16" f4 Orion Optics UK Dobsonian Project

[Moonshane]

After seeing John's Baader Steeltrack, I really had my heart set on that but at just under a third of the price, I really could could not resist Revelation Newtonian Crayford Focuser Dual Speed so I ordered one. the one on my 6" planetary dob is excellent and works really well.

[johninderby]

After seeing John's Baader Steeltrack, I really had my heart set on that but at just under a third of the price, I really could could not resist Revelation Newtonian Crayford Focuser Dual Speed so I ordered one. the one on my 6" planetary dob is excellent and works really well.

At that price I don't blame you. Wonder if they are going to discontinue it as it has all the hallmarks of a clearance sale. But still it's a good focuser though.

John

[spaceboy]

I was thinking it's because Astroboot is shut and you cant get the adapters cheap. Teleskop do them but with postage they cost a fortune in comparison. Not so much a problem for Shane as he is building from scratch but anyone looking to retro fit them to their SW, Orion or Celestron will have some stress when they find they don't fit. People aware that the adapter is needed will probably be holding off until astroboot is open and TH have noticed sales are slow and in need of a boost. At this price you can balance out the cost of the adapters from Teleskop. Probably me reading too much in to it as I do.

[johninderby]

You should be able to get the wide Skywatcher base-plate from FLO or Telescope House.

John

[spaceboy]

Your kidding! I waited 3 months for astroboot to open and in the end gave up and tried here. I put a post up asking if you can get them any where else. Where was you John?

Never mind it's not like I had much chance to use it with all the cloud anyway. They are a superb focuser for the regular price so anyone getting one in the sale will be over the moon "if FLO/TH can get an adapter".

[Moonshane]

progress report. I have mocked up in cardboard my mirror cell. See photo. This is based on 'the bible' Kreige and Berry although they obviously use a square box as the home for this. Mine will be a little more complicated as 1) my 'home' in round and 2) there's only half an inch space around the mirror.

The problem with point 2) is that this leaves no room for a sling which I would have preferred.

Therefore I am replacing the sling with a 10mm strip of teflon on the bottom half of the cell i.e. the part that takes the weight.

It seems to be pretty much OK although obviously the dimensions will be more accurate when it comes to welding. There will be a ring of 100mm x 3mm flat steel all round the cell into which the clips and tube fastenings will go. and to which the Teflon will be attached. This will all make for a very secure cell although it might be a little heavier than the OOUK version, it will be a lot cheaper - around £35 all in for an 18 point cell ain't too bad.

Also received my finder shoe (thanks FLO) and Telrad base (thanks Harrison's) and ordered my spider, holder and secondary mirror (thanks OO!).

[Moonshane]

ps

this is the first time I have made anything like this so any comments on obvious flaws in this design would be appreciated.

[Moonshane]

further progress made today. managed to find a local supplier of steel in central Manchester and collected some 25mm x 25mm box section mild steel, some 25mm x 5mm flat bar in mild steel and what I though was 100mm x 3mm in mild steel but is actually stainless. in fact stainless is better for the triangular supports for the planned 18 point cell but probably not for the ring of steel I was going to use in the main cell - more on this in due course. also, it's a blighter to cut and I found this out today when making the bars and triangular supports. I tried a jigsaw blade but only had some cheap ones and they were destroyed in a few minutes.

therefore it was over to the hacksaw and cutting by hand - not great fun but got it done in a couple of hours. then I ground them all to a pretty exact equal size and then rounded the corners to make them a little more aesthetically pleasing.

I also cut the bars to the appropriate length.

hopefully welding and drilling/tapping where appropriate and painting with black hammerite will all be done this weekend.

here's a pic with them all in the roughly correct positions.

[Moonshane]

Just got stung with the flocking from USA - £29 on a £80 product (inc postage).

Never mind, it will be here today.

[callump]

Looks like you are making good progress.

I am a little concerned about the teflon strip that you have all the way round the lower half. I am not saying it won't work, but it could lead to astigmatism - i'd suggest trying a two point contact (two small pads 45 degrees either side of centre). These could be teflon - ideally they are point contacts at the centre of gravity, but strips might be ok - there needs to be enough flexibility so that the mirror rides on these without being constrained.

