the example in the Obsession ads is more comparison with cheap scopes such as the Taiwanese dobs ?

[Stephen65]

I've looked through several generic Dobs and through a Dob with a premium (Mark Suchting) mirror. The premium mirror dob had tighter stars and better contrast. I know that's not the the same as a brighter image from a larger aperature but I'd take slightly dimmer with better sharpness and contrast over slightly brighter with grayer background and blobbier stars.

The other factor is that increases in perceived brightness don't go up linearly with aperature, for example, last weekend I was using my 10" and the ASV's 25" reflector. The 25" gathers about 6 times as much light, but that doesn't make targets 6 times brighter, it makes them noticeably brighter, but my scope had better sharpness and contrast. Where the 25" comes into its own is with the faint targets that are on the edge of a 10" scope's capacity.

[MrB]

Interesting, thanks for the scans mbaddah.
Their blurb definitely claims that it is the enhanced coatings that give an extra 2" of aperture over a 'standard' coating, Not some perceived gain from the 'experience' of using a 'premium' scope.

Not sure whether the primary is 12 or 12.5"? doesn't really matter...
96% primary coating and 98% secondary, 0.96x0.98 = 0.9408 or 94% total system reflectivity.
94.08% of a 12" = 11.64" equivalent perfect mirror(100% reflectivity.)

If a 11.64" 100% reflective mirror is worked out for a standard coating's 77.44% total system reflectivity(0.88x0.88 = 0.7744), the equivalent aperture is: 13.23"

So a relative gain of 1.23" of aperture.
Or, 30% more light reflected over a 'standard' coated 12"

[dhumpie]

On a side note the Vixen made Celestron newtonian's from the early to mid 80's made such claims as well, of course in comparison with the cheaper scopes from the far east. I think they were claiming this...two scopes side by side, one a premium and one a mass produced one with standard coatings clearly show visual differences at the eyepiece. I once had my Vixen/Celestron with its enhanced coatings next to a standard 6" f/5 chinese made scope and there was clearly a difference in brightness. In fact I can boldly claim that the brightness in this scope nearly equalled an 8" (well in comparison with an 8" SCT that is) so there is reason to believe that there is a difference and I believe what Dave at Obsession claims...

Darren

[Satchmo]

Talking about apparently larger relative apertures is erroneous because larger aperture implies greater resolution which a 12" cannot acheive regardless of how good the coatings are.

This is why I relegate the `2" more aperture ' as a piece of advertising spin...The only 'scopes around these days with all standard coatings are Taiwanese for about 1/5 the price, so I hardly think people in that budget range are going to swayed towards a premium dob due to a differnece in coatings.

[Satchmo]

Darren, Did you have each scope operating at a magnification giving the same exit pupil diameter? If not your impressions may have been erroneous.

Pre enhanced C8's before the 'Starbright' coating and magnesium flouride coated correcter had very poor transmission. Counting the star diagonal , 3 reflections at 88% as well as 4% loss each side of the correcter plate, plus a 40% secondary obstruction at the baffle.

This amounted to 0.88 X 0.88 X 0.88 X 0.92 X 0.86 = 0.54 or 54% total transmission at the eyepiece.

I'll never forget my first C8 in 1979 for which I'd cooked about 20,000 hamburgers to pay for. My homebuilt 6" F8 reflector blew it away :lol:

[MrB]

I totaly agree.
They'd be stupid to claim the resolution of a 2" larger aperture so I figured they meant increased light 'gathering' for a 14"-like 'brightness' of image.

[ausastronomer]

Hi Steven,

You are making some assumptions here which are not quite correct, based on a small amount of exposure with large aperture scopes. I have no doubt that you have correctly reported what you saw on that night at that time, however, that is not necessarily representative of the situation all the time.

Scopes in the 15" and larger aperture range suffer logarithmically increased image deterioration as a result of less than ideal conditions. This is because of the size of the "air cells" in relation to the aperture of the scope. Those deteriorating factors include "poor seeing" and the mirror not being at "thermal equilibrium". The thermal equilibrium issue is further compounded by the fact that the cooling time of a large mirror increases logarithmically in relation to its thickness/diameter/mass ratios. In simple terms, the bigger the scope the more it is affected by bad seeing, the longer it takes for the mirror to properly cool and the more noticeable the effects of a non cooled mirror. These effects are far more noticeable as magnification is increased.

I am sure you are aware that as the scopes' aperture increases its resolving power increases and the diameter of the airy disk (the size of the star you see) decreases as a result of the increased resolution.

For instance the size of the airy disk and Dawes Limit (resolving power) for some example apertures are:-

4 inch scope, 2.8", 1.1"
8 inch scope, 1.4", .57"
10 inch scope, 1.1", .46"
12.5 inch scope, .88", .36"
15 inch scope, .73", .31"
18 inch scope, .60", .25"
25 inch scope, .44", .18"

NB: The scope aperture is followed by the Airy disk diameter followed by the Dawes Limit.

What this means is that you need progressively better conditions for the scope to perform at its best and approach its resolution limit and give it's tightest star images.

As you can see from the data above a 4" scope will deliver its tightest stars with 3" seeing and best resolution in seeing that is just worse than 1". Further, being a small aperture instrument it will cool quickly. A 10" scope sits in limbo. It will deliver its tightest star images with seeing just worse than 1" and it will resolve to its theoretical limit with .5" seeing. An 18" scope needs .6" seeing to deliver its tightest stars. A 25" scope needs .44" seeing to deliver its tightest stars.

Why you perceived that the 10" scope gave sharper images with better contrast than the 25" scope was that the seeing and thermal equilbrium conditions were better than needed for the 10" to do its best, but clearly not close to the level needed for the 25" scope to do its best. I might add that this is a common occurence. Particularly early in the night while the large scope is still cooling and the seeing is still settling. Unfortunately it can sometimes take many hours, even all night for scopes in the 18" to 25" range to thermally stabilise if the temperature is dropping rapidly all night. This is specifically the reason several of the premium mirror makers worldwide are moving towards slightly thinner large mirrors.

You can take it to the bank that when the 25" scope is well collimated, properly cooled with good seeing, it will absolutely smash the 10" scope into the weeds in every single aspect, not just its ability to go deep and pop faint fuzzies. You will see lunar and planetary detail that it is physically impossible to see in a 10" scope. You will also split double stars that are beyond the physical limits of a smaller scope. This of course assumes both scopes to be of similar optical quality. I have seen this countless times. I regularly observe with my 18" Obsession beside a friends 25" Obsession and I regularly use smaller scopes from 4" to 12" aperture of high quality, including Astrophysics and Takahashi APO refractors.

Cheers,
John B

[dhumpie]

Yup I was using similar magnifications but what you say about the C8's having differing coatings may be true :)

Darren