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Posts Made By: Phil Barker

September 6, 2005 03:55 AM Forum: Telescope Making

thin plate mirrors

Posted By Phil Barker

Re the edge support I assumed at least 3 points would do am I far off the mark here?

Phil B


December 6, 2005 03:16 AM Forum: Telescope Making

18 inch Kriegsonian project

Posted By Phil Barker

here's the mirror box showing the inside and open back


December 7, 2005 04:53 PM Forum: Telescope Making

18 inch Kriegsonian project

Posted By Phil Barker

Thanks John a very good reference page. I'll be digesting its contents carfully.

The parabolising gets tougher the bigger you go but I reakon I can make a decent go of it the last 2 mirrors I did were 16 inches in aperture.

Phil B


September 17, 2012 06:36 PM Forum: Telescope Making

need advice on movement of mirror box

Posted By Phil Barker


My 17.5 inch truss dobb has a 34 pound mirror and 1 inch thick trusses.

I made my side bearings out of ply smaller than in the dobsonian handbook (kriege and berry)(18 inches)and to achieve balance they can be easily moved up or down on the mirror box.

I then use a chain on the back of the mirror box to get good balance. Works like a treat hope to get it out later this week.

I saved some weight with the mirror cell over the metal one using alu for the 18 point suspension and ply on the bottom of the box. This works well. My secondary cage is definetly over 5 pound with 2 inch focuser big secondary 50mm finder etc and a decent eyepiece.

I make this up with the heavy chain on the mirror box.


September 17, 2012 06:38 PM Forum: Telescope Making

need advice on movement of mirror box

Posted By Phil Barker

you can improve the movement of the bearings with car polish its better to be stiff than too loose.

No backlash is the sign of success. My setup is formica on teflon and is very smooth.

That dobsonian handbook could be useful to you it covers balance etc very well.

November 3, 2012 06:32 PM Forum: Telescope Making

how I make a pitch lap

Posted By Phil Barker

interestingly the recipe for the tar came from Gary Nankivell one of the great telescope makers.

He built the optics for the 1 metre f-6 corrected dall kirkham telescope at Mount John This was in the 1970's possible the first of its type in the world. He also did work on a 9.5 inch replacement doublet lens for the Cooke triplet at carter observatory before his death in 2001.

The origional lens had an unstable element that needed regular refiguring and was no longer useable.
There are still some of his 10 inch f-7 newtonians around which are suberbly corrected. I was lucky enough to learn a few things from him before his passing.

RE making the molds for the pouring of the hot pitch I recommend using tinfoil pressed into a master mold to replicate it. generally I make blocks around 6mm thich and 3 inches by 1 inch.

I can line up the foil molds ready to go. I use a spoon to ensure I put the same ammount of pitch in each 3 desert spoonfuls last time I did this.

good luck


November 4, 2012 05:30 AM Forum: Telescope Making

progress on grinding for 12.5 mirror

Posted By Phil Barker

It wouldn't hurt to do a sharpie type test on the mirror now you are closing in on polishing time.

I use a thin slurry and move the tool over the mirror a few times in all directions after drawing a grid pattern with a permanent marker on the mirror blank surface.

If you see for example the mirror is in contact at the edge but not the middle then you need more wets tool on top to bring them together and vice versa when the middle is in contact and not the edge.

Starting polishing especially on a big piece of glass like you have and seeing grey not go away through a non spherical surface can be disconcerting but it is easily avoidable.

I do sharpie type tests from 25 micron down and polishing always goes well.

re the scratch we all get them I now cover the tile tool with a pizzabox type setup if I'm leaving it for any length of time. You can't be too careful.






January 5, 2013 06:25 PM Forum: Solar System Observing

Saw all five of Jupiter's moons last night

Posted By Phil Barker

I was observing from New Zealand at 1000 universial time and I think the star was the far right object possibly a bit redish and airydisc visible versus disc in the other 3.

scope used an excellent 5 inch refractor with surplusshed f-9.4 lens. Plenty on Jupiter even though it is low in the sky here.


March 30, 2013 06:35 AM Forum: Telescope Making

12.5in mirror - Ronchigram after 5hr polishing

Posted By Phil Barker

well done

Early days close to a good sphere nothing to be concertned about.

A little bit of a hole in the centre and a slight tde.

keep to the basics press well don't polish for too long (as it can distort the lap) and alternate your stroke.
I usually do a 1/3 w mixed with number 8 and centre over centre also alternating mirror on top and tool on top.

I always warm her up with mirror on top to reduce risk of scratching.
You could try shortening the stroke a little closer to completion and that tde will reduce.

Pyrex is hard and rouge is slow but polishes beautifully. Its a good idea to use your back and straighten the arms as you can injure your arms and elbows if not careful repetitive strain injury.

I polished an 18 inch by hand once

keep up the good work

June 28, 2013 02:17 AM Forum: Telescope Making

Refractor Glass

Posted By Phil Barker

i'm doing a 5.8 inch f-15 lithrow cemented doublet 3/4 way thru grinding will be polishing when warmer its the middle of winter in new Zealand,

I want to mount on an heq5 so will be trying to save in weight etc

when I get a chance i'll show you the wedge tester and spherometer I made for the project which are simple but seem to do the job.

Have a go. picked up blanks 2nd hand for 180 dollars a bargain.