Posts Made By: Phil Barker

December 5, 2004 11:37 PM Forum: Telescope Making

polishing with aluminun oxide

Posted By Phil Barker

I always wondered if this would work.

As an experiment I added some 5 micron aluminun oxide to cerium and polished a mirror which had some grey near the edge and needed more fine grinding.

I did a few wets with good contact and was immediately aware of the rougher sound as I polished away.

This greatly speeds up the polishing 2 possibly 3 times as fast from what i experienced.

The grey went quickly basically i had a mirror that now didn't need any fine grinding.

A ronchi showed a rough but reasonably polished surface.

I didn't completly polish the mirror just went for 1 hour or so.

I then using the same lap which i cleaned thoroughly went to cerium and within 1 hour I could see no evidence of roughness in tests. I was well ahead of where I would be if the mirror was just polishing.

Basically this appears to be a way of speeding up polishing and if done carefully ie lap completly cleaned and returned to cerium has'nt degraded the polish at all.

Interested in comments.

Phil Barker
p.s the mirror is 16 inch f-5 and pyrex.

the lap is my own mixture 7/1 rosin and tar.


June 5, 2005 11:17 PM Forum: Home Observatories

9 foot paper mache dome

Posted By Phil Barker

Will be posting photos later but I'm building a 9 foot dome out of a ply frame with building paper chicken wire and paper mache.

the frame was cut with a hand zig after marking her out. All made from 1 sheet of ply screwed and wired together.

The mache is made with watered down pva glue. 6 layers are applied then 4 coats of paint.

The dome is finished except for the shutter which will be stainless steel sliding through plastic runners.

The dome structure is a 4 piece ring ring and 2 arches which are one piece. 18 ribs are screwed and wired into the shape along with cross braces between the arches. It is very strong.

Will sit atop a 1.8 metres building also 9 foot square.

PVA glue is amazing stuff when dried it is very strong. The chicken wire means hail just bounces off.

I got the recipe from a friend that build a dome that lasted over 20 years, The 17mm ply used is treated exterior ply. Should last decades.

Stray posted for the pickies.

Scope will be a 20cm lx200 and I will be doing imaging and photometery in the dome.

September 5, 2005 04:52 AM Forum: CCD Imaging and Processing/Deep Sky

ngc6781 with st7 and 8 inch sct

Posted By Phil Barker

at f-4 (f-6.3 meade with reducer)

6 minutes luminous and then 1 minute RG 90 sec B clours done with 2 x 2 binning

I'm new at this but the lx200 and camera are working flawlessly.

Hope to post some southern delights soon.

Phil Barker



September 5, 2005 10:05 PM Forum: Telescope Making

thin plate mirrors

Posted By Phil Barker

I'm almost finished figuring a friends 16 inch pyrex mirror and will be making a new scope.

I'm thinking 18 inch f-4.8 is within my capabilities.

The cost and risk in getting a big piece of pyrex here to New Zealand is a worry.

I can get 28mm thick plate at very reasonable cost. (around 1/4 of pyrex cost.)

I'm sure with a decent 18-27 point flotation cell it would work well.

Has anyone else tried a big plate mirror and had success???

I have seen a 20 inch made with 19mm plate which was ridden not surprisingly with astigmatism.
Phil B






September 15, 2005 08:21 PM Forum: Telescope Making

18 inch f-4.8 secondary

Posted By Phil Barker

Will a 3.1 inch secondary do with a low profile focuser and an 18 inch f-4.8 mirror?

I ask as I have access to one which I could use.

I'm thinking of a distance of 12 inches to focal point from the diagional.

I hope to use 2 inch eyepieces but 27mm panoptic is the larges I intend using.

I used to use Mel Bartels secondary sizing program but its no longer online.

Phil B

September 17, 2005 10:15 PM Forum: Telescope Making

the weight of plate versus pyrex

Posted By Phil Barker

It seems plate is slightly heavier than pyrex. I just received a very nice 18 inch 29.5 mm thick plate blank which comes in at a fraction over 26 pounds.

It seems to be about 10-15% heavier than pyrex of the same dimensions.

I bevell this baby later today then its off off and away on a new challenge.


December 6, 2005 03:12 AM Forum: Telescope Making

18 inch Kriegsonian project

Posted By Phil Barker

Getting along nicely with the truss tubed 18 inch f-5 scope I'm building.

I have just begun polishing the big 30mm thick plate blank.

I've also been making the scope itself out of a sheet of 17mm plywood.

All cut with hand held jig the bearings are 19 inches in diameter which is adequate in my opinion.

I'm keeping the costs down by making most of it including the mirror sling spider and possibly the finder but will be using a williams optical focuser.

Will be using formica on teflon for the bearing. i hope to cut out the rocker box later this week.

As to the mirror I will be testing figuring with the mirror slung. hope to do final startesting in the scope.

note the open back i will install a fan at some stage

December 8, 2005 09:11 AM Forum: Telescope Making

barnsite and 305 emery

Posted By Phil Barker

Just finished a spell on the big mirror I finished the grinding with some very ancient 305 emery left at te local observatory.

Been polishing with barnsite which I believe is no longer available.

Take it from me its very fast havent tested yet but this mirror is polishing a hell of a lot faster than the last 16 inch pyrex beast I did.

Does anyone know some info about barnsite the specs etc looks like rouge and changes colour while polishing starts out red and ends up at the end of the wet a similar colour to cerium.

Phil B

May 20, 2006 05:52 PM Forum: Telescope Making

18 inch kreigsonian first impressions

Posted By Phil Barker

For those that remember I was making an 18 inch f-4.8 newtonian.

The scope is truss tubed has a 3cm thick plate mirror and I built it all in the garage in the last year.

The finished scope is working extremely well. Optics figured using figure xp are superb at the eyepiece which is all that matters.

Omega centaurus ( I live in new zealand)through this scope is delightful when overhead.

I'll post picture of the big scope soon. Its like having a weight lifted off my shoulders finally completing this scope and I won't be building another for some time.

Phil B

April 22, 2009 03:37 AM Forum: Telescope Making

baffle for a nasmyth cassegrain

Posted By Phil Barker

I've built an 8 inch f-5.2 dall kirk with a elliptical flat situated around 100mm in front of the primary.

Is a baffle in front of and around the tertiary needed? I have to say I'm not getting the dark background I expected. The final focal ratio is f-19. The secondary is 65mm and the tertiary 45 mm in diameter.

The scope is an experimental design I am replacing the primary with a pyrex mirror I'm working on and the secondary with a smaller 60 convex plano lens from opto sigma. Tests on this from behind the lens with a ronchi show an excellant edge.

Overall the dall kirk gives a lovely flat sharp field with such a long primary.