Type of Dobsonian mirros and quality

[chrissara]

I hope not to start a flame war here, but I have just purchased a 12" Dob from Sirius Optics in Brissie. It is the Southern Cross one.

Whilst I am aware there are cheaper Dobs out there e.g. Bintel, Andrews Com etc, I understand these use BAK7 mirroras opposed to Pyrex mirror on the Southern Cross. (I understand the Southern Cross is a clone of the Orion models or made by the same people in Taiwan and that it has a 1/12 Wave Parabolic Pyrex Mirror)

Anyway, is it true Pyrex is a better type of mirror than the BAK7? Similar and no difference? What I understand is true is that this type of Dob does not need a fan due to its better temperaure control.

Just really wanted to get some thoughts. I have already purchased the unit and am sticking with it, and am just seeing what pro's/cons there are over the Andrews/Bintel type ones that go for $899-$1000.

[iceman (Mike)]

Hi Chissara.

The BAK7 is a fine glass for a mirror, especially in Australia where we don't get the extremes of cold temperature that they do in parts of America and Europe. For us, you will see absolutely no difference between Bak7 and Pyrex.

However, it's not true to say that you do not need a fan. It's important that the mirror be at ambient temperature for the best possible view. If the daytime and nighttime temperature are considerably different, you'll get use out of the fan to try and bring the mirror to ambient temperature quicker.

If you live in QLD where you get fairly constant temperatures most of the year, a "better" glass such as Pyrex makes even less of a difference.

Myself and others with GSO newtonians with Bak7 glass mirrors have produced fine high-resolution images where good quality, well-collimated optics are very important.

[Starkler (Geoff)]

Originally Posted by chrissara

I have already purchased the unit and am sticking with it, and am just seeing what pro's/cons there are over the Andrews/Bintel type ones that go for $899-$1000.

Same beast from the same factory.
I would pick the best priced variant with the options that you want from the above vendors.

At small mirror sizes 12" and under, the pyrex vs bk7 mirror is pretty much a non issue.

P.S. The crayford focuser and right angle finder options are well worth having, whilst anyone can add their own fan at little cost.

[chrissara]

Where I live in Mackay, the temperature rarely does below 12 degrees. Of course in Summer we hit the 35 degrees plus with 95% humidity (read Sauna). Someone did mention that a fan isn't necessary in Mackay.

NB Thanks for the advice.

[rmcpb (Rob)]

There is one instance here where the pyrex will beat the BK7 and that is in those winter nights when the temperature drops and just keeps dropping eg a Canberran winter night. A fan helps to keep the mirror close to the ambient but there will always be a difference. Enter the pyrex mirror with its better thermal expansion coefficient. This basically means that the mirror will distort less than a BK7 for a given temperature difference.

Most people will not even notice the difference but those doing high resolution astrophotography will get better performance from a pyrex mirror, under those conditions, than a BK7.

For us mere mortals a fan will do the job.

Cheers

[janoskiss (Steve H)]

Pyrex will help the mirror keep its shape when not in thermal equilibrium but it won't help with inversion layer on the mirror due to temperature difference. So you still might want a fan to stir things up after all. A fan will also help keep dew off both the primary and the secondary.