Max, read this thread. It may interest you. http://www.cloudynig...into-the-house/
Thanks Paul. Yes a 8” quartz mirror at ¾” thickness on a 9 point cell sounds gorgeous.
My Skywatcher has a 3 point cell which seems ok based on Plop software, but the mirror would need to be full thickness 1” and this adds to the boundary layer.
Alas, for highest-power, high-resolution viewing, the vibration of the fans may be visible
Fan vibrations have been a struggle.
It seems to help if long steel bolts are taped to the fan outer body, as this serves to increase mass and inertia resistance to vibration.
Also decoupling the reduced vibrations by inserting the fan into some 10mm foam that holds it very loosely (so the fan can wiggle easily) and connecting the foam to the OTA with two layers of Velcro hooks and loops does well. There’s no elongated airy disc at x240 at least.
--collimation. Make sure you have the right tools and do it well.
--cooling. Make sure you have fans and try to let it sit outside for a while to get closer to ambient before you start observing.
--conditions. Observe often…
It’s thanks to you Don saying the above that my f6 scope is autocollimated using a little hand mirror to stack the vanished triangles and has lots of night air blown into the tube.
On better nights, all 4 Jupiter’s moons appear as crisp little discs, thin blue festoons arch across and a little darker area in the GRS can be seen… from a cheap Synta scope!
My estimate of the typical/approximate heirarchy of bad things goes like this…
1) Pinched optics, very warm optics, ~1 - 2 waves
2) Moderate boundary layer/warm optics, bad edge support, ~0.5 - 1.0 waves
3) Gross optical errors / bad strain effects / poor edge support, ~0.5 waves
4) Moderate optical errors / mild strain effects, ~0.25 - 0.5 waves
5) Small optical errors / small strain effects / minor cell issues, 0.1 - 0.25 waves
Whatever astigmatism a fan may cause for a little while, it is dwarfed by the benefit I get from removing that boundary layer of air.
This is very useful in showing where efforts should be focussed to reduce wavefront error/ improve views.
Maybe the bigger the temperature difference between stored mirror and night air and the less time or patience you have in wanting high mag, high resolution views…
… Then the more fans on less thermal expansion glass is needed (plate glass and BK7 expands the most, Pyrex and Suprax and Supremax in the middle, Quartz or Zerodur the least) or thinner glass with better mirror cell support.
Thank you eyepiecedropper and others for solutions to this issue of cooling and mirror buying.
If [Chinese] BK7 is working well in so many scopes, anneal must be acceptable. Of course each example is different.
Imported mirrors are not BK7, they are faker BK7, BK7 like.
Meaning they probably use the same content but not the same annealing process.
The material to make BK7 is relatively cheap compared to the cost of processing it correctly so you can pay for the licensing fee to actually say you’re using real BK7.
Paul and Danny, I get the feeling that BK7 glass used for mass market scopes, just like the figuring process, needs only be good enough to do the job.
If 1/6 wave figures take more polishing time and full annealing takes more fossil fuel, then a company giving good value telescopes will reduce costs and go for “good enough acceptable.” So presumably BK7 annealing can be as much a “lottery” hit and miss as the finished mirror figure.
With this particular mirror, there’s too big a likelihood that the anneal is poor. Because whilst my eyes are a little astigmatic without a new prescription, the views in winter were *really* astigmatic. For example at low power it reveals twin overlapping Jupiters in the centre of FoV! Two for the price of one. Stars shaped like eagles’ wings flying towards you.
To paraphrase a famous quote in Blade Runner movie “I have seen things you people can scarcely believe”
Whereas in the last few weeks, with a much smaller temperature difference between indoors and outside, the low power views are now far less astigmatic.
The mirror figure on such a small 8” aperture surely cannot be ‘potato chipping’ furling and contracting so much by the side fan as to cause the astigmatism. So instead the cause could well be poor BK7 annealing.
Other faults with the mirror include undercorrection and a few millimeters of turned down edge (masked off). Considering the low cost of a new 8” mirror, a new replacement will be bought.
And with the unusual personal viewing circumstances (I have to store it indoors, often around 4C outside, short sessions, strong fans), it’ll be a lower expansion glass than BK7.
I really appreciate all of your advice and hope that this thread might help others too.
Edited by Max T, 17 May 2015 - 07:46 AM.