pyrex vs BK-7

[thersites]

Hi all
is pyrex better than a bk-7 plus fan in a 10 inch dob?
total novice here, any thoughts appreciated
ta

[ausastronomer (John Bambury)]

The simple answer to that is yes Pyrex is normally a better mirror substrate than BK-7 but only slightly. The pyrex mirror will cool slightly faster that is all, when the size and thickness are the same. I have an 18" Obsession with a pyrex mirror and a 10" GSO dob with a BK7 mirror and the smaller BK7 mirror cools infinitely faster than the larger pyrex mirror.

The 10" BK7 mirrors in the GSO dobs are very good, mine is actually outstanding for a mass produced mirror. Base your decision on which scope to choose on factors other than the mirror substrate.

CS-John B

[Starkler (Geoff)]

Originally Posted by thersites

Hi all
is pyrex better than a bk-7 plus fan in a 10 inch dob?

A bk-7 mirror with a fan will cool much quicker than a pyrex mirror without fan assist (if thats what you're asking).

[thersites]

thankyou, this answers my query

[janoskiss (Steve H)]

Pyrex is better but either will do the job. I'm very impressed with how almost immediately usable my 10" Dob with the Pyrex mirror is. Also as temperature fluctuates throughout the night a Pyrex mirror will be less affected.

My 10" pyrex is less demanding wrt cooling than my 8" BK7. I have fans behind both though. Pyrex will help the mirror maintain its ideal shape. It will not help with inversion layer and tube current. So you really need a fan for that in any case if you want your scope to perform at its best.

[Satchmo]

Originally Posted by ausastronomer

The 10" BK7 mirrors in the GSO dobs are very good, mine is actually outstanding for a mass produced mirror. Base your decision on which scope to choose on factors other than the mirror substrate.

CS-John B

BK7 bends its shape *3X* more than Pyrex for a given residual difference between mirror and air. Effect not so prominate on a lightweight 10" that is only a slight asphere..effect will be much more obvious on a larger slab of glass whose mass is goin up by the cube root of the aperture.

Mark

[Starkler (Geoff)]

Thats very interesting to know Mark, and now what you said in the 16" gso mirror thread makes sense.

[janoskiss (Steve H)]

Originally Posted by Mark Suchting

..effect will be much more obvious on a larger slab of glass whose mass is goin up by the cube root of the aperture.

I think you mean cube, not cube root. ...?

[ausastronomer (John Bambury)]

Originally Posted by Mark Suchting

BK7 bends its shape *3X* more than Pyrex for a given residual difference between mirror and air. Effect not so prominate on a lightweight 10" that is only a slight asphere..effect will be much more obvious on a larger slab of glass whose mass is goin up by the cube root of the aperture.

Mark

Mark,

Thats actually not quite correct. It's just over twice.

7740 Pyrex has a thermal co-efficient (TCE) of 3.25 as opposed to BK7 which has a TCE of 7.1. That is 2.18 times not 3. However, it gets more complicated than just looking at the TCE, as you know. BK7 has a somewhat higher specific heat than pyrex (18% or 858 as opposed to 726). While it takes longer to cool from a given temperature, it also takes longer and requires more heat to heat up to that temperature. The practical effect of this, is that the temperature differential between the mirror temperature and ambient is usually slightly less for a BK7 mirror than it is for pyrex. This depends on ambient thermal conditions. However, given consistent ambient temperatures for both mirrors throughout the day, the BK7 mirror is usually cooling from a slightly lower temperature than the pyrex mirror. Thermal conductivity of both is almost the same.

True, pyrex is an inherently better substrate than BK7 for large, thick front surface optics, no one is saying it's not. IMO it's getting way too technical to worry about when you're talking about mass produced Chinese and Taiwanese mirrors in 10" Aperture. If I was buying a premium 10" mirror, of course I would want it out of pyrex. However, when your talking a 38mm thick 10 " diameter "mass produced" mirror with a fan blowing on it, it doesn't really matter squat, what it's made out of. Base the decision on which scope to buy, around factors other than the mirror substrate. Who cares if it takes an extra 5 or 10 minutes for the BK7 mirror to deliver diffraction limited images, as opposed to the pyrex mirror ? I can guarantee that most "mass produced" 10" pyrex mirrors won't deliver lunar and planetary views as good as my 10" mass produced BK7 mirror, regardless of how cool they get or how many fans they have blowing on them. I got lucky and got a good one, but clearly optical quality out of the box is a lot more important an issue than what it's made out of.