On a similar thought, the clips that you show, these should just be a safety measure to stop the mirror falling out - with an 18 point cell, again the mirror is constrained by the points on the triangles and rides on the edge supports - and there needs to be enough flexibility for the whole arrangement to move when collimating (if your collimation bolts are ending up at the bars that support the triangles - a la K&.

Hope this makes some sense!

Callum

[Moonshane]

Looks like you are making good progress.

I am a little concerned about the teflon strip that you have all the way round the lower half. I am not saying it won't work, but it could lead to astigmatism - i'd suggest trying a two point contact (two small pads 45 degrees either side of centre). These could be teflon - ideally they are point contacts at the centre of gravity, but strips might be ok - there needs to be enough flexibility so that the mirror rides on these without being constrained.

On a similar thought, the clips that you show, these should just be a safety measure to stop the mirror falling out - with an 18 point cell, again the mirror is constrained by the points on the triangles and rides on the edge supports - and there needs to be enough flexibility for the whole arrangement to move when collimating (if your collimation bolts are ending up at the bars that support the triangles - a la K&.

Hope this makes some sense!

Callum

Thanks Callum, I really appreciate your comments as this is my first attempt at a mirror cell.

My 'problem' is that there's only about 15mm around the mirror to the tube. It's just possibly I might be able to put a sling in place but it will be very tight where the bolts go through the bars at the top. I will know more when I make the cell and put it together pre welding. The Teflon idea (it will be 10mm this and approx the width of the mirror's thickness) was that the mirror would be supported all the way around the bottom end and that it would 'slip' on the Teflon surface, thus not binding during collimation. The Teflon therefore has two main functions 1) hold the mirror centrally in the tube 2) provide even support around the bottom of the mirror where the weight ends up.

Yes, the clips would be just off the mirror (by 1mm approx) and there may be a need to have a gap in the Teflon 'sling' where the clips are fixed (which would effectively create shorter strips, more like your own description).

Does any of the above address your concerns? I'd appreciate further comments as matters progress. Maybe I should try and work on the sling approach and see what I can fit in. Perhaps I might just get away with an 8mm bolt (I suspect 6mm will be too lightweight and bend) but it will be very tight.

[callump]

There certainly does not sound to be a lot of space for a sling, though 'the experts' seem to think that the two point support is better than a sling, these days....

I think that you just don't need the teflon strip all the way round. Just two pads at the 45 degree points.

You can see a design with roller bearings at Jan Van Gastels website:

Jan van Gastel- Astronomy website-main menu/hoofdmenu

though you don't have space for the roller bearings, but I would try using teflon pads instead maybe 1cm wide (and the height of your mirror thickness), at the 45 deg points. I was going to try something like this (with teflon pads) on my 20" rebuild, but have since gone for a major redesign (and will be using rollerbearings now - as I have more space to play with).

Here is a pic from Jan's site.

http://members.ziggo.nl/jhm.vangastel/Astronomy/50cmscope/plaatjes50cm/bearingmirrorbox.jpg

There are quite a few atm'ers with similar cell designs, but I don't have the urls to hand.

Callum

[budski]

Lordy!, that's some tube. I assume the Saturn V engines are in there somewhere....

[Moonshane]

this is superb Callum - thanks!

I agree this makes sense and my concern was with astigmatism caused by the weight bearing on two points but it seems my concerns were unfounded.

I will try the slim teflon pad route and see how this works out.

pics will follow in due course.

cheers again

Shane

[Moonshane]

Lordy!, that's some tube. I assume the Saturn V engines are in there somewhere....

ha ha quite true. one possible future mod is to create a split tube to make it easier to transport but I'd like to keep the full tube if possible. It still amazingly fits into the back of my small hatchback with the rear seats down.

[Moonshane]

There certainly does not sound to be a lot of space for a sling, though 'the experts' seem to think that the two point support is better than a sling, these days....

I think that you just don't need the teflon strip all the way round. Just two pads at the 45 degree points.

