Where to source a large mirror set?

[callump]

HubbleOptic mirrors were said to be of poor quality in the early days, but seem to have a better reputation these days.

I 'won' a 12" in last weekends auction, and its listed as 'in-transit' in the postal tracking system, so will be interesting to see how long it will take to arrive. But i'm not desperate for it to arrive - lots of design things to think about before starting work...

They seem to have auctions every week, so if you're patient you might get a bargain on a big mirror...

/callump

[Glasspusher]

Regarding the quality of Hubble optics you might to have at look here...

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John

[marcopolo]

John, lets say those reviews stand and it is presumed there has been no improvement in the quality since the 3 years when that optical tessting was done (which for an up and coming company would surprise me) , then I guess you have to ask the questions 1) was the overcorrection common or was it bad luck , 2) At 1/3 wave p-v would it cause an appreciable degradation in image quailty for deep sky viewing (it's inteneded purpose) when wavefront errors of 1 wave to 1-2 wave are regularly caused by the atmosphere in the UKand 3) Is it worth it for a third or half of the cost of a comparably sized mirror.

Having seen that optical analysis I would quite possibly asking for a zygo cert with my mirror. But even then what sort of a guarantee that that is the mirror that got tested that was shipped to you.....

Tom, it's hard for me to put myself in your shoes and you wouldn't take the gamble on this? I may just throw a bid on that mirror just out of plain curiosity and get Orion optics to Zygo test it for me.

Marco.

[Beulah]

Wouldn't touch them with a bargepole. I've seen far better ronchi results on this forum by members who have ground their own mirrors.

[Glasspusher]

Callump, I hope the links I posted have not made you have second thoughts...there are positive reports about Hubble optics out there. My purpose in posting them was to raise awareness that there have been issues, as Marco points out maybe they have better quality control now.

John

[Peter Drew]

I think I would have some reservations re the long term stability of a sandwich mirror produced by this method. I also think that it is a myth that mirrors for predominately deep sky use need'nt be of good quality. Poor quality mirrors do not focus enough of their light into the Airy disc thereby reducing the brightness of the star image. This results in the faintest stars disappearing from view, a large no. of DSO's are composed of stars so you would be missing out on the best view.

[callump]

John, no i'm not put off at all - i'm fully aware of all the bad reports there were a few years back - so my eyes are wide open...

I was really after a 14" but they seemed to go at a price beyond which I was willing to have a punt on. But a 12" for around £200 seemed worth an experiment.

I have also been reading some satisfied user reports on CN - and it seems to me they about balance...

I quite agree with Pete too - a good quality mirror will give better DSO views.

To some extent it comes down to what is 'good enough' - but in the end if you are happy with the view through your scope then its good enough.

Regarding long term stability - i'm not sure they have been around long enough to tell - time will tell.

To some extent with the HO mirrors, I just want to see for myself...

Callum

[marcopolo]

I also think that it is a myth that mirrors for predominately deep sky use need'nt be of good quality. Poor quality mirrors do not focus enough of their light into the Airy disc thereby reducing the brightness of the star image

It's more the fact that used at lower powers you are not close to resolving teh Airy disc.

Granted it will reduce the pinpoint light of fainter stars but there is a MAJOR complication that you havn't factored in. Namely the 1st and 2nd diffractions rings as seen in fast newtonians due to the large central obstruction actually throws quite a bit of light outside the airy disc (in reinforcing the 1st and 2nd diffraction rings) to the extent that I would argue that the effect of reducing the pinpoint image of a star is not as pronounced as you think in a 'poor' mirror with respect to a 'good' mirror.

Also I don't know about DSO observers but I spend 80% of the time looking at Galaxy and Nebula rather than globulars or open cluster.

As for long term satbility. Bonding is for life, not just for Christmas. LOL!!!! i don't know; bonding agents can outlive the surfaces they are bonding especially if it is really flat on the back...really not sure in this case.

Marco

[Glasspusher]

The disks used by Hubble optics are fused to the posts that seperate the two disks by heating the glass to high temperature. The pros have experimented with this for years, but it has never really found favour. In the sizes Hubble are offering, monolithic blanks have a proven track record, I don't understand the need for sandwich blanks in this size range. They might argue that the ventilation offered by the arrangement is an advantage when it comes to cool down time.

John

[Nictorax]

A better figured, smooth mirror will be more apt at taking advantage of the available contrast between an extended object and the background sky. Saying that a low quality large mirror is sufficient for extended objects is wrong. The difference between a 16" low quality mirror and a 16" well figured smooth mirror 1/6 to 1/8 wavefront can mean the difference between seeing spiral arms in a galaxy and just seeing a fuzzy blob.

Regards

[marcopolo]

Saying that a low quality large mirror is sufficient for extended objects is wrong.

Nicotrax, you are too black and white and misunderstand the more subtle points of this debate.

If I were to give you 2 mirrors one with 1/3 wv and 1/4 wv wavefront errors you would of course choose to use the latter. But i'll bet on 95% of nights used on the sky for DSO you wouldn't be able to tell the difference. (see previous posts about diffraction secondaries etc...) In fact I bet you could only tell the difference from quantitive 'lab' measurement and that it would take a very expert eye on a VERY good night to spot any real world difference.

lets take it to an extreme which would qualify your statement. I grind my fist ever mirror where the wavefront is barely batter than 1-1/2 wv and have another I purchased with research grade true diffaction limited optics at 1/10th wave or better. Yes of course the latter will give better views on most nights that would be visible to most people.

However rearely are these extremes seen in comparing 2 mirrors and the former case would be more likely when purchasing one from say the easern supplier listed over say for example a UK optical mirror at a guaranteed 1/4 wv.

So I satnd by assertion that you would get better bang for your buck were you to gamble and were it actually arrive on it's long 10 week travels! It really doesn't take much of a mental leap to understand that paying more for something will generally get you a better product so by the same token here spending more on a well known optical supplier may get you crisper views on nights of very good seeing IF were talking about big differences in the wavefront error. If not then you really are getting excellent value for money.

cheers,

Marco.