Pyrex or Not

Where did you here this? There is no reason to use BK-7 as mirrors for
anything but mangin (rear surface mirrors) in telescopes. Pyrex has
much less thernal expansion and is cheaper and tougher.



http://www.handsonoptics.com/astrono...es/debate.html

jon

Where did you here this? There is no reason to use BK-7 as mirrors for
anything but mangin (rear surface mirrors) in telescopes. Pyrex has
much less thernal expansion and is cheaper and tougher.



http://www.handsonoptics.com/astrono...es/debate.html

jon

As a telescope maker, I can tell you that I would never make any reflector with
BK7. The material is fine if you can get an accurate figure on it. However, it
takes a lot of testing time to make sure that you really have a parabola on the
surface because the figure will change violently for 15 - 20 minutes after
coming off the pitch lap. Pyrex has a much lower expansion co-efficient, and
the figure is much easier to judge during the parabolizing stage.

Assuming equal quality figure has been achieved, in actual use and if the
temperature is stable, there won't be any difference in the image. However, if
the air temperasture is steadily falling, the BK7 mirror will lag far behind
the Pyrex mirror in terms of spherical correction. This won't make much
difference to the deep sky observer at 10 - 15 x per inch magnification, but it
will trouble the dedicated planetary observer at higher powers.

The main reason BK7 is used in Chinese made mirrors is the lack of availability
and relatively high cost of Pyrex in China.

Roland Christen

Where did you here this? There is no reason to use BK-7 as mirrors for
anything but mangin (rear surface mirrors) in telescopes. Pyrex has
much less thernal expansion and is cheaper and tougher.



http://www.handsonoptics.com/astrono...es/debate.html

jon

As a telescope maker, I can tell you that I would never make any reflector with
BK7. The material is fine if you can get an accurate figure on it. However, it
takes a lot of testing time to make sure that you really have a parabola on the
surface because the figure will change violently for 15 - 20 minutes after
coming off the pitch lap. Pyrex has a much lower expansion co-efficient, and
the figure is much easier to judge during the parabolizing stage.

Assuming equal quality figure has been achieved, in actual use and if the
temperature is stable, there won't be any difference in the image. However, if
the air temperasture is steadily falling, the BK7 mirror will lag far behind
the Pyrex mirror in terms of spherical correction. This won't make much
difference to the deep sky observer at 10 - 15 x per inch magnification, but it
will trouble the dedicated planetary observer at higher powers.

The main reason BK7 is used in Chinese made mirrors is the lack of availability
and relatively high cost of Pyrex in China.

Roland Christen

The main reason BK7 is used in Chinese made mirrors is the lack of
availability
and relatively high cost of Pyrex in China.

Roland Christen

No doubt that Pyrex is the better material, I was pointing out that some
Chinese mirrors are BK-7 which would not have the green tint and might be
identified as Pyrex.

I think it is interesting to consider why it is that Pyrex is such a good glass
for telescopes when it was developed for a different purpose, handling high
levels of thermal shock without fracturing. Doing this requires a glass with a
low coefficient of thermal expansion so that when it subject to a thermal shock
the stresses are below the level which causes fracture.

This of course is what makes it ideal for cool down though it appears that
fused quartz would be much better than either.

jon

The main reason BK7 is used in Chinese made mirrors is the lack of
availability
and relatively high cost of Pyrex in China.

Roland Christen

No doubt that Pyrex is the better material, I was pointing out that some
Chinese mirrors are BK-7 which would not have the green tint and might be
identified as Pyrex.

I think it is interesting to consider why it is that Pyrex is such a good glass
for telescopes when it was developed for a different purpose, handling high
levels of thermal shock without fracturing. Doing this requires a glass with a
low coefficient of thermal expansion so that when it subject to a thermal shock
the stresses are below the level which causes fracture.

This of course is what makes it ideal for cool down though it appears that
fused quartz would be much better than either.

jon