You can see a design with roller bearings at Jan Van Gastels website:

Jan van Gastel- Astronomy website-main menu/hoofdmenu

though you don't have space for the roller bearings, but I would try using teflon pads instead maybe 1cm wide (and the height of your mirror thickness), at the 45 deg points. I was going to try something like this (with teflon pads) on my 20" rebuild, but have since gone for a major redesign (and will be using rollerbearings now - as I have more space to play with).

Here is a pic from Jan's site.

http://members.ziggo.nl/jhm.vangastel/Astronomy/50cmscope/plaatjes50cm/bearingmirrorbox.jpg

There are quite a few atm'ers with similar cell designs, but I don't have the urls to hand.

Callum

ps, I should mention that one of my concerns was that I will (if I retain the solid tube) transport the scope horizontally (maybe with a slight incline - focuser end slightly up). this was also the logic of the teflon sling but from what you say this should not have an effect.

do you think it would be safer to take the mirror out for transport?

[callump]

this is superb Callum - thanks!

I agree this makes sense and my concern was with astigmatism caused by the weight bearing on two points but it seems my concerns were unfounded.

I will try the slim teflon pad route and see how this works out.

pics will follow in due course.

cheers again

Shane

If it does not seem to work out, you will always have the option of going to your long strip.

Callum

[callump]

ps, I should mention that one of my concerns was that I will (if I retain the solid tube) transport the scope horizontally (maybe with a slight incline - focuser end slightly up). this was also the logic of the teflon sling but from what you say this should not have an effect.

do you think it would be safer to take the mirror out for transport?

If you trust your clips, it should not be a problem....

I think you would have the same concern with your teflon sling or using two pads.

It would probably be safer to take the mirror out for transport, but then you have a new danger when setting up and taking down, of moving the mirror. I'd probably do a few gentle tests of the mirror in the cell to see how secure it is.

Callum

[Moonshane]

cheers Callum

I suppose a 16" mirror is not that heavy really.

I'll do as you suggest and try it carefully (although I am always careful anyhow and surround the OTA with pillows and deflated old airbeds to retain in position when traveling).

[Moonshane]

hi all

after some thinking and discussion with various people I decided to change my plan for the cell and rather than try and fit a square peg in a round hole, I decided to fit a triangular peg in a round hole which is much simpler.

I still wished to retain the partial ring of steel that will allow me to secure the cell to the tube wall, add clips and also some teflon strips to act as buffers at the bottom of the tube.

The result after an afternoon's welding with my father in law (thanks Ken!!) is as shown below.

This will be an extremely strong cell and there will be no flex at all.

The retaining ring will also provide further strength and the gaps will also make putting the mirror in a breeze too.

I temporarily placed the support triangles etc on the cell for effect and there's also a pic with the cell in the bottom of the tube to show that it's an excellent fit.

Now just need to add some welded nuts, tap the holes for the collimation bolts and drill and final fix the supports, add the clips and teflon pads and we are ready to go (bar the rest of the work:

make alt bearings (30mm plywood) - half 'cartwheel'.

make rocker box (30mm plywood)

make ground board (30mm plywood)

make feet (possibly adjustable)

flock tube

fit focuser

fit secondary

fit finder bases

paint cell

fit cell and primary

star test and use for years of enjoyment!

cheers

Shane

[johninderby]

Love the bit of modern art you've created. Is it going to be hung on the wall for display?

Seriously though it certainly looks good and solid. A proper job.

John

[Moonshane]

ha ha maybe John, I do believe that a large dob is a thing of great beauty after all

[Astrokev]

Looks really solid Shane. I'm struggling to see how you intend to fit the collimation bolts, but I'm sure it'll become clear with your next update. Good luck.

[spaceboy]

Looking good Shane. Like little pieces of the jigsaw coming together. Keep up the good work!

[Moonshane]

cheers guys. the collimation bolts will be set into threads which I need to tap into the holes drilled through the cross members. you can see the three holes in the pics. each triangle is on a bar and the collimation bolt will pass through the centre of this and the cross member.

I'll be using kitchen style adjustable feet so I'll be able to stand the OTA on these before dropping onto the base if required.

that said, I'll be making some wheelbarrow handles for this so there will be a lot less lifting.

although rock solid, this cell is a lot lighter than I expected it would be which is a bonus.