I guess there were enough Newtonians and Newtonian primaries discussed here, I thought of adding a bit more spice ...
A few pics of the progress of my 12" CDK project. Making a secondary of any Cassegrain is quite involving, as one needs to finish TWO surfaces before even starting with the secondary itself. You need to grind and polish a "window" (in my case back of the reference concave tool) so you can see interference patterns later. I used the mirror itself to grind both of the backs really flat (to within one micron saggita). There was of course no need to get that precise, but I guess why not
Polishing went quickly - I stuck polishing pads to the back of the secondary itself (now very flat) and got the required transparency in roughly one hour. You can still see some remaining pits towards the edge, but it will do.
Getting the curve right was another matter. Even having a spherometer that could read one micron, there is still enough guessing to make things er - interesting. I did manage to hit the required ROC to within 3mm in the end with polishing pads, which is close enough as I'm yet to cut the glass with pitch (my plaster tools are still drying out). I used first a convex curve on the secondary as a tool (stuck the pads onto it) to polish the concave side; then used polished concave reference to stick the pads onto it and polish the convex mirror itself. Sounds brave, but I was confident enough after using the pads for many other projects before.
Reference sphere only got about an hour of polish before pads started to move so I had to abandon early. What looks like TDE in the Ronchigram below (snapped just outside ROC) is in fact the outer 5mm that is yet to be fully polished. The secondary itself (convex) got solid 3 hours of polish with pads and it is more or less fully polished. I used Zeiss type machine and 5kg weight on the pin which makes pads cut really fast.
Despite non-conforming pads, concave sphere only shows a minor bump in the center and some groovy zones, which I'm quite confident will disappear quickly once I attack it with proper pitch (Acculap).
Here's the design outlined in gory detail.
305 diameter, 1700mm efl, f/5.6 (geometric). 146mm secondary, with baffle that makes it 49% obstruction - but this is an astrograph, not a planetary scope.
22mm unvignetted field, then a gradual drop towards the edge. System is designed to cover a large 35mm chip with diffraction limited spots 430 to 800 nm (well, NEARLY - it is just over 9 micron RMS at the edge of the 43mm circle; Airy disc size is 8.1 micron outlined as a black circle in spot diagram. But my chip has 9 micron pixels so I'm fine; kind of pixel limited instead if diffraction limited).
Design is all spherical, bar gentle elliptical primary (e=0.67) and should be relatively easy to make. Especially if we mention that solution is based on off the shelf lenses, so corrector is in fact ready!
Another advantage is those lenses have an excellent broadband antireflection coating, something that is impossible to do in ATM environment.
It will have secondary mirror focusing, that was the easiest way to ensure zero flexure. Secondary assembly rides on 6 linear bearings ensuring no tilt during travel. Assembly is aluminium, but upper and lower assembly will be connected via unidirectional carbon tubes, minimizing the thermal expansion. In theory at least (mirrors will change focus themselves somewhat as they cycle, being Pyrex - but an internal thermometer will give readings to intelligent focuser program which should then track and refocus as temperature drifts).
I have been using the same 22" glass tile tool for grinding 2 x 22" and 3 x 25" mirrors with varying f ratios. The second to last 25" ground was f4.28. For the last one of the 3, I wanted to produce an f5.0-f5.3 result. As the tile tool had too much reverse curvature, I decided to use my hogging tool on it to reduce the curvature. Using the same stroke, as I used to hog out the mirror, I was able to achieve the curvature reduction, and thereby, control the sagitta. At the end of the grind, (#800 finish), on the current 25"er, I have the Radius of Curvature at 258", or f5.16, for the 25" of clear aperture.
Note: Having experimented with both ceramic, and glass tile tools, I found that ceramic wears much too quickly, compared to the glass. Also the cutting action of glass on glass, is quicker than ceramic on glass.
Over the years there has been a lot of discussion regarding the durability of silver when used to coat first surface mirrors (as in telescope mirrors). Thought to share my own experience. Its just on a year since I "spray silvered" a pair of larger mirrors, (22" and 25"). I might add this was my first attempt at home silvering. The cost of materials used for the two mirrors was around $40.
Contrary to popular belief a raw silvered surface is actually quite resilient to all sorts of abuse. At around 6 months into its life I had to do a clean (after a 150km house shift [25" OTA was transported in vertical orientation in open trailer alone some roading that was loose metal/gravel]). To put it mildly the mirror looked a mess. To clean or not to clean? Well not to clean was not an option, so mirror out and with the garden hose most of the crud was removed. A little help with lightly warm water, suds, and clean piece of cloth the remaining dirt was persuaded to leave. Finally a spray down with de-ionized water.
Pictures one and two show the washing down and cleaned mirror returned to the scope. Picture three: the mirror surface at 11 months.
Hi Stephen, nice work!
Wondering where you purchased the silvering chemicals?
Last time I searched for these I was only able to find silvering kits, with a lifetimes supply, for approx $600 US!
