The 22" mapstar mirror

[Ajohn]

 I wouldn't laugh to hard about Texereau's comments on mount weight and scope size and some of his other comments too such as mirror thickness and accuracy too really.

Perhaps Damian should consider using his mirror in a scope at max magnification for viewing say mars and being able to touch and focus the scope or even keep it pointing at it. Most people would quickly find that conditions can mean that the view was hopeless anyway at this sort of size so that mag would have to be dropped way back. Accuracy comes into that as well. Not just the mirror but the entire structure of the scope. He still makes a number of very fair points really but they have to be taken in context. Change to a relatively  low mag light bucket and things are different but not as much as some might think especially in relationship to mirror thickness that will hold the sort of accuracy he is after. The other point about accuracy is that light buckets don't really need it unless they are going to be used at max magnification but on the other hand too serious a departure will spoil the view of a number of things. The term diffraction limited has changed a bit too -  he really means it. With a 1/10 wave PV and not RMS PV and the sort of obstruction sizes he suggests things are very close to it. His book is also more or less totally aimed at visual work but he does touch on the photographic problems of his day. That area is now totally different but the same principles apply.

Making mirrors - nothing has changed really as far as the actually making is concerned. There are numerous methods of testing mirrors but really his comments about ronchi still hold. From memory MB proposes it but follows it with star testing - that ain't as simply as people might think. Some one posted on here ronchi so far and then Foucault.  There is also a variation on Ronchi where a supplementary lens is used to make the lines straight when the mirror is parabolic. Then there is the Dall null test and the other with a different name that is based on it but the lens goes in front of both the knife and the source.  The lens for the latter one is pretty expensive. Sounds great until people realise just how accurately it has to be positioned in either case. Both can be used with a Ronchi screen as well. Some one has also come up with a "Ronchi screen" that consists of circles that stay round when the mirror is parabolic or something like that. Most of this type of testing has the same problem - things are set to give the correct result with a very specific mirror that has to have an exact focal ratio not what it just happens to turn out to be. Foucault software works on the basis of the best fit parabola it finds when the measurements are taken.  Maybe the is a match the ronchi pattern set up where the screen is just moved a precise amount either side of the ROC and software analyses the image and it comes up with the best fit parabola. If so that would be a bit different but there would still be a need to know exactly where the ROC was.

The pro's often use supplementary optics in what is essentially a Foucault test but the optics cause the mirror to null out when it has the correct shape, no shadow reading - eg Hubble  . They usually use several lenses in the tester though not just the usual one used by amateurs. They have also use the Hartmann test sometimes just as a final check. It's probably possible to do that one with a DSLR. 

Foucault - easy to make the gear that is needed and no real drawbacks that can't be circumvented. The usual complaint about Texereau is not taking his suggestions on scope size etc in context and also the sums used to work out the mirror profile. I posted links to several packages that will do the sums. Another comment is sometimes pre Dobson but in real terms that has nothing to do with mirror making His comments about size and usability in typical urban situations are not far off the truth as far as resolution goes. 8in F6 was a bold step in his day. The usual was plate glass 6in F8 and Rubbish instructions as well. No one in the USA had even heard of Plossl's until his book was translated. It's a pity good ones aren't easy to come by and that they had to go super etc. I understand good ones are expensive to make and have little eye relief at shorter focal lengths.

 If anyone does it don't make the source too bright - after several hours of minor figuring and testing I looked in the mirror and found one pupil was the size of pin [removed word] and the other as big as it could be. It  took a while to get back to normal.

 Hope Damian doesn't finish up like this

[barkis]

I would love a big Dob, a 36" f3.5 will do nicely thank you.

Housed in a weatherproof Obs, an a simple Alt Az  Swing through Fork Mount to gain access to the Zenith.

Not too high to climb to the focuser, and no awkward eyepiece positions to annoy me.

Just wait for the Cosmos to roll by me nightly through the years,  and pour the starlight down to the big collecting eye, and back up to mine.

Nice thought.

Sorry Damian, I digress in your thread.

Ron.

[mapstar]

Sorry Damian, I digress in your thread.

Ron.

Not as much as some do Ron, but at least it's helpful digression 

Edited March 29, 2015 by mapstar

[barkis]

Try to imagine William Herschel figuring that 49" Speculum metal mirror of his, way back when  Damian.

I think he used a star as his light source, and lugged the mirror in and out of the huge truss to make corrections to the 

surface as required. Some Job Heh?

Carry on yor good work sir, I wish I wasn't so far away from Barnsley, I'd love to pop in to watch progress, and maybe even get a 

few wets in. You've got me yearning to get back on the grind again you bad lad      .

Ron.

[gooseholla]

Yeah, but he got paid well for the privilege!

John
 

[mapstar]

Try to imagine William Herschel figuring that 49" Speculum metal mirror of his, way back when  Damian.

