test report for OO 12" f6 SPX newtonian

[neil phillips]

Hi all, My scope will be delivered this week, Ive just had my first look at the test report, which quite frankly looks astounding, I'm no expert at reading these things but from what i do know, its certainly one of ( if not the best ) report Ive ever seen from Orion. It may be overkill but clearly this mirror will never be a limiting factor for lunar and planetary imaging. Only the atmosphere will be.

I'm pretty excited by this report, Though i know some have reservations about the validity of a zygo test, i Am certainly interested in what others think, if they have seen any OO reports, or any others come to that. I cant wait to get this under the night sky. Need to think about wind breakers as the scope is 8" longer than my 10" which used to blow around quite a lot.( though that was on a vixen GP all being equal I'm guessing the see saw effect will be similar to that. but hopefully not any worse considering I'm now using a EQ6, Pictures of my new setup i will probably post next week. Scary strehl ratio

[johninderby]

Hi

Congratulations on the new scope. It will give you some fantastic views. The test results are about what I would expect from an OO 12" f/6 with 1/10PV mirror. The results are within the normal range. As the size goes up and focal lengths get faster it's a bit harder to get higher Strehl numbers.

Don't let people try to put you off the validity of a Zygo test. They are deadly accurate and are the only way to verify the quality of a mirror, but the testing equipment and associated expenses mean a lot of mirror makers can't afford it and they only have the Ronchi and Foucault tests to estimate the quality of their mirrors.

Here's a quote from Optical Mechanics (they make dobs up to 48 inches) that sums it up pretty well:

"interferometer is the final impartial go/no-go point in the quality control chain. It is a completely objective and accurate assessment of the quality of the optic under test.

Although it is possible to produce diffraction limited mirrors using methods such as the Ronchi and Foucault test, it is impossible to accurately verify the quality of the entire wavefront to a fraction of a wave without interferometry. The Ronchi and Foucault tests are excellent evaluation tools during mirror fabrication as they show the general shape of the wavefront as well as localized and high frequency errors extremely well. These tests can show localized errors as small as 1/100 wave. This makes them indispensable tools during mirror figuring. However, unlike interferometry, they do not provide a means of accurately quantifying the whole wavefront because they only measure a few points. Interferometry on the other hand, is an extremely strict statistical analysis that assures the customer of a truly diffraction limited optic over its entire wavefront."

John

[neil phillips]

Hi John thanks for the information. I know its a hotly debated thing old versus new measuring techniques. Both sides can get quite hot under the collar suggesting there right. But i mentioned it befor that debate flaired up, Probably still will lol.

I hope the figures for this mirror are not what you would expect for a 12" 1/10th pv mirror as its a 1/12th pv mirror with strehl figures to match. I have seen a test report from OO on a 1/10th pv mirror on this forum. And this report to me ( im no expert mind ) looks quite a bit better in every way, which im glad about as i didnt get this figure for free.

[johninderby]

Hi

The 1/10PV figure given by OO really means better than 1/10PV, so an actual measurement of 1/11 is very common with some up to 1/12PV. Average Strehl on a f/6 version should also be between about 0.992 and 0.995. In other words your's is at the higher end and so you've got yourself a very good example.

You'l genrally find that the larger the size of the mirror and the shorter the focal length (particularily the focal length) the harder it is to get the highest Strehl numbers. For exmple I had an OO 8" f/8 that was 0.998 Strehl and now have an OO 14" f/4.6 is 0.990 Strehl and 1/11.5 PV.

The relationship between the Strehl number and the PV does vary a bit depending on just how smooth the mirror is.

Regards

John

[neil phillips]

Thanks for that extra information John i understand your comments more fully now

[John]

I looks as if it will be a cracker Neil - I'm looking forward to reading your 1st light report

At F/6 collimation will be that much easier as well - my old Skywatcher 8" F/6 was a doddle to collimate and keep in collimation compared with my F/5 Lightbridge 12".

John

[barkis]

You will be the owner of a superb optical masterpiece. We will cross our fingers you will at some time get the conditions to fully exploit it's perfection. It will be an excellent photo visual telescope, and I look to the wondeful planetary images it produces.

Ron.

[neil phillips]

Thanks Guys, the conditions for great lunar and planetary havent been to good of late so ive been reading, but that can change at any time. So my first light report hopefully wont be a report at all but a series of photos heres hoping for good seeing.

[tom.yates]

One thing about OO the mirrors are superb even if the build quality is a bit suspect but saying that they are improving the new mirror cell is great.

[albedo0.39]

At F/6 collimation will be that much easier as well - my old Skywatcher 8" F/6 was a doddle to collimate and keep in collimation compared with my F/5 Lightbridge 12".

Deep joy... so an F4.5 is going to keep me busy I gather. What did you find as the "best" way to check colimation on your LB ? Cheshire ? Laser ? Pinhole ? - I'd be interested. Thanks

[John]

Deep joy... so an F4.5 is going to keep me busy I gather. What did you find as the "best" way to check colimation on your LB ? Cheshire ? Laser ? Pinhole ? - I'd be interested. Thanks

I use a simple plastic cheshire eyepiece to get things near enough and do the final tweaks under the stars.

John

[Glasspusher]

Interferomertry is of course the last word in optical testing, but like any other test it requires experience to master it. The reliability of the test result is dependant on the number of points used in the analysis, as the strehl ratio is a measure of the total surface accuracy in theory an infinite number of data points is desirable, in reality this is not practicable. The mirror should also be rotated in order to catch any astigamtism, air turbulance in the test cavity can also affect the results. There is one test that can equal interferometry, that is looking through the telescope at astronomical objects!! Looks like you have an excellent mirror on the way.

John