parabolic primary mirrors

[Budding Star Gazer]

hi all,

it's been some time since i was last on this site but my interest in astronomy has grown quite a lot and i'm now ready to invest money in some new equipment.

i've settled on either a meade 12" dobsonian or a skywatcher 12"/14" dobsonian with goto but that all depends on a few questions i have about both models.

after a lot of searching over the last week i'm having trouble finding the make of the mirror set for the skywatcher, i know the meade lightbridge uses BK7 parabolic diffraction limited optics for which i have read good things, my understanding is they are pretty much up there and set the standard for future newtonians but i can't find anything on the skywatcher at all, any and all help would be much appreciated, so my questions are:

1. are the meade primary and secondary mirrors on the dobsonians as good as they say they are and if not what sort or problems have you had.

2. can anyone provide information on the skywatcher dobsonian mirror sets, a link will do but i'd rather first hand experience.

3. can anyone provide genuine comparisons between the two, a link will do but first had experience would be better.

4. ( after thought ) can the meade lightbridge be fitted with an off the shelf goto system, or similar.

both have some very good features, meade is a very trust worthy name as is skywatcher but i must admit i'm drawn towards the skywatcher without the mirror info because i can get more for my money. the 14" goto version would be at the top of my list without knowing more, i've never forgot my first astronomy lesson: if you invest in equipment buy as big as you can afford.

thanks for reading

budding star gazer

[nephilim]

All I know is that the Skywatcher PDS range are parabolic.

[ronin]

The glass type makes little difference, the light does not pass through the glass so from that aspect it can be anything, does not even need to be transparent. Why BK7, nice optical glass but a slab of cheap plate glass will be as good. What you want in the glass is low thermal expansion. Just because they quote a glass type does not to a great extent make any difference, good mirrors are made from Pyrex that is used to make casserole dishes.

Diffraction limited, go search CN there is talk on what this means and ultimately no one is quite sure. Also what measurement are given, are they good or bad or just a number that will "impress" you and so get you to buy.

If Meade follow some of their eyepieces then it will be a GSO mirror.

The secondary simply has to be flat, what flatness and PV do they quote ?

Most Dobsonians cannot be retro fitted with goto.

Buy the biggest is OK as long as it is still small enough to be used easily and often, otherwise it is a lump.

[Budding Star Gazer]

high ronin,

numbers as well as all other technical info would be good but i can't find any that is the reason for my post.

i've been searching for almost a week, all i can find is reviews so all i have to go on is other peoples word, i want more, it would be nice to read the technical info for myself.

[Islander]

As above, mirrors are front silvered so the us of specialised low diffraction glass means nothing. The use of low expansion borosilicate glass (Pyrex by trade name) has far more significance for the performance of the mirror.

I've got a Lightbridge 16 and I'm happy with its performance. I've seen no obvious errors with the optics. I've not used a Skywatcher newt but they get excellent reviews. Both are manufactured in China.

There are a couple of off the shelf GoTo and track systems that can be retrofitted to dobsonians. They ain't cheap though!

Argo Navis is one:

http://www.wildcard-innovations.com.au/argo_specs.html

Stellarcat is another:

http://www.stellarcat.com/

[E621Keith]

Actually glass type does matter for mirror, but not as much as lens. A mirror needs to be thermally stable to keep its figure. Glass like other material expands when heated and contract when cooled.

Pyrex is a better glass than BK7 for mirror because it has a lower thermal expansion coefficient. It's the same property which made it ideally suited for heat resistant laboratory glassware. (Current Pyrex branded kitchen glassware are fakes made from tempered soda lime glass rather the real borosilicate Pyrex)

Just a quick summary:

Sitall/Zerodur>Fuzed Quatz>Pyrex>BK7>plate glass

Just for your interest, modern space telescope mirrors are made from silicon carbide (HST, AKARI) or Beryllium (JST). Neither are transparent

PS: I believe SW mirrors are Pyrex. Also more importantly, I believe SW mirrors have a protective over coat, where as the GSO mirror in the Meade doesn't. As a result, Meade's mirror coating will deteriorate quicker than SW's

[Moonshane]

My summary would be that almost without exception the main brands provide excellent quality and value for money and assuming good condition of coatings, even second hand telescopes will be excellent. You are unlikely to be disappointed with any of them. I have used Celestron and Skywatcher newts and they have all been superb. I prefer Orion Optics UK (used only as I cannot afford new prices) as the mirror quality is excellent the build quality acceptable. I tend to make my own dob bases as I am not keen on the skywatcher design which is both heavy and oversized.

I can only comment subjectively as I have no facilities for checking optical quality other than my eyes.

[J_M_Franklin]

Skywatchers website states...well nothing...

However the UK importer has said on the phone that the Mirrors, to the best of their knowledge, are Low Expansion Pyrex and the aluminised coating is over coated to protect it from the effects of UV and impurities in the air...

They do not stock Meade, but said that the two mirror sets are on a par with each other and the only difference will be emotive when used visually and will depend on the grade of equipment and software when used for Astrophotography..

So the choice is yours....

[Budding Star Gazer]

thanks for the replies all,

it is a little frustrating that no technical info is published by the skywatcher, meade websites or pretty much any website for that matter.

i've been looking at the cost of a retrofit goto system for the meade and as already pointed out you are talking mega bucks so for that reason the skywatcher comes out trumps, the 14" would be nice but the bank balance says 12"

i think just for the sake of it i'm going to call both and see if i can get my hands on a tech sheet but i have a feeling thats a well guarded i will have to kill you if i tell you secret.

regards all.