I think he used a star as his light source, and lugged the mirror in and out of the huge truss to make corrections to the 

surface as required. Some Job Heh?

Carry on yor good work sir, I wish I wasn't so far away from Barnsley, I'd love to pop in to watch progress, and maybe even get a 

few wets in. You've got me yearning to get back on the grind again you bad lad      .

Ron.

Thank you Ron, a very encouraging comment and thank fully he got it finished using the ultimate star test. Bet he was as happy as I'll be once I get my shaving mirror complete    

You could also have a few wets if you could wrestle the lap away from me   although I think after 5 mins we'd be having a cup of tea and you'd be reminiscing about your own mirror making exploits and acquaintances. 

I've done three hours today and maybe a little change to the edge 

Inside ROC

Outside ROC

Not much to report and the battle continue's maybe tomorrow. A well deserved beer now and some relaxing to think of strategies for tomorrow

Keep watching

Damian

[barkis]

Yeah, but he got paid well for the privilege!

John

I don't think King George III would miss the £4,000 he granted for the telescope John.

Cheap for the 40 foot monster I would say.

Consider the discoveries made with it. I don't think 

there would be much for William and his Sister Caroline,  (Who helped him) to pocket after paying for the materials.

It was the largest scope in the World for 50 years. Believed to have helped in the discovery of Saturn's sixth and seventh Moons, Enceladus, and Mimas.

Anyway, I think we have hogged Damians thread too much as it is.

He must be getting fed up with interruptions  :icon_mrgreen:. 

Ron.

[barkis]

Thank you Ron, a very encouraging comment and thank fully he got it finished using the ultimate star test. Bet he was as happy as I'll be once I get my shaving mirror complete    

You could also have a few wets if you could wrestle the lap away from me   although I think after 5 mins we'd be having a cup of tea and you'd be reminiscing about your own mirror making exploits and acquaintances. 

I've done three hours today and maybe a little change to the edge 

Inside ROC

SDC13299.JPG

Outside ROC

SDC13289.JPG

Not much to report and the battle continue's maybe tomorrow. A well deserved beer now and some relaxing to think of strategies for tomorrow

Keep watching

Damian

You're certainly approaching the tape Damian. Carry on the excellent work, and I'll recede from the workplace and let you

get to Nirvana.

I'll be back to join in the celebrations. I'll send you a pack of Jaffa Cakes, SGL's usual celebratory snack   .

Best of luck on the Final Approach Matey.

Ron.

[mapstar]

I don't think King George III would miss the £4,000 he granted for the telescope John.

Cheap for the 40 foot monster I would say.

Consider the discoveries made with it. I don't think 

there would be much for William and his Sister Caroline,  (Who helped him) to pocket after paying for the materials.

It was the largest scope in the World for 50 years. Believed to have helped in the discovery of Saturn's sixth and seventh Moons, Enceladus, and Mimas.

Anyway, I think we have hogged Damians thread too much as it is.

He must be getting fed up with interruptions  :icon_mrgreen:. 

Ron.

And to also add John the history lesson is free too so the £4000 goes a long way 

I don't mind interruptions so carry on guys it breaks up what has turned into an edge rectifying thread 

Damian

[barkis]

Good luck on the run in Damian.

I'll be back to join in the celebrations, and I may send you a Pack of Jaffa Cakes, SGL's 

usual celebratory snack   .

Best of Luck Matey.

Ron.

[mapstar]

Good luck on the run in Damian.

I'll be back to join in the celebrations, and I may send you a Pack of Jaffa Cakes, SGL's

usual celebratory snack .

Best of Luck Matey.

Ron.

Cheers Ron I'll hold you to the Jaffa's

Damian

[barkis]

Cheers Ron I'll hold you to the Jaffa's

Damian

For Sure matey  .

[mapstar]

After a bit of downtime I have nore work today and maybe an update later on progress.

Damian

[Soupy]

Look forward to seeing the progress Damian.

[mapstar]

Evening viewers

Work has been at a stop for the last couple of days due to the unfortunate interference of my job 

Today I managed to get back to it though and decided to change something I've not been happy with for a while and that is the work table I use.

The top is made out of a piece of kitchen worktop and is what I've been using from day #1 to grind then polish on. Anyway this was looking rather worse for wear and with all the best intentions about sealing it the chip board had swollen especially where the three clamps were mounted through to stop movement of the mirror. 

I had a thought about this and decided that I would find a 600mm2  flat flag stone from the local DIY store and change it for that. So I scoured the rack and came up with one

Once home it was out with the toys to do a bit of modding. I drilled it through in 6 places then countersunk the screws. I sealed these with silicone.

I then grabbed a bit of american hard maple from a previous project and made some new clamps mounting these with M6 countersunk bolts right through the flag. Because of the shape of the flag I put 4 stop blocks/clamps on which actually give a loose fit so not to pinch the mirror. To finish off I then  rounded the corners of the flag with a diamond disc.

