Is that vivid around the edge. Did the edge end up turned?
It'll be interesting to see how well it holds collimation with that cell setup, is it sitting on bubble wrap?
This is not my mirror just an image I found. I understand many mirrors used for high definition imaging are blacked out along 5mm of the outside edge to improved image quality and nothing to do with bad edges.
Yep sitting on bubble wrap which is what I'll do initially to see how well it works and a number of other options including an 18 pointer.
I've made an 18 point cell for mine but I've found its the edge supports that needs to be done really right. My first effort caused a little pinching and produce a slightly triangle star. With a mirror that tall i would say the edge support is going to be very important.
If you're going to mask the edge of yours then i would recommend making a ring rather than vivid as in that image. There should only be a need to mask a 1mm off the edge unless the edge is rolled.
Even my Royce mirror works better with the last .5-1mm masked off with a ring i made in my lathe.
I've made an 18 point cell for mine but I've found its the edge supports that needs to be done really right. My first effort caused a little pinching and produce a slightly triangle star. With a mirror that tall i would say the edge support is going to be very important.
If you're going to mask the edge of yours then i would recommend making a ring rather than vivid as in that image. There should only be a need to mask a 1mm off the edge unless the edge is rolled.
Even my Royce mirror works better with the last .5-1mm masked off with a ring i made in my lathe.
Good info thanks, what did you make your ring from?
I'm sure there will be very little edge roll from Normand but 1mm is what I'll start with, any more than that and it will be costing me lots of wasted $.
The edge support will be a work in progress for some while unless I get lucky, I will start with 2 wires and 12 supports both anchored E & W.
Interesting John, though many refer to Dob support the idea that edge can have more effect on the mirror than cell needs more thought. My top and bottom mirror plates are the same shape and size 508mm x 16mm.
After fusion of the pillars to both plates the second fusion is done with the mirror inverted over a mould allowing the mirror to slump to the correct shape F3.95 in my case which gives a FL 2008mm.
Update on my mirror from Normand, figuring is going well and now at 1 wave another week it will be ready.
Phil , did you talk to Bird about the test results I got on his three 16" F4 mirrors including one a Fullham sandwich mirror ? PM me if you need any clarification .
I understand many mirrors used for high definition imaging are blacked out along 5mm of the outside edge to improved image quality and nothing to do with bad edges.
Hmmm...
Can you explain the logic of this?
I can see only one reason to do this and it has everything to do with bad edges.
You can ignore the blackout around the edge of the mirror Phil showed earlier -that's my 16" Fullum mirror, and it was blacked out while I was chasing the source of overcorrection I was seeing in my images, but blacking out the edge made no difference. In the end the problem was the barlow and nothing to do with the mirror.
The mirror does have a slight turned edge so I keep an edge mask on there that's a few mm wide, for planetary work you don't want any scattered light from a turned edge, no matter how small.
Yes, it seems that pretty much all barlows add some over-correction, even if their marketing department says otherwise... probably due in large part to my f/4 mirror being right at the edge of the design criteria and using the barlow for more magnification than intended.
It was very handy to have 3 different mirrors on hand for the testing - all of them gave the same over-corrected images which helped to isolate the problem, but for a while there I wondered if they all had turned edges so I asked Mark to check them out. There is a very slight turned edge on all of them that can be covered by a mask a few mm wide, so now I do that always as a matter of course. Kills off stray reflections too from the edge of the glass.
The Titan does indeed look small compared to this monster, it will just have to grin and bear it. I will however make sure it is near perfectly balanced so I don't break it's back.
I don't think it will be a case of breaking it's back, instead will it break your back getting it on there!
Quote:
Originally Posted by ags_
The Titan does indeed look small compared to this monster, it will just have to grin and bear it. I will however make sure it is near perfectly balanced so I don't break it's back.
The mirror from Normand arrives today, as expected it was packaged very well and looks wonderful. Manufacture of cell support and secondary holder with spider will commence soon.
I couldn't help noticing the 7 zone foucault test claiming an 0.99 Strehl ratio. Such tests are useful for small slower mirrors but in larger fast mirrors are often idealized in relation to the reality as measuring the all important edge zones edge zones is much harder this way and of course complex figure distortions ( not just simple astigmatism ) more common with large fast and laminated mirrors are pretty much invisible to the maker using these methods. Do trust your eyes and star test diligently before making decisions on the optical quality.
I found the 16" F4 Fullham sandwich mirror I tested that was mentioned earlier in this thread , looked at with sensitive null interferometry to have results nothing like the documents presented here though I didn't see original foucault test results it was supplied with . I do wish you the best results with your new optics and as always trust your eyes.
lol
