Mirror Spec Obsession

[glend]

Having been involved in amateur astronomy 40+ years ago as a young man in Canada, it has been a real eye opener to return to this world last year now that I'm retired, and see all the changes that have (for better or worse) brought new technology, large numbers of accessible suppliers, and the internet as a place where it can all be discussed (to death).

The hobby has come along way from ship porthole mirror blanks, and even the budget production scopes on offer today seem to have a level of capability that was sadly lacking in the past. And yet, early astronomers like Dunlop, Messier, etc accomplished their discoveries with fairly basic equipment by today's standards.

This brings me to my topic: the obsession with mirror specs (he said as he kicked the hornet's nest).

Sure, it would be nice to have say a Zambuto mirror in my 16" dob but is it going to be a noticable improvement to my old eyes, and one that can be justified in terms of expense, or simply something for people to brag about to others; and preach about on forums. Expense is relative of course, and the primary mirror is the single most important item in any Newtonian, so spend your money as you see fit. But given a dark site, is there really any difference between a good production mirror and these exotic custom made ones with nice names and a list of test results to rave about. Can the average (old) eye use the difference, or is it more a case of wishful seeing.
;)

[wavelandscott]

This is a great question and my answer is that it depends...

Can there be a measurable and seeable difference...absolutely yes. Will you notice it? Maybe...it depends...

Where and what will you view? How hard will you push your gear? Will you use it every night? What other gear will you have in terms of quality eyepieces? How experienced an observer are you? How good is your seeing...

The answer for you will depend on your responses to the above...

I believe that in good seeing from a dark place with good quality gear that a reasonable person could see a difference if they look...

[AstroJunk]

With a mass produced mirror you may get a great one, an ok one or a dud. You won't know until you get it! I have looked through a number of skywatcher mirrors of late and they have been very good indeed. Times have changed.

I am a sucker though for a hand crafted mirror (and have a superb Zambuto 8" travel scope). You pretty much guarantee a top mirror and can send it back if its not good and that's why you pay the $$$

Ironically though, to keep the weight down, Skywatcher have cast very light conical mirrors on their bigger dobs which cool down much quicker than traditional mirrors! If I had my way, I'd take a new 16" stock Skywatcher mirror and send it to Mark Suchting to figure to perfection...

[AG Hybrid]

Well my skywatcher 12 can perform at at least 40x mag per inch if seeing allows. So unless the view in the eyepiece is physically making you angry with an upturned edge I wouldn't bother refiguring it. Mirror temperature and atmospheric seeing will ruin the image long before the mirror figure does.

[Camelopardalis]

Naive question...what would one look for to tell the difference? Or is it a question of sitting them down side by side and comparing :question:

[AstroJunk]

Kind of. A much loved eyepiece under great sky will soon weed out the great from the ordinary, but those great skies don't come around too often (for me!)

Look for pinpoint stars in the centre of the field, in a fast Newtonian you may see coma at the edges, but that is expected, and check out the contrast in bright nebulae or on planets.

In addition, there are various tests that can be applied to add rigor both on the bench and under the stars - google 'star testing telescopes' for some examples of how to spot basic faults

Oh, and one thing. If you find that a mates mirror is a dud, unless asked, its polite not to mention it as some observers have quite poor eyesight ;)

[Peter.M]

Honestly, I think if you need to "startest" the scope to find a defect then maybe it isn't a defect after all. That is the point of this thread, the practical difference between a high end mirror and a cheap one. I myself have a high precision mirror in my imaging scope, the main reason for which is that the price difference between a basic mirror and a "ultimate" one was nothing after shipping it here.

[SkyWatch]

I've looked with all sorts of mirrors (including ones I have made myself), and you can certainly tell the difference. It comes out in clarity of view, sharpness of the image and contrast- at any magnification; although it is most noticeable at higher powers on a good night- and particularly noticeable with objects like Jupiter at high power.
The view through some mirrors just looks "soft" due to the light being spread over a wider area, and you simply can't get really sharp images.
The mass-produced mirrors are way better than they were (I use a GSO 12" in my other-wise home-made scope); but there is still the problem of quality control, and you can definitely pick the 1/8 wave or better optics of the better hand-figured mirrors against the "diffraction limited" mass-produced versions.
I find it is a bit like when I look at the sky with, and without, my glasses. My eyes work OK generally, although I have some astigmatism, but when I put my glasses on at night I can probably see 1-2 magnitudes fainter...
- and I agree with "Astrojunk": never criticise someone else's scope unless asked! Any scope and any mirror is better than none!!

[Satchmo]

Glen - you've asked this question before on the forum - so I fear you are going to get the same answers. Without knowing how good your eyes are , what your collimation standards are , how you prepare the scope for viewing , and whether you use high power in a skilled way Its hard to answer. With a well mounted optic , that is properly cooled and collimated on a night of reasonable seeing the focus will have a fair amount of snap even up to a 1mm pupil .

A mirror of 1/2 wave or more PV wavfront error will not show this behaviour. With high expansion glass as used in the GSO's , there may be thermal issues as well. As I pointed out in the recent 16" GSO thread - I've measured three 16" mirrors that were around the advertised 1/12 wave RMS surface which is around 2/3 wave P/V wavfront error - Strehl would have been around 0.5 or less and definitely no contrasty snap at any power. These mirrors would not be showing you fine detail on the planets, but for a 16" aperture scope complete they are good value if you are after value.

Are you happy with your images in view of the money you invested ? If so then I'd suggest you have good value.

The experience of observing is certainly not all about the largest aperture . My favorite views at the last IIS camp were through a 12.5" Conical primary F6 and there were a number of larger scopes there up to 24" . With a 13mm Ethos there was sufficient steadiness, snap and contrast to give the image a `reality' that belied that fact we were looking through an atmosphere ( a fact I could not forget looking through the larger scopes ! )

[Camelopardalis]

That explains why folk are polite around me on observing nights :lol:

[glend]

Well I admit this has me stumped. I just went back through all the threads I started and I can't find a reference to the general question that I asked at the beginning of this one. This isn't about a particular brand, type, etc nor about the specifics of what is or what isn't a good mirror - what it was intended to be about is the building obsessiveness surrounding mirror specifications and what relevance it has to real world observations.

It would be nice to know where I have posted this question before and the context of that thread. If your right I am happy to get this one deleted.

[Satchmo]

Glen - as far as I remember you made the question before inside another thread or a PM about whether you would see the difference between your GSO and a Zambuto or a bespoke mirror - I don't think it was one you started . Anyway sorry, didn't mean to sound impatient - all these things are well worth discussing again :)

[torana68]

I'm no expert however I've looked through mass production mirrors and better ones and I can see a difference (on planets). I've attached an 8"'scope comparison that shows what I've seen, apologies to the owner whoever he is I forget where I found this. I have an Astro Optical 12" here with a Strehl of .954 so IM hoping for good things from that :)

edit , that attachment shows up very poorly , Ill have to look for the original :(

[Shiraz]

this is a useful alternative Roger

http://www.damianpeach.com/simulation.htm

[torana68]

yep that's a great site thanks!