mirror technical data

[Budding Star Gazer]

i've been looking at buying a new scope and want to know i'm being wise with my money so i have been checking out the technical aspect of mirrors & the glass used to produce the blanks, some might ask why well i guess it's because the mirror set is the most important part and one of the most expensive parts of a telescope.

after a bit of searching i found this

http://www.oldham-optical.co.uk/Glass.htm

the chart at the bottom give a table of types of glass and their thermal coefficient properties.

my reason for this post is i've seen a few posts asking the same questions about mirrors that i have and the fact that very few sites have technical data when it comes to mirrors, let alone what the mirror sets are made off.

i just thought this would be a handy bit of information for those wanting to know more about what they are buying.

regards.

[Mr Q]

The article is an excellent one IF you are going to make your own mirror(s). Otherwise, the info is somewhat useless because when looking at telescope ads, you will rarely, if ever, see any mention of mirror material/quality.

It was very educational just the same and thanks for posting it

[John]

Orion Optics do specify the type of glass used for their mirrors:

http://www.orionoptics.co.uk/OPTICS/opticsinformatio.html

[Budding Star Gazer]

thanks for the link john, i have already read up on the skyquest and as orion telescopes are the only one's i have found that actually state the glass type i've opted for the xx12i or even the 14" if the man at the bank says yes.

i am starting to wonder why some of the top names do not publish what everyone should or wants to know.

but all the same i'm learning a lot i never knew before in my hunt for the right scope so that can't be bad.

[Budding Star Gazer]

one thing i would like to know that i can't seem to find is information regarding the range at which a mirror is considered optimal, if heating and cooling has the same effect then there must be a set point at which a mirror is prime.

it may seem a bit much wanting to know so much but i have got right into the nitty gritty of it now and wish to learn all i can so if anyone has a link please post.

cheers.

[Budding Star Gazer]

i did come across a page with info on wave lengh and refelctivity of each type but lost it, as soon a s i find it again i will post it here.

[John]

thanks for the link john, i have already read up on the skyquest and as orion telescopes are the only one's i have found that actually state the glass type i've opted for the xx12i or even the 14" if the man at the bank says yes.

i am starting to wonder why some of the top names do not publish what everyone should or wants to know.

but all the same i'm learning a lot i never knew before in my hunt for the right scope so that can't be bad.

You may be confusing Orion Optics (UK) with Orion (USA) here. The former are entirely UK made scopes (that was the link I provided) while the latter have a range including many scopes made by Synta who also make the Skywatcher Scopes. Optically the Skywatcher and Orion (USA) newtonians are the same.

[Budding Star Gazer]

my bad, i did read that synta bought out orion usa but had no idea they made skywatcher, thanks for that info.

[John]

my bad, i did read that synta bought out orion usa but had no idea they made skywatcher, thanks for that info.

Celestron too now. Saxon is another Synta brand name as well in some parts of the world.

[Budding Star Gazer]

http://www.bbastrodesigns.com/ratemirrors.html

after some more skulling around the vastness of the www i found this page which i thought was quite a good read, it may or may not interest others.

i have just had quite a good customer experience with orion usa via their little live chat tab, very handy and quite good i thought, i would say they could have been a little more helpful but i think the guy on the other end did the best he could.

the context of the convo was about the mirrors in the xx12 & 14i but i'm not sure if i should say anything else as i guess they would deem it private, but that said if you do have a question or two then don't hesitate to use the live chat tab.

[Budding Star Gazer]

welcome to synta worl ay john, i did try to look them up but all that comes up is contact skywatcher!

[cs1cjc]

Orion UK is of course independent and not connected with Synta.

I should not worry about mirror material unduly. The change in figure as a mirror cools is small. However the mirror has to be close to ambient temperature otherwise air currents will distort the image, though a fan will reduce this.

[John]

Orion UK is of course independent and not connected with Synta.

I should not worry about mirror material unduly. The change in figure as a mirror cools is small. However the mirror has to be close to ambient temperature otherwise air currents will distort the image, though a fan will reduce this.

Good point Chris - I agree with this

[Budding Star Gazer]

i think now it's more about me understanding the equipment and how it all works and why really, educational stuff when you start to look closer at what goes into the production of a telescope.

i've just learned that the expansion & contraction of the tube materials it's self can have a huge impact on focal lengh & focal ratio which will in turn have an impact on the mirror and it's clarity.

lot's more to read yet i guess.

[Peter Drew]

It think expansion and contraction of the tube will have a negligible affect on the focal length and hence the focal ratio, it could become a problem during long exposure photography where precise focus must be maintained. Within reason, glass types are more of a consideration for the mirror manufacturers than the end users. Common plate glass was used for most of the high quality results from amateurs in the past.

[umadog]

The material from which the mirror is made (commonly called the "substrate") really isn't that much of a big deal. The main consideration is the quality of the figure and the polish. i.e. the mirror's shape.

The expansion and contraction of the mirror with temperature isn't a problem per se. The mirror will perform as well at -10 C as it will at +40 C. The problems occur WHILST the mirror is changing shape. Because it's a big heavy piece of glass, different regions will be at different temperatures whilst it's cooling down for the night (the core will be warmer than the outer regions). That creates stress in the glass which causes it to very slightly change shape. It's this difference in shape that alters the figure and so may degrade the view a little at high power. When the mirror has acclimatised the problem has gone. The mirror will cool faster if it's thinner. The substrate is only half the equation. If the anneal is bad then you will get bigger problems during cool down.

Low expansion glass will deform less during temperature changes so these effects are minimised.

The other side of the coin is mirror manufacture. During the final figuring process, the mirror must be regularly tested on an optical rig. The grinding process warms the glass so the mirror must be tested when it's thoroughly equilibrated to ambient temperature. Thus, lower expansion glass is easier to work with because it changes shape to a lesser degree.

In summary, you can can excellent plate glass mirrors, excellent supramax mirrors, and excellent Pyrex (or equivalent, Pyrex is no longer made) mirrors. The glass has different thermal properties but if it's annealed right and figured well then you can have an excellent mirror.

"i've just learned that the expansion & contraction of the tube materials it's self can have a huge impact on focal lengh & focal ratio which will in turn have an impact on the mirror and it's clarity.

lot's more to read yet i guess."

I doubt "huge impact" is right. It'll be fractions of a mm in focal length changes. What can happen, however, is that the collimation can shift when the scope changes temperature.