[John]

For what it's worth I've owned A Meade Lightbridge 12" and the 8" and 10" Skywatcher dobsonians and looked through a number of each type I'd say the optical quality was pretty consistent with all of them.

The Lightbridges do use GSO optics and mirror cells as do the Revelation dobsonians and the ones marketed under the Telescope Services (TS) brand by the German based dealer of the same name. The Orion (USA) dobsonians are made by Synta who also make the Skywatchers.

I've seen enough evidence of coating deterioration with GSO sourced mirrors to agree that was at least a period when their overcoatings were inferior to the Skywatcher ones, as Keith says.

With regards to the optical quality (figure) of the mirrors, over the years I've been reading about them there seems to be a concensus that the GSO and Synta ones average around 1/4-1/6th wave PV with the odd example falling below that and occasionally better ones as well. A member here had a Skywatcher 8" F/5 mirror independently tested a while back and I seem to recall the results showed that it was a decent mirror around 1/6th wave PV.

With the exception of those that specify differently (eg: the Skywatcher 250PX) I assume the mirrors are made from plate glass but I could be mistaken.

[scarp15]

Here is some additional technical spec on the primary and secondary mirrors of a 300P flextube

Primary

F4.9

1500 Focal length

Grade A anealed optical glass

The centre-spotted mirror is ground with computer controlled accuracy

Multicoated with aluminium and titanium dioxide for high reflectivity and then overcoated with quartz for long life

Secondary

2.75" mm diagonal mirror polished flat to diffraction limited accuracy

glass type and coatings same as primary

For a fuller account of more detailed specifications of this scope visit www.astronomics.com

[Budding Star Gazer]

thanks for the link scarp

[neil phillips]

Hi Me personally from what ive seen and read i would, plum for skywatcher, But GSO mirrors can also be excellent, a member here often posts excellent planetary images with a 10" goto SW dob

Another i know also produces world class images with a 12" GSO newt. I am presently myself getting a secondhand 300p SW scope next week. i will give my thoughts when ive had time to fully see what it can do.

But i bumped into this site recently ( bottom of page ) Which took me back a bit, as the image quality is extraordinary for such a low cost mass produced design. one of the best planetary images ive seen so far with these optics. other images on the site are also excellent. But the top image, could have been taken by any number of very expensive high end instruments. I think its that good. If the consistency of these mirrors holds true. ive read both views. But mostly folks say they are consistent, then clearly these are worth the money. Mine is costing me less than the camera im using to image with it

http://www.ssmassey.com/saturn.html

[umadog]

Here's the deal:

Don't worry about mirror quality. Just buy whatever scope looks best for you. All mirrors in these mass produced scopes are made by one of two Asian companies: Synta or GSO. Both churn out reasonable quality mirrors. Their definition of "diffraction limited" is a little loose but it does NOT mean that their mirrors are the best you can get. A Synta/GSO mirror that scrapes past the standard may not be very nice (this is documented, BTW) but how bad it is depends on the sorts of aberrations it exhibits. The PV (wave) measure doesn't tell you how the mirror will perform in practice. Synta and GSO are not setting the gold standard; ignore any advertising that states that. The standard is being set by custom optical houses in the west and they are much more expensive.

You will learn almost nothing about the optical quality from the specs in the adverts. The consistency of GSO and Synta mirrors is pretty good but it is a little variable. There are some great mirrors and there are some not-so-great mirrors. The only way you'll know which you have is to look through the instrument or pay someone who knows what they're doing to test it. There are a lot of figures that matter: more than just the PV measure. It don't mean to spook you, though. For the money you're paying, this is excellent glass.

Finally, the glass type. Firstly: none of these mirrors are Pyrex because Pyrex is no longer being made. As far as you're concerned, the glass type doesn't matter. The thermal expansion stuff is a red herring because you only notice expansion/contraction issues while the mirror is equilibrating to ambient temperature. During this time any substrate (including Pyrex) can exhibit spherical aberration and all will have a nasty turbulent boundary layer of warm air. The latter is the biggest problem during cool-down. Once the mirror has cooled all of this goes away (no matter what the mirror is made from). How long it takes to cool depends on the square of the mirror's thickness and not the substrate material. It is true that glass such as Pyrex will deform less as it warms and cools. Where this most makes a difference is during manufacture as the mirror needs to be at ambient to be tested during polishing. So polishing can proceed faster (and possibly more accurately) with a Pyrex-type mirror. However, all of this is in vain if the substrate wasn't annealed properly in the first place. A badly annealed mirror will exhibit uneven stresses during cool-down/warm-up that will lead to big changes in its shape. Any glass can do this and not spec sheet will tell you if it was done properly.

[neil phillips]

The only way you'll know which you have is to look through the instrument or pay someone who knows what they're doing to test it.

Or some consistent time imaging with the scope. Over time the images will betray how well the mirror is performing. This can take some time to fully gauge, Though as mentioned star tests and Ronchi gratings can be used much quicker.But for me being a imager at very high powers, the final acid test is always the quality of the images.

Some problems with mirrors are more destructive than others. And for the most part all will have there own variance to some degree. But the important point is, this will not stop many of them performing well, either visually or imaging, So for the price they can not be beaten. Unless of course some problem with the mirror surface hinder this ideal.

But from what ive read so far ( and its obviously incomplete ) Bad ones are not happening a lot. ive only read one such report so far. There is a discussion on CN where a mirror maker who refigures lots of chinese optics. was asked how many SW mirrors he gets coming in, he replied that they have rarely ever had to refigure a Synta mirror. My own personal experiance should be happening soon. I hope they are as consistent as many suggest.