8" Newts - optical figure

[chiltonstar]

Apart from Orion Optics UK, details on the exact optical figure (eg wavelength/8, wavength/12 etc) of the primary mirrors of scopes seem a bit vague/non-existant.

What are the likely values for 8" (200mm) Newtonians/dobs for common manufacturers?

Chris

[dweller25]

The mass produced mirrors are around 1/4th wave usually

[NGC 1502]

Apart from Orion Optics UK, details on the exact optical figure (eg wavelength/8, wavength/12 etc) of the primary mirrors of scopes seem a bit vague/non-existant.

What are the likely values for 8" (200mm) Newtonians/dobs for common manufacturers?

Chris

Hi Chris.  I used to get hung up on all the stated figures for optical quality.  I'm not at all saying it doesn't matter, but interpreting those stated specs is not a simple process.

When I had a Skywatcher 200p Dob, I was at my club's dark site, another member had his Orion Optics 200mm Dob with higher spec mirror. We had both been there for a couple of hours, so both scopes were cooled, and collimation was good.

We viewed Saturn through both scopes at the same magnification.  The views were very sharp and contrasty, and we couldn't pick a winner.......

Perhaps in excellent conditions at very high power, a difference may have emerged.

I'd say that most of the commonly available " Asian scopes " are quite good enough for most people most of the time.

Collimate, cool, enjoy 

I only switched to my OO Dob because it was available second hand, and at a good price from someone I know.

Regards, Ed.

[John]

One of our members had a Skywatcher 8" F/5 tested a couple of years back and that turned out to be around 1/6th wave PV. I suspect the vast majority are at least diffraction limited so  a range between 1/4 wave PV and 1/6th wave PV with the occasional peak above that and the odd "dog" would seem reasonable.

[rl]

Optical specs are an absolute minefield. What matters are total errors in the focal plane. These are the sums of errors on both the primary and the flat. Then you have to consider the wavelength of light used (which ideally is about 500-550 nm where the eye/ camera is most sensitive). OO themselves are slightly guilty of specmanship here since they measure at 632 nm (red light) which tends to flatter the results a bit...a 1/10 wave mirror is in reality closer to 1/8 because of the wavelength discrepancy...it's a feature of the laser fitted in the zygo machine.

Also most important is the area of the mirror over which the errors are present... a 2 wavelength error sounds really bad but if it's present over 1% of the surface while the rest is good to 1/10 wave then you're not going to care about it! Any general figure like 1/4 wave assumes usually that the errors are evenly and randomly averaged over the whole surface which may or may not be true on an individual mirror.

More important and informative is the Strehl Ratio....this tells you how much of the light actually ends up in the right place. 80% Strehl  corresponds to 1/4 wavelength p-v errors in the focal plane. Strehl for a Newtonian is more complicated than just the mirror value since for the whole instrument it has to include the scattering off the secondary. Typically the secondary scattering might be 15-30% drop in Strehl depending on the seondary size, which means you need mirrors with strelh better than 95% to get the whole instrument up to the 80% representative of 1/4 wave.

1/4 wave is the Rayleigh criterion to seperate 2 double stars of equal brightness with the 1st dark band between them.

As I said...it's a minefield....

This website (in french) is one of the best review sites since they actually measure the errors for the whole instrument:

http://www.cieletespace.fr/instruments

There is a box in each report with the test results and an interferogram. There are a couple of 8" scopes on the site. Hope this helps......

RL

[AlexB67]

My feeling is exactly what John said from what I read.  With regard to buying an OO, it I were buying such a scope it would be one of the main reasons to get one with a high grade mirror of 1/8 or better, otherwise for price and bang for buck would stick with a Chinese scope if buying new, unless you can easily afford or justify the price. I would be looking second hand in any case personally.

I've never had the privilege so far to look through one of the resreach grade mirrors, but I expect you would be able to tell the improved performance based on simulated picture and reports I've read. Everything else being equal in a Chinese scope in terms of aperture and f ratio, It may need a good night and skies to see it, but that would be my prediction in term of planetary details, when splitting doubles etc when putting it to the test given some time.   

For a lot of the deep sky though in that aperture range, would I buy an extra 2 inches of aperture and a standard Chinese made mirror at 10 inch versus a research grade 8 inch, hard call, but me thinks the former without ever having tested it. Of course the best of both is always nice in an ideal world :0)

[Merlin]

Does anyone know if Synta and GSO mirrors are hand-polished and parabolised, or machine produced from start to finish?

[rl]

AlexB 67 speaks a lot of truth here...

I guess there are 2 ways of looking at it:

The first says that the atmosphere becomes the limiting factor on 49 nights out of 50. In this case no scope will resolve better than 1 arcsecond so a 5" scope shows all the detail you're going to see therefore arguing between 1/4 and 1/6 wavelength on an 8" scope is pretty pointless. This is true and probably explains why the many OO-Skywatcher comparisons usually return a draw.

The other point of view says that 2% of nights will be really good and it's best to be prepared for these rare occasions with the best kit. Similarly even on a bad night you can get fleeting moments of excellent seeing when a bigger or more accurate mirror will show more. I reckon I've seen the spokes in Saturn's rings on one such night with a 14". My own scope is a 12"F/4 using OO 1/10 wave mirrors with a small secondary and on a rare good night the planetry views are indeed magic even with a scope this fast. Double stars ain't bad either...I have very occasionally resolved to 0.7 arcsec. But there again GSO and Skywatcher owners will tell you the same thing....