All in an hour. Once the piece of foam matt is back on it's ready to polish again although I may add some form of channel around the side to stop all the slurry running everywhere as it did with the old one. I'm still thinking on this though?.

Damian

Edited April 1, 2015 by mapstar

[nicks90]

might be a daft question.......actually two questions.....

does the temperature of the glass influence the polishing of the mirror? Does the mirror get warm when you polish away due to friction, therefore would the big slab cool down the mirror a lot more than a bit of old kitchen worktop and will that make a difference to the grind? 

does/can the mirror flex under the weight of the tool? if so, is there a chance the scabby swollen worktop was allowing flexure and causing your polishing problems?

[Physopto]

Hi Damian,

You could try bonding some plastic sheet around the edge of the flag, raised edge 1/2". Then drill a hole or two through the flag to let the slurry out though a bonded in pipe (15 mm copper or plastic). just put a bucket underneath to catch it all. I use Soudal Fixall bonding for loads of things, it is very strong and slightly flexible. Most importantly unlike silicon you can just cut it off and re-bond on top again if necessary. Can't do that with silicon.

Can't use polythene as it won't bond may get away with upvc as used in flat type guttering and it's cheap, just cut with a grinder. Better still some aluminium strip. Stronger and you could fit a rubber edge over the upright edge for safety. It would last a lot longer.

Derek

Edited April 1, 2015 by Physopto

[mapstar]

Hi Damian,

You could try bonding some plastic sheet around the edge of the flag, raised edge 1/2". Then drill a hole or two through the flag to let the slurry out though a bonded in pipe (15 mm copper or plastic). just put a bucket underneath to catch it all. I use Soudal Fixall bonding for loads of things, it is very strong and slightly flexible. Most importantly unlike silicon you can just cut it off and re-bond on top again if necessary. Can't do that with silicon.

Can't use polythene as it won't bond may get away with upvc as used in flat type guttering and it's cheap, just cut with a grinder. Better still some aluminium strip. Stronger and you could fit a rubber edge over the upright edge for safety. It would last a lot longer.

Derek

I'd thought of something similar Derek with the bonding although it was going to be silicone as I had it in stock  drilling the flag again to allow the slurry drainage was another thought (great minds and all that). Plastic would be better as metal anywhere near the glass at this stage fills me with dread. Upvc with a stanley is probably an option and I've just remembered I've got some square drain pipe somewhere 

[mapstar]

might be a daft question.......actually two questions.....

does the temperature of the glass influence the polishing of the mirror? Does the mirror get warm when you polish away due to friction, therefore would the big slab cool down the mirror a lot more than a bit of old kitchen worktop and will that make a difference to the grind? 

does/can the mirror flex under the weight of the tool? if so, is there a chance the scabby swollen worktop was allowing flexure and causing your polishing problems?

Cheers for the questions Nick and no question is daft as it maybe something obvious I've overlooked

At this stage there is no difference in temperature to the touch but it will have raised slightly after polishing. If you handle the mirror, then where your hands have been it will expand causing high spot problems when you polish, and I'm sure some Texereau Know it all will be along soon to point it out! It's one thing I wanted to eliminate before progressing further but there is a piece of anti fatigue matting between the mirror and table so it would in fact thermally insulate it from the table surface. 

You can see the matt in the back ground against the radiator

Edited April 1, 2015 by mapstar

[Physopto]

I'd thought of something similar Derek with the bonding although it was going to be silicone as I had it in stock  drilling the flag again to allow the slurry drainage was another thought (great minds and all that). Plastic would be better as metal anywhere near the glass at this stage fills me with dread. Upvc with a stanley is probably an option and I've just remembered I've got some square drain pipe somewhere 

Use an angle grinder with a stone disc. Being one or two fingers short might inhibit your grinding technique a bit.  Seeing a red moon is one thing but just stained .... well not quite the same is it?

 On a serious point though, upvc  does not bond very well with anything. Not silicon or Soudal.  That is why I suggested Aluminium with a rubber edge cover. You know the type fitted over car door edges to hide the metal to the interior. It fits over the edge well and is not damaged by a knife or a knock. You could also bond that on. I use Metal Supermarkets for my supplier for aluminium. You can get 2" wide by 4 mm thick in lengths.

 Derek

[mapstar]

The grinder I may have to give a miss as tIe risk of contamination with the grinding disk is there although small.

I've had a thought about facing the flag with something like a sheet of pvc or rubber? something similar must be 600mm2 or bigger I could just run a bead of silicone around the outside then?

Less chance of picking up something off the flag when moving the mirror to clean.

[Soupy]

New table top is looking great Damian, and I am sure Texereau would approve

[Soupy]

What about using damp proof membrane for covering the flag Damian, you can get heavy duty stuff pretty cheap

Edited April 1, 2015 by Soupy

[mapstar]

I think I may have some somewhere

[gooseholla]

Man you seem to have scraps of everything just lying about!!
John