crayford bk7

[davidpretorius]

Hi all, i assume with andrews ltd 10" and 12" dobs that the crayford is a good quality focuser? Are they worth the extra

and what is the BK7?

[elusiver]

bk7 is better then pyrex, and is what the primary is made of. The crayfords are the way to go in terms of a focuser.. alot better than the standard r & p.

el :)

[ballaratdragons]

Well,

Not quite right El. Sorry.

Dave, Pyrex is far better than BK7.

Cheaper mirrors are made of Plate Glass which is easy to recognise by it's green tinge (in the glass, not in the reflective aluminium coating).

BK7 is a higher grade 'optical glass' used worlwide as a very good mirror material and a lower thermal co-efficient to plate glass (quicker cooling) and does not have the green tinge to it, it is clear. It is a very good quality Glass.

Pyrex is recognisable usually by it's price!!! Although unrecognisable from BK7, Pyrex is hardly affected by thermal changes which gives it a very short cool-down time and is a far better quality material.

As far as weather in Australia, we do not have the extremes as do other colder countries. So, BK7 is perfect for the Australian climate.

As for Crayford, they are far, far better than Rack & Pinion focusers as they are much smoother, easier to focus, have adjustable friction, a focus lock, have nil backlash and are better made.

They are most definately worth the extra!!!!

Hope that helps.

[davidpretorius]

great, thanks for that, also for tasmanian weather, will i actually put the cooling fan that comes with the ltd versions to use all that much? It would be highly unlikely to get above 15 degrees at night even in summer

[ausastronomer]

Hi,

As Ken explained Pyrex is a better mirror substrate than BK7 which is an optical glass, however BK7 also makes very nice mirrors and the end user would be hard pressed to detect a difference which will only be in cooling time not visual quality. Yes, you will use the cooling fan frequently and the Crayford focuser is way better than the Rack and pinion.

CS-John B

[ballaratdragons]

Yes Dave, you will need the Fan in Tasmania. Some people fit their own made from computer fans.

Another use I found for my fan is if the mirror dews up fairly badly you can help it dry out by taking the scope inside (or wherever you will store it), leave it sitting horizontal (so that the mirror is vertical) and turn the fan on. Even with extremely heavy dew I have dried my mirror in around 10 minutes this way.

My fan has helped prevent fogging of the mirrors when viewing too! If I see the beginnings of dew on the mirrors I turn the fan on and the fogging goes within a minute.

[davidpretorius]

thanks again, great advice

[MiG]

As far as mirrors go, BK7 is hardly better than plate glass. Its light transmission characteristics are better but that is irrelevant for a mirror. Its thermal characteristics are only a bit better.

[ausastronomer]

Thats not quite correct either. In terms of thermal properties BK7 falls between pyrex and soda lime (plate or float or whatever you want to call it). In addition to having marginally better thermal properties than plate glass, BK7 is harder and can take a better polish than plate glass. Also its co- efficient of elasticity is lot lower than plate glass therefore it is less susceptible to flexure from physical stresses (poor cell design etc). Also its surface deviation is significantly lower than plate as the mirror expands or contracts due to temperature changes. In other words a non temperature stabilised BK7 mirror will not change its figure as much as plate during the stabilisation process.

Most importantly of all however BK7 has a significantly higher consistency in terms of its quality than float glass. Float glass with poor annealing can be subjected to flaws and internal fractures and stresses and particularly air bubbles which are a pain in the ars_ to a mirror maker. There is nothing worse than spending countless hours figuring a mirror only to find an air bubble and go back to square 1. This doesn't happen with BK7.

Simple fact is BK7 is more expensive than Plate glass and if it wasn't better in many ways than plate the manufuacturer wouldn't be using it. Same with pyrex, it is more expensive than Plate and BK7 and if it wasn't better than the others the premium mirror makers wouldn't be using it.

The above having been said, there have been some exceptional mirrors made with all 3 substrates. The early 8" Starsplitters with Zambuto mirrors are actually plate glass and they are as good as it gets. So the mirror substrate shouldn't really be a determining factor in deciding which scope to buy, although for thermal reasons pyrex is the best, keeping the exotic unaffordable substrates out of the equation.

