P or PX?

[m37]

Hi, just a quickie

What does the X denote on the end of the 250PX dob? the 200 and 300 are both just P

cheers

[knobby]

Not sure but I think it maybe as bit uses pyrex for the mirror

[GuillermoBarrancos]

The X stands for Pyrex Mirror glass Version.

[m37]

Thanks chaps!

[Paul73]

So that is good is it??

My granny had some Pyrex pie dishes.

She made very good pies....

Not sure how that translates into good optics though. Something to do with minimal expansion/contraction with temperature change??

Joking apart. It would be interesting to know. Especially as the 200 and 300 models seem to be non Pyrex?

Paul

[ollypenrice]

Pyrex is thermally stable, which is something your granny might have appreciated! (Nobody likes cracked glass fragments in their pie!!! Well, I don't...)

Mirrors are fantastically sensitive to cool down and warm up and need to change shape as little as possible during these processes. This is the Pyrex positive, I think.

Olly

[m37]

Thanks all. I remember my Mum's brown pyrex mugs in the 80s. We could drop these without fear of breakage. I assume the same is true of a Pyrex mirror Also I suppose it means they are microwave and dishwasher safe.

[GuillermoBarrancos]

I have no idea why only the 250 size in the dobsionian series have a Pyrex Mirror and the rest not.

I guess it has to do with pricing of the different sizes mirrors. Likely a larger volume of the 250mm pyrex Mirrors are produced and so better priced, than the other sizes.

Or they still have a very large back stock of the other sizes dobs to clear out, before replacing them with Pyrex Mirror versions. Who knows.

The SW Quattro imaging Newtonians come in 8, 10 and 12 inch and all have Pyrex Mirrors. But these are also priced accordingly.

[knobby]

I love the image of sticking my 250 PX mirror in the dishwasher... If I ever win the lottery I will have to try it :-). And you tube it :-)

[Paul73]

So we are fairly safe in assuming that Pyrex is the way forward.

I guess that it it the coating that provides the reflection. So we are looking for a material that is relatively easy to work, holds it's form in changing conditions and is strong.......

Interestingly, Pyrex has less that half the Thermal expansion coefficient than glass (good). However, Diamond, Wood, Mica and Silicon are better in this respect! But, as an all rounder, Pyrex looks pretty good!

Paul

PS. My qualifications as a Materials Engineer total about 20 mins at the the Wiki University.

[m37]

Are you suggesting a wooden mirror? Not saying it's a bad idea necessarily. Diamond might be an option, probably still cheaper than a Tak frac!

I wonder if they had the 200 and the 300 models out for ages so had lots of stock but perhaps the 250 was a newer in-between model so there was less mirror stock.

Not that it really matters, I'm getting the 250 whether it's made of Pyrex, spam or hair. Not wood though.

[Paul73]

Good choice! A good balance between Apature and Practicality. I have a Flextube version which is a little heavier but for storage and transportation it makes a lot of sense.

Paul