Mirror wave

[estwing]

mulling over a few options is a great way to pass time..so can anyone tell me if i will notice "visually" the difference between 1/4 wave and 1/10 wave or is this more for AP. For e.g. 16"1/10 v 18" 1/4..yes,yes app rules in most cases,but give the e.g. where would the greatest reward be to the eye.It's going to be the once in a lifetime purchase,so to get it wrong may result in quite a bit of cussin'.Also is there a sweet-spot for Focal ratio?sorry i'm asking what might be an impossible question to answer completly,the options with a custom dob are drowning me!

[laser_jock99]

Visually the difference may become apparent only at very high powers. If you doing critical visual planetary work then it may be worth the extra cost but 90% of the time bad seeing will probably affect the image far more. For imaging work the 10th wave optics is also a nice-to-have at high powers.

[Plantins]

If you can afford it then go with the 1/10 wave. There won't be much in it at the eyepiece but it will stop you constantly asking the question....what if!!!

[estwing]

if i was imaging... the answer is simpler,but it's a road i'm not going down.

[Mike73]

Good questions Calv.

I'd say that it just depends on how much money you have to throw at it, if you have £5000 for a custom 16" 1/10 Pv then go for it but exactly the same custom scope but with a 1/4 Pv primary will cost half that, is it worth the extra money well I dont know I'II let someone answer that who has actually used a 1/10th.

My Sumerian is a f/4.5 with the same GSO primary as the Meade LB, EP height at the zenith is 5'10" but as I never observe near the zenith there isnt any real problem in staying seated but ideally I'd like that EP height lower so in the future I'II probably change my primary for an f/4 Huygens mirror which will lower that EP height even more, for me comfort at the EP is high on my priority list.

Another advantage that I'II get when I change primary will be that the scope will be lighter still, at the moment fully assembled it weighs 33Kg with a thinner premium mirror it will weigh less than 30Kg and will cool down even quicker!

[estwing]

i know that at F4 its a demanding "zone" for ep's but people aren't going to spend wise on the scope,then not on the ep's...it's not a massive price difference between the waves...just is it worth it for strictly visual...arrrrrggghhhh

[Mike73]

I think it depends on what you want to view really, for the moon, planets and splitting doubles then I'd say its worth it, if its just for DSO's then maybe not.

[Astrobits]

Quite a few years ago Peter Ceravolo made three? telescopes with 1/10, 1/4 and ( if memory serves ) 1/2 wave errors and invited a number of observers to judge the difference. It was written up in Sky & Telescope which I have somewhere. Perhaps anyone who has the searchable index could find the article?

Nigel

[estwing]

is this it Nigel?http://neilenglish.net/thoughts-on-diffraction-limited-optics/

[Astralstroll]

It would be well worth seeing that article if its still available. Always nice to have a side by side comparison to settle a discussion. Im hoping from a purely selfish perspective that there is little difference between the three mirrors when used on near identical scopes in typica UK viewing conditions.

And if not, the majority of my viewing is DSOs so if there is a difference, I can still have the cheaper mirror and be OK. Having said that, I do a bit of lunar observing and the very occasional bit of planetary, so it would be good to have the best mirror for those too.

At the end of the day, I am looking to buy, build a 16" dob so the mirror choice will determine to a large extent how much it costs, or looking at it another way, what spec 16" mirror i can afford.

Dave.

[estwing]

i didn't realise that DSO's weren't that fussy on quality,and our poor sky conditions...so anything over say 1/6 wave maybe overspend...this is deeper than i thought!

[Astralstroll]

I think that as long as we make the most of whatevr scope we have then all is well. Its good to dream of better equipment, but a well used pair of binos will beat the most expensive acope that nevr sees the stars.

Dave.

[Mike73]

i didn't realise that DSO's weren't that fussy on quality,and our poor sky conditions...so anything over say 1/6 wave maybe overspend...this is deeper than i thought!

Hopefully Shane will chime in at some point to confirm but I think I'm right in saying that better optics will improve the views when using higher mags, my most used EP's for DSO's are 26mm Nagler and the 17.3mm Delos.

[estwing]

"optical quality in telescopes" by peter ceravolo in s&t march 1992..now this is where it gets scary,someone has tried to post this piece before but cann't find it anywhere on the web and were told that if you were to copy this article without permission...prepare to have action taken against them....i only wont to spend some money!

[Moonshane]

hi mate

my 16" dob has a stated 1/8 pv and 0.984 strehl so it's a good mirror. it's very hard to say whether or not my scope shows more detail than a scope with a lesser figure as I have never looked through another 16". there's another factor to consider too and that's smoothness. some mirror makers (check out John Nicol's site http://www.nicholoptical.co.uk/index.htm and http://www.nicholoptical.co.uk/Optical%20Standards.pdf. As I understand it John makes mirrors which might not necessarily be the same PV as e.g. the 1/10PV from OOUK but his mirrors are said to be extremely smooth, reducing light scatter on e.g. Jupiter at high powers.

