Thermal expansion of BK7 cf Pyrex question

Hi,

Can anyone tell me what are the thermal expansion coefficients of BK7 glass and
Pyrex for comparative purposes ? Looking at a couple of dobs and trying to
weigh up the pros and cons...

Thanks,

--
Boo

Can anyone tell me what are the thermal expansion coefficients of BK7
glass and Pyrex for comparative purposes ? Looking at a couple of dobs
and trying to weigh up the pros and cons...

While thermal changes can change the figure, there is far
greater damage to the figure from air currents caused by the
glass not being at the same temperature as the air. So the
mirror cell construction --- how quickly it allows the mirror
to cool --- would be more important than the type of glass.
Also, if it isn't Pyrex, it is probably plate glass and not BK7.

You might enjoy the article at
http://skyandtelescope.com/printable...rticle_508.asp

Clear Skies

Chuck Taylor
Do you observe the moon? If so, try
http://groups.yahoo.com/group/lunar-observing/

If you enjoy optics, try
http://groups.yahoo.com/group/ATM_Optics_Software/
*********************************************

Chuck Taylor wrote:
Can anyone tell me what are the thermal expansion coefficients of BK7
glass and Pyrex for comparative purposes ? Looking at a couple of dobs
and trying to weigh up the pros and cons...

7.1*10^-6 mm/mmC vs 3.2*10^-6 mm/mmC


While thermal changes can change the figure, there is far
greater damage to the figure from air currents caused by the
glass not being at the same temperature as the air. So the
mirror cell construction --- how quickly it allows the mirror
to cool --- would be more important than the type of glass.
Also, if it isn't Pyrex, it is probably plate glass and not BK7.


In fact the choice of substrate is quite critical in achieving good
thermal transfer and BK7 (or, even worse, plate) glass is considerably
worse than Pyrex (which is considerably worse than Zerodur which is
worse than ULE quartz). while you can change the cell quite easily you
can't replace the primary. If anything I'd suggest the OP to look for
the primary thickness *and* the substrate material as key parameters.

Andrea T.

the type of glass. Also, if it isn't Pyrex, it is probably plate glass
and not BK7.

The spec says BK7.


You might enjoy the article at
http://skyandtelescope.com/printable...rticle_508.asp

Yes thanks, I learned a lot from that site.

--
Boo

7.1*10^-6 mm/mmC vs 3.2*10^-6 mm/mmC

Thanks !


While thermal changes can change the figure, there is far
greater damage to the figure from air currents caused by the
glass not being at the same temperature as the air. So the
mirror cell construction --- how quickly it allows the mirror
to cool --- would be more important than the type of glass.
Also, if it isn't Pyrex, it is probably plate glass and not BK7.


In fact the choice of substrate is quite critical in achieving good
thermal transfer and BK7 (or, even worse, plate) glass is considerably
worse than Pyrex (which is considerably worse than Zerodur which is
worse than ULE quartz). while you can change the cell quite easily you
can't replace the primary. If anything I'd suggest the OP to look for
the primary thickness *and* the substrate material as key parameters.

Interesting point. I've actually got so wide a range of things to consider that
the mirror cooling is maybe a bit more esoteric than I should really be
considering anyway.

I'm going to post more specific questions regarding which scope to choose
tomorrow if it's not too much trouble for you both to keep an eye out.

Many thanks,

--
Boo

The spec says BK7.

What scopes are you looking at?

Clear Skies

Chuck Taylor
Do you observe the moon? If so, try
http://groups.yahoo.com/group/lunar-observing/

If you enjoy optics, try
http://groups.yahoo.com/group/ATM_Optics_Software/
*********************************************

Chuck Taylor wrote:
The spec says BK7.

What scopes are you looking at?

Hi Chuck,

See my other thread "scopes question".

Cheers,

--
Boo