Mirror wave

[Moonshane]

that's a decent mirror. 1/12th wave is roughly equivalent to 1/6 PV.

[estwing]

that's a decent mirror. 1/12th wave is roughly equivalent to 1/6 PV.

..i stand corrected on mass production,sorry.

[Mike73]

GSO mirrors vary wildly from very smooth and 'nearly' 1/6Pv to very very rough...and I mean rough! Its pot luck as to which you get.

[John]

Claiming 1/12 wave without any clarification is a bit misleading. Telescope House initially used to show that figure when they marketed the Revelation dobs but quite quickly dropped it.

I've read of a Meade LB 16" that was tested at barely 1/2 wave PV.

[Moonshane]

I think that's the problem John, that it's a lottery. although China certainly seem to have improved their mirrors, their quality control cannot be as good as a smaller business that checks and tests every item.

[estwing]

ahhh the people from the companies must of smelt a lawsuit coming...from what they were claiming?

[John]

I suspect that the vast majority of mirrors in Skywatcher and scopes with GSO optics are good though. You will get the odd "rogue" but the consistency is pretty good I reckon. You don't hear many complaints about them and there are 1000's out there

[estwing]

well you learn something every day,so much information on a piece of glass! thanks guys.

[Mike73]

I've read of a Meade LB 16" that was tested at barely 1/2 wave PV.

I had a very similar GSO mirror arrive with my Sumerian when I first got it John. Imagine a 16" mirror giving views as bright as a 8"! I didnt cry but I was close!

Both TS and Sumerian reacted in a positive way though and the problem was sorted quickly. TS now test each 16" GSO before its sold, yep you have to pay for the Zygo but you will get a mirror somewhere between 1/4Pv and 1/6Pv.

[russ.will]

I had a very similar GSO mirror arrive with my Sumerian when I first got it John. Imagine a 16" mirror giving views as bright as a 8"! I didnt cry but I was close!

I suspect that the vast majority of mirrors in Skywatcher and scopes with GSO optics are good though. You will get the odd "rogue" but the consistency is pretty good I reckon. You don't hear many complaints about them and there are 1000's out there

Mmm. Unless my 12" FlexTube starts giving up some crisper views than my old Explorer 200p in fairly short order, I may be number 1001.

However, the latter was used across 2011 which had a number of really crisp, clear nights, whereas the 300p was bought in May last year. It really hasn't had much of a chance. Two things spring to mind though. One, I recently had the best views of Jupiter I've ever had. Two, I've yet to see a view of M81 & M82 that compares to the view the 200p regularly threw up.

I'd also tentatively suggest that a significant percentage of those 1000s have no reference point, to know whether there scope is a goodun or a badun. Are all of this years second hand 200p Dobs, bought on the back of last years BCE, simply the bad ones, whilst the good ones kept people in the hobby? I think we should be told!

Russell

[John]

....I'd also tentatively suggest that a significant percentage of those 1000s have no reference point, to know whether there scope is a goodun or a badun.....

There is probably some truth in that Russell.

[E621Keith]

Just out of interest, how much does thermal expansion affects mirror figure? All material expands when heated and contract when cooled.

As such a mirror tested at room temperature will have a different figure to when it is in a cold winter night.

or is that why you can't get pyrex mirror better than 1/10th pv or plate glass mirror better than 1/4 pv?

[estwing]

it must get very warm to move the glass?..close to 900f and pyrex is very stubborn

[Astrobits]

Interestingly my 12" GSO has 1/12th wave glass at the business end!!!

http://www.gs-telesc...tent.asp?id=114

Interesting that their "Optics" page quotes 1/16th wave or better RMS.

That RMS is important. 1/16th RMS could easily be 1/4 PV on the wavefront!!!!

Like this: 1/4 PV wavefront comes from 1/8 PV error on the glass. Take the deviation about the median line and 1/8 PV is +/- 1/16 on the glass. Go for RMS and the figure can become even smaller because RMS averages the deviations at many points over the surface and a few large errors can be swamped by many more small errors. To simplify the idea, consider the four "measurements" 4,8,8,8. The RMS ( Root Mean Square, the Root and Square bits just mathematically remove negatives ) is simply the average of these four numbers, =28/4 =7. So a mirror with one reading of 1/4 plus three 1/8's has an RMS of 1/7.

Using a red laser to do the RMS can shrink the "error" a bit more because of the longer wavelength of red light.

Even so, one 1/4 PV mirror can perform differently to another. It depends on which area of the mirror has the worst error. Mirrors are tested in isolation ( normally not after they are installed in a telescope ) so if the 1/4 wave error is near the centre of the mirror it will have little effect on the image and it may even be obscured by the secondary, while a 1/4 wave error near the edge of the mirror will produce a poorer performance.

You need much more info than just PV on the figure of a mirror to make an assessment of it's likely performance without actually putting it into a telescope and using it. That, of course, is the final test.

Nigel

[Peter Drew]

+1 for Nigel's analysis.

[estwing]

this amount of care when grinding,testing and assessing the reports of these mirrors and then perhaps repeating the process over and over,points me towards experts like John Nichol/OO and others a-like..and so the RMS report is better at finding the true quality of the mirror? what RMS figure should i be looking for? It all makes sense when you start spending £250+ on single eyepieces..think on..

[John]

If it's any help, the figures on the Zygo report that came with my OO 12" F/5.3 are as follows:

PV Wavefront: 0.113 wave

RMS: 0.018 wave

AstMag: 0.035 wave

Strehl: 0.987

I believe the first figure equates to 1/8.8th wave PV and the Strehl is a good figure.

As I've not had first light with it as yet I'm in no position to comment as to how the optical set as a whole actually performs (the above results are for the primary) of course or whether it will deliver any noticeable performance improvement over a standard Skywatcher or Meade 12" F/5 mirror

[estwing]

Please let it be outstanding John..for what i know,the strehl figure does look good..

[Damo636]

+2 for Nigels analysis, I haven't got the foggiest what any of it means but enjoyed reading it :-)

[estwing]

+2 for Nigels analysis, I haven't got the foggiest what any of it means but enjoyed reading it :-)

shhh Damo this is a great place to pretend to be clever,just reply.."nice one mate, intresting piece there" they'll never know.Thats what i do!!

[Damo636]

Sounds like a plan bud

[Astrobits]

John's mirror details exemplify what I posted. Btw. that looks like a very good mirror there John.

Wavefront PV 1/8.8 or 1/55.5 RMS. For the same mirror!. When you want to flog your mirror which number would you choose to use? and I have seen all the variants at one time or another.

Strehl Ratio is a good guide to the optical performance as it is based on the RMS error of the wavefront and thus takes account of the quality of the whole of the wavefront and not just the maximum error.

Nigel

[barkis]

I don't think our overall sky conditions are good enough to

cater for the high accuracy mirrors anyway.

Just enjoy what you've invested in, and stop fretting about it's wavefront

Just make sure your collimation is good enough to allow it to deliver

what it is capable of .

Ron.

[estwing]

it's nice to get the best quality mirror that you can afford tho Ron?

[Mike73]

it's nice to get the best quality mirror that you can afford tho Ron?

I guess its all down to the individual and what objects they want to view but personally a 1/10 Pv mirror would be wasted on the objects I use my 16" on.

If and when I upgrade to an f/4 it will only be 1/4 or maybe 1/6Pv with a high lux coating mirror, oh and it will HAVE to have a full Zygo report so I know exactly what I'm getting!