I bought mine as it came up for sale when I was looking at either a 16" or a 18" mirror from John and I have not been disappointed at all. The views at high powers are spectacular when seeing allows. If you ever fancy a trip over here (or possibly I might be able to come to you at some point) then you'd be welcome to look through my scope.

It really depends I think if you love fuzzy hunting or solar system and double stars. at the same aperture I doubt (but don't know) that the image differ but at higher powers then a better mirror will yield better results.

the other thing to consider is size of the scope and observing height. I wanted to be on the ground when observing as ladders are a faff. therefore it was up to 16"f5 or 18" f4.5 that I wanted. my scope ended up at f4 and you do need a paracorr I think at this ratio. at 4.5 you'd get away without if you have good eyepieces at f5 you'd be fine I think.

finally, think about width of true field. I am not a wide field junkie and am content at about 1 degree. if you crave wider fields then the faster focal ratio will achieve this of course.

not sure if this helps or not but hope it does.

[estwing]

paracorr, as you say shane will have to be adressed at F4..that i don't mind that, it's getting the best mirror thats within my budget...and i too am no fan of "step-astro". The nicholoptic..thats one of the mirrors on David Lukehurst site? correct?

[HD-AP]

Hi

I'd go for the smoothest ,highest accuracy optics i could afford ,the caveat is only if the mirror /.telescope is a keeper As the depreciation is vertical even on the best optics out there .

Telescope hardware is easily upgradable improving the accuracy of an existing mirror can be done and is expensive but sometimes not worth it.

.And it can be a minefield in that accuracy and smoothness of figure don't always go hand in hand ,you can have a very high surface accuracy but a very rough surface /with primary and secondary microripple /dogbiscuit which will be outperformed by the same accuracy but smoother figure

I observe mainly moon and planets so can justify sort of the higher accuracy but the uk is seeing limited ,so the number of nights when you can exploit an apeture above 10 inches is limited if you are lucky to half a dozen nights per year ( i wish )

Dso's and a good set of eyepieces and it might be harder to tell the difference .It's a big financial outlay but it should be a non recuring cost ,What to do only you can answer that ,i get the best i can at the time /circumstances allowing .

What does your gut instinct tell you ?

All the best with you new scope

[kirkster501]

if i was imaging... the answer is simpler,but it's a road i'm not going down.

Ahhh, the number of times I heard people say that - myself included - and end up flogging stuff as a result since they star to dabble in imaging and get hooked on it!!!!

[Scooot]

Well I coffed up for 1/10 wave, from memory I don't think it was a lot more, although I seem to remember it does start to get more expensive as the mirrors get larger. I haven't experience of looking through others but I've been delighted with the views I get, especially given the light polution here. So over time I have formed the view that my optics are pretty good, and like others have said, I think it's more obvious with the higher powers. So I'm very pleased with the decision but I would have probably been none the wiser if I'd gone for 1/8 or 1/6. I took the view it was a long term decision and if I spread the extra cost over the life of the scope it would work out cheap. Hope this helps.

[John]

My understanding is that a smooth figure is the one to go for rather than the ultimate Peak / Valley value. I'm no expert but I thought that the strehl ratio is intended to be a sounder way to indicate optical quality ?

I guess the accuracy of the whole optical system is important too - a great primary can be "undone" by a mediocre secondary I reckon.

I think I've read a precis of the Ceravolo experiment mentioned above that suggested the results were that folks could see a difference between the 1/2 wave mirror and the 1/4 wave one but struggled to see much difference between 1/4 and 1/10 wave.

I think most people will take the best optics they can get for their budget though. Not that figures are available for mass production scopes.

[scarp15]

From a visual perspective, my understanding is that a 1/10 wave mirror, may subtly outperfom a mirror at a lower wave frequency, at high magnification during evenings of excellent seeing conditions. Evenings such as these, at least in the UK though, are few and far between.

[Astrobits]

is this it Nigel?http://neilenglish.n...limited-optics/

That's the one. I have just dug out my copy of S & T 1992 March. I note that copies of S & T articles for "Personal use" are now allowed without prior authorisation. So if anyone wants a copy they will have to find the mag and copy it themselves. My mag is available and I have a copier

Nigel

[estwing]

mass produced stuff skywatcher/meade are good for the A to B i'm not knockin' them at all there great i know i've got one,the mirrors in these cann't be top notch because of their price,but when one wants a keeper for life you've got to go down the quality street it may cost at first but to my mind it's a sound investment for later in life..just like EP's once the top stuffs in the case it stays there..we only chop and change for curiosity,but then still buy top of the range.

[estwing]

That's the one. I have just dug out my copy of S & T 1992 March. I note that copies of S & T articles for "Personal use" are now allowed without prior authorisation. So if anyone wants a copy they will have to find the mag and copy it themselves. My mag is available and I have a copier

Nigel

...sounds like a great read

[Damo636]

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

http://www.gs-telescope.com/content.asp?id=114