If you're buying GSO or Skywatcher all will be well and the metalwork will probably be better than OO even if the mirrors are marginally inferior..

rl

[chiltonstar]

Optical specs are an absolute minefield. What matters are total errors in the focal plane. These are the sums of errors on both the primary and the flat. Then you have to consider the wavelength of light used (which ideally is about 500-550 nm where the eye/ camera is most sensitive). OO themselves are slightly guilty of specmanship here since they measure at 632 nm (red light) which tends to flatter the results a bit...a 1/10 wave mirror is in reality closer to 1/8 because of the wavelength discrepancy...it's a feature of the laser fitted in the zygo machine.

Also most important is the area of the mirror over which the errors are present... a 2 wavelength error sounds really bad but if it's present over 1% of the surface while the rest is good to 1/10 wave then you're not going to care about it! Any general figure like 1/4 wave assumes usually that the errors are evenly and randomly averaged over the whole surface which may or may not be true on an individual mirror.

More important and informative is the Strehl Ratio....this tells you how much of the light actually ends up in the right place. 80% Strehl  corresponds to 1/4 wavelength p-v errors in the focal plane. Strehl for a Newtonian is more complicated than just the mirror value since for the whole instrument it has to include the scattering off the secondary. Typically the secondary scattering might be 15-30% drop in Strehl depending on the seondary size, which means you need mirrors with strelh better than 95% to get the whole instrument up to the 80% representative of 1/4 wave.

1/4 wave is the Rayleigh criterion to seperate 2 double stars of equal brightness with the 1st dark band between them.

As I said...it's a minefield....

This website (in french) is one of the best review sites since they actually measure the errors for the whole instrument:

http://www.cieletespace.fr/instruments

There is a box in each report with the test results and an interferogram. There are a couple of 8" scopes on the site. Hope this helps......

RL

Many thanks for this, and the other replies. I do love an objective test with real numbers!

Interesting how "bad" the Orion XT8 is in terms of optical figure (is this typical of cheap 200mm scopes?), and how well eg the SW 150 Mak PRO performs optically!

Chris

[Peter Drew]

I have an Orion XT8 and the optics are superb by any standard, luck of the draw for mass produced optics? 

[chiltonstar]

I have an Orion XT8 and the optics are superb by any standard, luck of the draw for mass produced optics? 

Hmmm.....It is often said that one particular manufacturing nation tends to carry out its Quality Control on the customer, unlike for example the Japanese who are famed for their in-house and excellent QC approaches. Adds a bit to the cost of course, but.....

Chris

[Astrobits]

Another site that you can look at is this one:

Интерферометрические тесты, Levenhuk, Левенгук, SkyWatcher, Celestron, DeepSky, Tal-250K, Meade

It shows an interferometric analysis of complete telescopes using auto-collimation. It's in Russian but Google will give you a translation (that also needs translating!!!).

If you are of a nervous disposition don't look at this site

Nigel

[John]

I remember reading a thought provoking piece in an edition of the magazine "Astronomy" from the 1980's. The gist of the story was that a keen amateur astronomer and member of a US astro society made his own 8" dobsonian scope including grinding the mirrors. He was very proud of his scope, used it on every clear night and saw and shared some wonderful sights with it. One day a well meaning fellow society member offered to test the primary mirror for him and, out of politeness, he agreed. 

In due course the test results came back and showed it to be a reasonable but not great amateur made mirror, around 1/3  wave PV I seem to recall. The chap who made the scope took the results quite well but started to use the scope less often and eventually the scope gathered cobwebs at the back of his garage and I believe he dropped out of the hobby.

I don't think there is a moral to the story other than perhaps that getting on with using what we have to the best of our ability is possibly better than agonising over it's technical ranking in the optical pecking order.

The article was called "Seeing the ghosts in the mirror" or something like that but I can't recall the author. I'll dig out the magazine and find it again I think.

I reckon the mass produced scopes that are on the market today are really good and cost relatively little compared to what was available to the amateur 20+ years ago. Lucky us 

[rl]

very good point...rl

[chiltonstar]

...indeed. Maybe the old maxim that "the best scope is the one you use most often" applies as well....

Chris.

[SkyGibbon]

I have an Orion XT8 and the optics are superb by any standard, luck of the draw for mass produced optics? 

Not at all. I also have an xt8, plus xt 10 and both put up great views. Acoording to suiters star testing book, they come in about 1/6 th wave or a bit better.

[AlexB67]

Not at all. I also have an xt8, plus xt 10 and both put up great views. Acoording to suiters star testing book, they come in about 1/6 th wave or a bit better.

The SWs being the same optics I am very pleased with both of my mirrors. I have no idea what mine is in terms of strehl, PV or RMS and I'll never know. it could be a 1/4 PV or 1/6 PV or it could be 1/ 20 tomatoes    

What I do know is when my 10 inch it has been out for several hours and really settled down when it gets into best performance zone it has given me some marvelous views of the planets without knowing what a better mirror would give me. I've been able to push that 10 inch to some very high mags with some excellent results on planets and moon I feel. The same is true of the smaller little heritage 130p I have, which settles a lot more quickly. Even that little puppy has allwed me to se some fine details on Mars and pushing the mag well above  200 times on one night of very good seeing recently, that is close to its max theoretical recommended mag, but it allowed me to see some extra details without getting washed away in contrast loss and other noise I didn't even think possible I could stretch it that far. 

Amazing really when you think the heritage is about 130 pounds what we are blessed with today for very little money.

[Merlin]

Let's not also forget the appalling labour conditions ( long hours, toxic fumes, no unions ) and poor pay that some of the producers of these cheap exports have to endure.