Edit: Another question, I dismissed the idea of silvering because I live very close to the ocean(~100m) and figured the salty air would damage the coating too quickly, however I noticed the town you live in is also near the ocean, is the salt air not an issue for you?
Wondering where you purchased the silvering chemicals?
Edit: Another question, I dismissed the idea of silvering because I live very close to the ocean(~100m) and figured the salty air would damage the coating too quickly, however I noticed the town you live in is also near the ocean, is the salt air not an issue for you?
Hi Simon,
Google up "Angel Gilding" for affordable and easy to use silvering chemicals.
I currently live about 5km from the ocean. My scopes are stored in a large closed fairly air tight shed. Mirror boxes/OTAs are closed when not in use.
I'm thinking of grinding a mirror and am just wondering were I would be able to get the grinding powders, pitch, etc, from? I need to find out some prices to see if I've got enough money
Astro Optics were getting their abrasives from http://www.naxosabrasives.com.au/ for many decades before they stopped selling mirror kits ;-) but you have to know what you are after !
If you cant acquire Silicon Carbide grits elsewhere, I have surplus stocks of #40 to #1000 for reasonable prices. Prepared to ship to Oz (but shipping extra).
If you cant acquire Silicon Carbide grits elsewhere, I have surplus stocks of #40 to #1000 for reasonable prices. Prepared to ship to Oz (but shipping extra).
Thanks, that is worth some serious thought, I was thinking of maybe getting one of the 12" mirror blanks that you have in the classifieds so if I did you could send it over with it
I would obviously need some pitch, would it be best to buy that locally? What shops would stock it do you reckon?
I understand that a 12" isn't usually recommended for a first time mirror grind but I'm pretty sure I could do it.
Thanks, that is worth some serious thought, I was thinking of maybe getting one of the 12" mirror blanks that you have in the classifieds so if I did you could send it over with it
I would obviously need some pitch, would it be best to buy that locally? What shops would stock it do you reckon?
I understand that a 12" isn't usually recommended for a first time mirror grind but I'm pretty sure I could do it.
Have been working on a pair of 25" by 1 1/2" thick plate glass blanks, (ex Sea World Aquarium [Napier NZ]). The dolphins had their turn looking through this glass. Now in its recycling mode will be turned to focusing in on the cosmos.
About 85 hours of work (all by hand) have gone into these two mirrors so far, from hog to preliminary figuring. Had intended the pair to be close to parfocal at around f5.15. However my mechanical method for measuring sagittas was perhaps a little loose. Ended up at f5.3 for #1 and f5.05 for #2. Wasnt planing on a binoscope so no worries.
Had previously ground a 25"f4.68 from 3/4" plate, but have in practice found it difficult to use due to its lack of stiffness. As stiffness goes up with the cube of thickness, the current thicker mirrors have an edge stiffness about 8x , and a central stiffness of over 20x that of the old 3/4" glass.
Have got both mirrors to about the same stage figure wise. Picture[Ronchi] is of #1 taken outside ROC. Some work to be done but its a start.
It's been a while, and I got sidetracked by other things a bit but there's been some progress to report.
The spherical test plate has been finished to reasonable sphere (conservatively 1/10 wave ptv). It does show some TDE but test plate is nearly 20mm larger than a secondary itself so I'm not concerned that much. First image shows a Ronchi snap - smooth and straight. Foucault shows that any zonal differences are very hard to measure, less than 50 microns of LSA in any case.
The convex secondary was then finished to match the test plate. It is still a bit more convex (approx 1/4 wave) but it is a sphere, which is all it counts.
When secondary is placed in near contact against test plate this is a true interference test, so I could use OpenFringe to analyse it. It gave better than 1/30 wave RMS on the wavefront. This is not going to be a planetary scope so I'm happy with that.
The rough appearance of interference lines is an artifact of test apparatus - the 45 deg semi transparent mirror has been made from some sort of foil which is quite uneven optically. When secondary/test plate stack is viewed directly lines are much more smooth, but I can't take pictures of it easily...
The 315mm primary has been now polished with pads, ready for figuring. It again shows a reasonably smooth sphere, as seen in this Ronchigram. All it needs is now a smooth 67% ellipse...
The optical design is now finalised, I could squeeze just about diffraction limited performance over full 35mm chip at rather fast for a Cassegrain f/5.5, with minimal vignetting. Of course, obstruction is nearing 48%, but that's the price I'm quite willing to pay. Most commercial Cassegrain systems have much worse vignetting profile, and users don't complain much
The mechanics are also coming together, I'll snap some pics to share soon.
Grits.
of 25" by 1 1/2" thick plate glass blanks, (ex Sea World Aquarium [Napier NZ]). The dolphins had their turn looking through this glass. Now in its recycling mode will be turned to focusing in on the cosmos. 
loose. Ended up at f5.3 for #1 and f5.05 for #2. Wasnt planing on a binoscope so no worries.