CS-John B

[davidpretorius]

that is exactly what i thought, the bk7 thingy and versus the sand stuff and all that. I bags being on ausastronomer's table / team for any quizes etc.

In all seriousness , thanks for info

[ballaratdragons]

Hmmpph! I gave you the correct facts too! Don't you want to sit with me.

I'll remember this Dave!!

[slice of heaven]

Another good post John. Clearly defines the differences .

That post should be hanging in the front window of the beginners forum for all to see.

[davidpretorius]

oops, Ken, ummmm, ummmm. Genius is when you can say the same thing in less words????? You said it in less words!! So yes, you can sit at my table of champions. I will hold the drinks whilst you guys win me a prize!

[ballaratdragons]

Thank you Dave,

Just for that I will let you sit at my table too, at the Snake Valley Star Camp. Drag yourself over here for it and bring some other Tazzies with you!!!!

[davidpretorius]

cool Ken, my wife's family is in gisbourne, so i can drop them off there and keep coming your way. What time of year is the camp. I am sure I can make a business trip out of it? There are a few boys from around here. If those pesky northerners (ie north of wodonga) have their little get together, then the southern states (SA, VIC and TAS) should to. I will take the boat to bring my pride and joys, oh and my family as well.

Snkae valley would be a very central meeting point for all concerned in all honesty. Mt Langi Guiran wines are pretty close from memory!!!

[ballaratdragons]

Dave, I can tell you don't read through all the sections in here. You naughty boy!!

Look here . . . http://www.iceinspace.com.au/forum/showthread.php?t=2590

[davidpretorius]

you've got my attention. i will talk to my wife, check boat schedule and flights and get back to you on this.

[ballaratdragons]

Dave,

If you are into wineries, there is one about 10 minutes drive from the camp. It is called Yellowglen Wines.

[Starkler]

To answer your original question, it is worth paying the extra, and mainly for the crayford focuser which is streets ahead of the 'standard' sloppy one.

[davidpretorius]

thanks geoff, i am ordering tomorrow the 10" LTD with crayford

[MiG]

That was an interesting post. I had forgotten to consider things other than thermal properties.
I found a site that has the properties of mirror substrates and the steady and transient thermal distortion characteristics of BK7 are between 1.5 and 2 times as good as plate glass. But just stiffness is not much higher (73.1 vs 81 GPa).
http://www.minerals.sk.ca/atm_design/materials/plate.html
http://www.minerals.sk.ca/atm_design/materials/bk7.html
Substrates page (http://www.minerals.sk.ca/atm_design/mirror_substrate.html)
An interesting thing is this sentence at the top of the page:

"As a telescope mirror material, BK7 is still much more expensive and inferior to many other possible choices."

I know this may make me look argumentative, but I like to get to the bottom of things when involved in discussions of a quantitative nature. I don't mean to disrespect you or anyone else.

[ausastronomer]

MIG,

No offence taken.

Doug Miller is a small time Amateur Astronomer and mirror maker in the USA. That statement is his opinion which he is entitled to have, doesn't mean its correct. He makes good mirrors but does not have the reputation of people like Carl Zambuto, Bob Royce, Steve Swayze or Larry Hardin. If you also look at the properties of BK7 glass on the Melles Griot website you will notice some of the figures are significantly different to what Doug quotes, particularly Knoop's hardness which Doug quotes at 520 and Melles Griot at 610. This compares with plate glass at 470. A couple of other numbers worth looking at are Steady State Thermal distortion and Transient Thermal distortion where BK7 fares infinitely better than plate glass.

http://www.mellesgriot.com/products/optics/mp_3_1.htm#BK7

FWIW Bob Royce indicates that he quite likes BK7 as a primary mirror substrate but goes on to say that he has also made a number of excellent plate glass mirrors in the past.

What does all this mean, not a lot really other than "who makes the mirror" is infinitely more important than "what its made from" which is what I also indicated in my previous post.

CS-John B