To Strehl or not to Strehl?

[Darkplague]

Hi,

I have been thinking about either getting a high quality mirror for my current dob or buying a premium dob such as obsession or starmaster in 15" format.

Seeing as I have never looked through a Zambuto or royce or pegasus before, can anyone describe how they are better than say a GSO mirror?

I know that the higher the strehl ratio and wave ratio, the better the detail you will see on planets , but how would the 2 mirrors differ on say Orion Nebula and DSO's?

If you had my 12" GSO with a strehl of say .85 side by side with a Zambuto 12" .97 strehl, and looked at the Orion Nebula and compared the 2, what would the difference be?

BTW if anyone has a premium dob 4 sale, can you message me? Noone is willing to ship on astromart for some reason.

[Robert_T]

Sounds like a question for Bird.

I'm not sure myself what "strehl" actually means though I'm familiar with the fraction of wave rating for most optics. I have a Takahashi Dall Kirkham with mirrors figured to 1/20th of a wave. It's great at high power on planets. That said, my understanding is that once you get much better than say 1/4 wave optics the returns in terms of improved images diminish rapidly. For low power work I'd be surprised it there is big gains to be made from figure accuracies much beyond this.

cheers,

[ballaratdragons (Ken)]

Bintel informed me that the GS 12" is .94 - .95 strehl rated.

[astrogeek (Leon)]

What the hell is strehl?

[ballaratdragons (Ken)]

Geek, it is a scale used to measure the performance of an optical surface.

This link will explain it better. http://www.rfroyce.com/standards.htm and scroll down to 'The Strehl Ratio'.

[Darkplague]

Originally Posted by ballaratdragons

Bintel informed me that the GS 12" is .94 - .95 strehl rated.

A Refigurer in Adelaide told me he had done quite a few of them and as standard they were around .87 strehl

I always thought that around .95 strehl was Zambuto/Royce/Pegasus territory?

[Starkler (Geoff)]

From what I hear, the difference between a good mirror and an exceptional one wont be noticable unless you also have excellent seeing conditions.

Zambuto is American, here in Australia Mark Suchting is the man to talk to if you want an exceptional mirror. He has a well established reputation here.

[ausastronomer (John Bambury)]

The link Ken gave you to Bob Royce's website is very good.

The Strehl ratio is the % of light concentrated in the airy disk and not put out into the 1st and subsequent diffraction rings. The higher the % the better. Any mirror with a strehl over .95 is outstanding. Any mirror with a Strehl over .88 is good. The GSO mirrors are very very good for mass produced mirrors with Strehl's somewhere between about .88 as a worst sample and about .96 or .97 for their best mirrors.

What you will find with a premium mirror is that the level of polish is higher than a mass produced mirror and the mirror ultimately is "smoother". This helps contrast as there is less light scatter from the mirror surface itself. Importantly also it will allow you to push the scope to higher powers when seeing permits, without any image breakdown. What does this all mean in layman's terms ? Not a lot until you get those nights of excellent seeing. On those rare dozen or so nights a year an experienced observer can pick the premium mirror. The rest of the time you would be hard pressed to pick much difference between them.

If you wish to buy a large premium mirror in Australia, Mark Suchting at Deep Sky Optics in NSW, makes mirrors as good as anyone in the world. But like anything else that stands out from the rest, as being a premium high quality item, they are not cheap, but worth the money if an oustanding mirror is what you are seeking.

http://deepsky.cia.com.au/optics/

The reason Mark is not as well known as the likes of Zambuto and Royce is that he is not American and he does not supply 7 large scale premium telescope makers like Zambuto, who now has several hundred telescopes on the ground if not a lot more.

Mark's mirrors are every bit as good as Zambuto's.

CS-John B

[Darkplague]

On the website it states he makes mirrors up to 75cm,
Im guessing he would also make the larger mirrors 15" +.

Ill try him and see the price difference. Ive got a quote from US for US$2250 for a Torus 15" mirror. This seems a little high when you still have to add on customs after as well

[ausastronomer (John Bambury)]

Originally Posted by Darkplague

On the website it states he makes mirrors up to 75cm,
Im guessing he would also make the larger mirrors 15" +

75cm is 30" how much bigger do you want it ?

Originally Posted by Darkplague

Ill try him and see the price difference. Ive got a quote from US for US$2250 for a Torus 15" mirror.

This seems a little high when you still have to add on customs after as well

I have heard several bad reports about some Torus mirrors being of marginal quality and in fact issued with inaccurate optical quality certifications, so I would steer well clear of them. I could try to find the article if you like and send it to you.

Why compare the price of a Torus mirror with one of Mark's mirrors? the quality is not comparable so why should the price be comparable ? Mark is arguably 1 of the top 3 mirror makers in the world, Torus may make the top 300 on a good day.

CS-John B

[janoskiss (Steve H)]

Originally Posted by ausastronomer

75cm is 30" how much bigger do you want it ?

Well, that's almost big enough! You need to have magnification of about 120x or more to get all that light in your eyeball. I suppose a 1-1.2m mirror might be in order for specialised visual use (fainter galaxies & planetaries) in the 200x+ range. Any bigger than that is for CCD/astrophotography use only.

[Darkplague]

Oops, for some reason I read it as 7.5cm
Doh!

Anyway I havent heard of this Mark guy up until today so cut me some slack!


I have emailed him anyway and well see what he comes up with.

So far I am thinking of an astrosystems 15" dob kit. This comes in at $2300 plus
whatever mirror I can source.

[davidpretorius]

is mark = the username lambda????

i have a feeling birds 13" is getting made there

[ausastronomer (John Bambury)]

Originally Posted by Darkplague

Anyway I havent heard of this Mark guy up until today so cut me some slack!

No problem, I can appreciate that you haven't heard of Mark !.

My puzzlement was more at why you contacted Torus in the USA. What made you approach them ? Did someone recommend them to you ?

If I was considering contacting a US mirror maker, there would be 20 I would contact before Torus got a mention.

CS-John B

[Darkplague]

Originally Posted by ausastronomer

No problem, I can appreciate that you haven't heard of Mark !.

My puzzlement was more at why you contacted Torus in the USA. What made you approach them ? Did someone recommend them to you ?

If I was considering contacting a US mirror maker, there would be 20 I would contact before Torus got a mention.

CS-John B

I contacted Torus because the well regarded Obsession telescopes use a lot of their mirrors, so they can't be that bad right?

I have only heard great things about the mirrors on Cloudynights and such.
At this stage I am only sending out feelers to get prices.

Mark emailed me with some figures and at this stage I will probably order through him and get the astrosystems dob kit to go with it.

[ausastronomer (John Bambury)]

Hi,

Just didn't want you jumping in head 1st and ending up with a dud mirror and a non local supplier. Torus changed their name from Torus to Optical Mechanics Inc (OMI)about 3 years ago, you aren't likely too see Carl Zambuto or Bob Royce do that !! Read into that what you wish. I also suggest you read this story:-

http://www.ux.his.no/saf/ej-telescope.htm

Torus obviously produce some very fine mirrors but obviously the odd lemon gets through as well. What you don't need are those issues to resolve with a supplier 10,000 miles away.

Good luck whichever way you go.

CS-John B