Something else to consider, a lot of people, not me, observe planets by letting them drift across the FOV from edge to edge. The drift method. As you are well aware a large part of the FOV in an F5 newt is not diffraction limited anyway, because of off axis comatic blur. Yet people still observe with that part of the mirror. Other factors which IMO are more important. How well is the scope collimated ? Is it really a diffraction limited mirror, or close to it ? What is the skill of the observer? How accute is his eyesight? What are the quality of eyepieces being used? How good is the seeing?

There are a lot more things to affect optical performance in such a scope, than worrying about what substrate a 10" mass produced mirror is made out of.

CS-John B

[janoskiss (Steve H)]

Originally Posted by ausastronomer

... BK7 has a somewhat higher specific heat than pyrex (18% or 858 as opposed to 726). While it takes longer to cool from a given temperature, it also takes longer and requires more heat to heat up to that temperature. The practical effect of this, is that the temperature differential between the mirror temperature and ambient is usually slightly less for a BK7 mirror than it is for pyrex.

That's not quite right either. The material with the lower specific heat will follow changes in ambient temperature more closely. The material with the higher specific heat will be more stable wrt fluctuations in ambient, i.e. not follow them so closely and therefore in general have greater difference in temperature from ambient.

[iceman (Mike)]

Originally Posted by ausastronomer

There are a lot more things to affect optical performance in such a scope, than worrying about what substrate a 10" mass produced mirror is made out of.

I'd agree with this, regardless of the other technical data in this thread. Great advice right there.

In practise, in 95% or more of your observing time, you wouldn't know or care what substrate the mirror is made of.

[Satchmo]

Originally Posted by ausastronomer

There are a lot more things to affect optical performance in such a scope, than worrying about what substrate a 10" mass produced mirror is made out of.
CS-John B

Thanks John, for that detailed treatment, I hadn't really noticed that the emphasis of the original thread was particularly about 10" mirrors, so I stand corrected in that case.

One needs to bear in mind that cooling issues are even more significant in the manufacture too. Pitch polishing generates heat which bends the shape even more than normal use. For the limited time these mass production mirror have to be figured , one can imagine that the final figure is something that the optician guesses at because they simply don't have the time to sit around and equalise properly during figuring. I find when putting the finishing touches on large mirrors that they need to be left overnight and not touched to reveal their final figure.

Its easy for the uninitiated to extrapolate comments to apply to all sizes of mirror so I'm emphasising that as the volume of glass escalates fast in the larger sizes the figuring and using will become far more of a challenge. . I was thinking more about low expansion vs. high expansion larger mirrors. My experience figuring and using a 50mm thick 16" mirror was very poor and B270 crown ( as distinct from BK7 Crown ) glass of the larger Chinese mirrors is around 2.9 X the expansion of Pyrex.

Mark

[Satchmo]

Originally Posted by iceman

In practise, in 95% or more of your observing time, you wouldn't know or care what substrate the mirror is made of.

Mike, Just to clarify , you are talking about 10" mirrors ?

Mark

[iceman (Mike)]

Yes Mark, you're right.

[ausastronomer (John Bambury)]

Mark,

I appreciate the point you are trying to make regarding the issues involved in cooling large thick mirrors. As you correctly point out, the issues increase logarithmically as the aperture and thickness of the mirror inceases. I agree 100%, that in these situations the selection of the mirror substrate is very important. You also correctly point this out on the thread concerning the 16" GSO mirrors.

Reading between the lines the question the original poster is asking is this. Should I buy a 10"/F5 GSO dob with a BK7 mirror and supplied with a cooling fan. Or, should I buy a 10"/F4.7 Synta dob with a pyrex mirror and no cooling fan. The answer is, for all intents and purposes it doesn't matter and he should base his decision around factors other than the mirror substrate.

CS-John B

[ausastronomer (John Bambury)]

Originally Posted by Mark Suchting

I find when putting the finishing touches on large mirrors that they need to be left overnight and not touched to reveal their final figure.

Mark,

I respect your comments in this regard and have a couple of questions if I may. Do you find you can work pyrex for longer time periods before having to allow it to cool, compared to plate ? Do you let all mirrors cool overnight or only the larger ones ?

Its easy for the uninitiated to extrapolate comments to apply to all sizes of mirror so I'm emphasising that as the volume of glass escalates fast in the larger sizes the figuring and using will become far more of a challenge.

Very true and an easy conclusion to jump to for those that have had good experiences with smaller GSO mirrors. As you point out in the other thread, everything that applies to the smaller mirrors goes out the window with a 16" diameter plate mirror. The 2" thick pyrex mirror in the 18" Obsession can lose heat all night. The day and night time temperature extremes at Lostock, while a severe test of any mirror, showed this. I can only imagine that a plate mirror of this size would never get close to equilibrium under such conditions.

I appreciate your comments on this. The only problem is we confused the hell out of the poor guy trying to buy his 1st telescope.

CS-John B