The Strehl ratio

h0ughy (David) · #1 12-09-2005, 12:38 PM

http://www.rfroyce.com/standards.htm

In my quest for optical perfection

I have encountered a term I was not familiar with (amoungst the several hundred) and decided to find out what it was and how it would effect my scope purchase. I was looking at a scope with values of 1/6.2 wave, 94% Strehl, resolution 0.890 arc-seconds.

Now there are several Aficionados out there, can they help fill in the blanks?

ausastronomer (John Bambury) · #2 12-09-2005, 12:53 PM

Houghy,

For a multiple Optic System thats outstanding. You would be happy with that in an Apochromatic Refractor.

In addition Nat wouldn't have quoted the figures if it wasn't.

I am not 100% sure what the difference is between the 500 and 503 model but I don;t think its much. Same horse different jockey I guess. I think the 500 model was what was released to the European Market and the 503 was the model released to USA. Regardless the 500 model sells new in the UK for about £650 to £700 and you have no guarantee to get 1 this good optically. What part of "BUY THE SCOPE" are you not getting

hehehe

If you don't hurry I am gonna buy it and I am serious

CS-John B

ausastronomer (John Bambury) · #3 12-09-2005, 01:02 PM

Sorry Houghy I should elaborate on the Strehl ratio a little. It is basically the % of light that is concentrated into the airy disk from the theoretical maximum of 100%.

In Bob Royces table at the very bottom of his article he indicates that a strehl of .94 is very good and anything over .95 is excellent. Remember of course that in Bob's table he is talking about the primary mirror only (1 optical surface) . That Mak Cass has a "system" strehl of .94 after encountering 3 optical surfaces, the corrector plate, the primary mirror and the secondary mirror, so you can't ask for any better than that, its right at the top of the tree.

CS-John B

h0ughy (David) · #4 12-09-2005, 01:06 PM

Quote:

Originally Posted by ausastronomer

Houghy,

For a multiple Optic System thats outstanding. You would be happy with that in an Apochromatic Refractor.

In addition Nat wouldn't have quoted the figures if it wasn't.

I am not 100% sure what the difference is between the 500 and 503 model but I don;t think its much. Same horse different jockey I guess. I think the 500 model was what was released to the European Market and the 503 was the model released to USA. Regardless the 500 model sells new in the UK for about £650 to £700 and you have no guarantee to get 1 this good optically. What part of "BUY THE SCOPE" are you not getting

hehehe

If you don't hurry I am gonna buy it and I am serious

CS-John B

Thanks John, I have been hunting high and low for information on these scopes. it sounds good to me and I intend on purchasing the scope. I spoke with Nat today. i also pointed him to iceinspace as he had not heard of it. i hope we get another member. with just about every purchase I do I refer or ask if they have heard of iceinspace, have had a few people look at the site, and join I think.

Anyway, fear of the unknown is riding me at the moment as well. i am tossing up as to this scope and the one from Andrews Comms (8" MC for $1199) it is double the price, but 8" as opposed to 5". but quality and portability and usefullness, ruggedness and all the rest of the nesses

are coming into play. What does a guy do?

ausastronomer (John Bambury) · #5 12-09-2005, 02:32 PM

Quote:

Originally Posted by h0ughy

I am tossing up as to this scope and the one from Andrews Comms (8" MC for $1199) it is double the price, but 8" as opposed to 5". but quality and portability and usefullness, ruggedness and all the rest of the nesses

are coming into play. What does a guy do?

Think of the decision like this Houghy, you are deciding between an 8" TASCO achromatic refractor for $1,200 and a 5" Astro Physics Apochromatic refractor for $600. On the basis that you already own a 12" LX 200 I think you would get a lot more use out of the smaller scope and its quality is infinitely superior in every respect to the Chinese made scopes.

