8" Homemade planetary dob - The Project

[AstroLuke]

Wow! Good luck. Hope it will be great. I can imagine it will be a special scope when having built it yourself.

[earth titan]

Just received a copy of 'The dobsonian telescope' by Kriege and Berry. Loads of useful info, especially on the style of scope I intend. 470 pages...... Should while away a few evenings.

[earth titan]

Back to the grind

As a bit of relaxation and a break from tiling the kitchen, I got a few hours rough grinding of the primary this weekend.

I'd forgotten about some of those muscles......

After two sessions, roughly 2.5 hours total, I have made good progress. You can start to feel the mirror beginning to develop a curve during grinding as it moves under your hands and I have measured it at about .75mm (only about 1.2mm to go).

The curve does not cover the whole face yet, and there is still an area where the orginal moulded blank can be seen (photo below), but not far to go until this is gone.

I have been using a chordal stroke with a fair amount of overhang and as I understand it, I should begin to reduce this once closer to the target sagita.

My grinding technique, or lack thereof (and my untidy garage / depository for the families unwanted rubbish / me in scummy tiling clothes) can be seen in the following video:

I have put a grinding bench together, on top of a workmate loaded down with balast but it has a small amount of flex. Will have to try to improve this in case the mirror becomes astygmatic but up on the bench the work takes considerably longer as I have to keep moving the tool rather than walk round.

I have managed to reclaim loads of grit from the washing bucket and have mainly used this, as during this phase the largest grit I can end up with is the rough grit I started with.

Tool seems to hold pretty steady on just a wet paper towel, but a wet rag used later (not in this video) improved it still further.

Any suggestions?

[earth titan]

After another hour last night, I'm down to my target sagitta of 1.98mm and the face is ground all the way across.

To check if I have a sphere, I marked the tool face all over with a sharpy marker and rubbed the mirror over the top a couple of times. I don't have a spherical surface just yet, as it appears deeper in the middle, so the next phase is to grind the edges away a little more. I shall use a 1/3 centre over centre stroke, mirror on top.

Can anyone tell me the best place to source hogging out grit? I'm down to my last few wets (which could be enough) but the recommended 80 grade carbo seems rare. Ordered some rough carbo from Galvoptics and it arrived as 180 grit, which was finer than I had expected. They tell me this is the coursest they have and their website shows this as the coursest grit in their kits. I would be there for a month of Sundays with that grade. You live and learn.

[ponytale]

Hi ET this is where i got my grit Vacuum Coatings Scientific Mirrors - Pricelists

[Glasspusher]

Looks like you are making good progress, I like the video. Yes, 180 grit is too fine for hogging out, you need 80 or even courser. I recently discovered this supplier...Cerium Oxide, Silicon Carbibe, Tin Oxide, Rough Rock, Plastic Pellets for Stone and Metal Polishing and Tumbling. a good source for the courser abrasives. Following your progress with interest...nice to see people making their own optics. First light with your own optic is something really special.

John

[Gina]

That's what I call real dedication Great stuff

[Vicky]

Wow! I'm in awe!

My ex-boyfriend's grandfather made his own telescope in the war. He told me all about the grinding, changing the grade of grit- you must be extremely dedicated!

Well done, and keep going- can't wait to hear about first light!

Best wishes,

Vicky

[earth titan]

I'm away now for the week (work to do, bills to pay) but before I left I had another brief session on the mirror. Much more spherical now, only a few more wets to go and still preserving the depth I need.

Thanks for all the encouragement folks. So far, thoroughly enjoyable and fascinating. Keep hoping for cloudy evening now, so I can can crack on and this reminds me of the quote I read recently which went something along the lines 'build your own optics, and never look through the eyepiece again.....', suggesting it will become addictive. Don't know about that, but there is something very satisfying about it.

Thanks John for the links for grits and polishing supplies and it also look as though Vacuum Coatings is a good supplier.

[earth titan]

Grinding coming up this weekend. Aiming to finish with 80 grit and move up to 120 but I need to get it spherical. This is proving difficult, although it is nearly there.

I have been reading the guidance on mirror grinding in Kriege and Berry, which suggests short W strokes with 120 grit are better at achieving a spherical surface than centre over centre. It seems to make sense as the grits move around more freely. What do folks think?

[Demonperformer]

NE Howard states "The strokes from this time [120 grit] on will be 1/3 W strokes, although they should actually be a little shorter than 1/3 of the diameter".

HTH

[earth titan]

Agreed. Howard suggests W stroke after spherical surface achieved. Kriege suggests W to achieve it.

[Glasspusher]

W stroke or through the centre, both will achieve sphericty if the stroke length is right. A good test for sphericity is the pencil or 'sharpie' test. Mark the surface of the mirror or blank with a series of lines edge to edge then same at right angles to make a grid on the surface. Rub the blank and tool together for a while then examine the lines to see where they are worm away.....uniform wear means sphericty. You probably new that trick anyway!! If the tool and blank are the same size working 50% of the time tool on top (TOT) and 50 % mirror on top (MOT) will help to bring the mirror to sphericty using the strokes mentioned above.

John

[Glasspusher]

For sphericty read sphericity, tongue twister that one.

John

[earth titan]

Thanks for all the help folks. The grind went well over the weekend, with only one minor hiccup!

Dispensed with the 80 grit and went down to the 120. Around 8 or 9 wets and I broke through a small bubble. Could have been done using the 80, but it wasn't until I started using the 120 that I could see it.

Switched back over to 80 for a few wets to get rid of the bubble and then back to 120 again. The bubble is now a very small 'pit' which you only make out using a 25mm eyepiee as a loupe, so after another few wets with the 120, all else appears fairly pit free.

Good clean up, then started with the 220. After a couple of wets the surface is becoming nice a mat in appearance and the abrasives are lasting longer each time. Still have one or two very minor pits from the last abrasive, so will carry on with 220 until these have all gone.

I've been using a 1/3 W stroke - 50% tool on top / 50 % tool on bottom so still preserving the sagitta of around 1.98

Surface is becoming obviously spherical now, with small bubbles appearing between tool and mirror, evenly over the surfaces.

Got to go away for work for a few days and the new kitchen worktops are arriving Friday, so could be a while before I get much more done. Just have to watch this space.

[IanLinux]

Calling earth titan - Tom, just looked at some of your images. Love the deep sky stuff. Are you using your Altair Astro 8" f4 imaging newtonian for this? They seem quite affordable at £400

[earth titan]

I am currently fiddling with it to try and improve on the images. Work is getting in the way though as I have to fly out to beruit next week, after just getting back from other far flung places.

Can't really comment as result, as I've only had 3 sessions with it.

Sent from my X10i

[IanLinux]

I am currently fiddling with it to try and improve on the images. Work is getting in the way though as I have to fly out to beruit next week, after just getting back from other far flung places.

Can't really comment as result, as I've only had 3 sessions with it.

You also have many scopes Tom. It'll be interesting to understand the rationale behind each one. Do you have a link to another thread discussing this?

Cheers

Ian