CS-John B

h0ughy (David) · #6 12-09-2005, 02:47 PM

Done deal!

now it will rain!

atalas · #7 12-09-2005, 05:13 PM

Oh well there is no point in me putting in my 2cents worth ! congrats are in order David .
You will get some great shots through your Mak-newt and the stars through It will blow you away ! I must add David that you should really send It to me when you get so I can
check It out for you.....as a favour of cause .

Louie

h0ughy (David) · #8 12-09-2005, 07:17 PM

Quote:

Originally Posted by atalas

Oh well there is no point in me putting in my 2cents worth ! congrats are in order David .
You will get some great shots through your Mak-newt and the stars through It will blow you away ! I must add David that you should really send It to me when you get so I can
check It out for you.....as a favour of cause .

Louie

please do louie, it is worth more than gold mate

#9 13-09-2005, 01:48 AM

Wish I could find an AP 5" APO for $600, I think it should be closer to $6,000, even then there is the 6yr wait..

ballaratdragons (Ken) · #10 13-09-2005, 08:02 PM

Interesting that you ask about Strehl Ratio Houghy.
When I was looking around before I bought my scope I came across the term back then.

Bintel was one of the shops that mentioned it in relation to the GS 12". Even though I ended up getting mine from Andrews I was impressed at the GS Strehl rating.

It is reported, according to Bintel, that GS 12" optics train is between 9.4 and 9.6

That sent me off on a Google search for what 'Strehls' are! After reading up about the Strehl Ratio I am more than happy to know that GS mirrors are of a very high quality.

Congrats to the GS factory for pumping out mass-produced optics at such a high Strehl Ratio.

Some other brands were lucky to make 9.2 on the chart I was looking at. Apparently that level is not good.

h0ughy (David) · #11 13-09-2005, 09:11 PM

It is all a new thing to me but looking at the article that describes it it does seem to make some sense. But it does need to be done for each individual scope. At least with the one I am getting it has been certified and that is realy what made the difference for me, knowing that I was going to get a quality scope.

mch62 (Mark) · #12 14-09-2005, 08:41 AM

The GS optics tend to be of better quality than the chinese optics.
Taiwan is attempting to out do there over powering neighbours in quality controll to win market share .They use an international QA standard system.
A political thing if you understand the problem over there.
I think every one knows the hit and miss of QC of Chinese products.
The Stehyl ratio quoted is that for the mirror only or the complete optical system?
Now all they have to do is get the machanical quality up a bit better.

ausastronomer (John Bambury) · #13 14-09-2005, 01:10 PM

Mark,

GS have always claimed their Optics to meet or exceed 1/12th wave, this is for the Primary Optic only. This was sprouted on their website some time ago and is still listed on their website.

http://www.gs-telescope.com/

(Click on Dobsonian)

Jim Sheng owner of GSO also commented on the Skyquest Telescopes Forum in 2003 in regard to this, in response to a post by Robert Bruce Thompson, author of the recently released publication "Astronomy Hacks"

http://groups.yahoo.com/group/skyque.../message/14046

I have not seen GS quote their optics as .94 strehl and I assume that feedback has come from Bintel or Andrews who have access to this test data either independently or from GSO.

It sounds about right as GSO quote a "minimum" 1/12th wave Peak to Valley surface accuracy which equates to a wavefront error of 1/6th wave at the wavefront which according to Bob Royces table is equivalent to a Strehl of .92. I have used a reasonable number of GSO mirrored scopes and all of the mirrors are very good with a couple being outstanding, particulalry so considering they are mass produced optics.

CS-John B

Starkler (Geoff) · #14 14-09-2005, 07:03 PM

Quote:

Originally Posted by mhodson

The GS optics tend to be of better quality than the chinese optics.

A few years ago I used to own a Saxon dob, 8 inch f5. Whilst the views were nice enough a star test showed obvious spherical abberation. I have heard this is par for the course with this model.
On the other hand my GS dob showed an identical set of diffraction rings both sides of focus on a rare night of exceptionally good seeing. The GS mirror is way better !

Still I do miss the little 8inch f5 sometimes, as it takes up much less space than the 10 and could possibly travel with me on work trips.