20" f/4 Dob build

[swamp thing]

I've just about reached the point of deciding on a focuser for the Dob. With feathertouch just edging it over moonlite, when someone threw me a curve ball.

SI-SIPS-KIT---Starlight integrated Paracorr System(SIPS) Coma Corrector w/FTF2015BCR focuser

Now I know I'm gonna need a Paracorr at f/4 so this looks good on paper but how's it in the flesh?

I've asked on CN but thought I'd ask on here. Anyone got any thoughts?

It's just the idea of not having to adjust a Paracorr for each eyepiece that's really appealing.

Regards Steve

[Adz]

Wow, i've never seen a focuser like that before. I guess its worth considering it changes the properties of all eyepieces used with it slightly doesn't it? That and collimation with it would still be okay.

[nytecam]

Very nice project Steve How do you stop the support plates rotating out of alignment and will you use a strap/sling to support the mirror edge?

Only managed a demountable 17.5" dob conversion awhile back but good fun @ WPO - big dobsonian and used a novel flexible ground board to dob

[swamp thing]

Very nice project Steve How do you stop the support plates rotating out of alignment and will you use a strap/sling to support the mirror edge?

Thanks.

The Black plastic circles that are taped to the support triangles prevent the connecting triangles from rotating. As they cannot rotate without the support triangles moving and the tape prevents this.

The two split bolts at the sides of the mirror (on the centre rung of the tailgate) will accommodate the support strap.

Love the simplicity of your 17.5" Dob. I've often wondered with a design like that how do you keep dew and stray light to a minimum?

Regards Steve

[swamp thing]

Wow, i've never seen a focuser like that before. I guess its worth considering it changes the properties of all eyepieces used with it slightly doesn't it? That and collimation with it would still be okay.

Yeah it's a new one on me too. As I said its a bit of a curve ball. Definitely an unknown.

If it works well it could solve a lot of headaches in one go though.

[Moonshane]

I am sure it's a beautiful focuser but very expensive. Assuming you can afford the cost then I bet it will be superb. Collimation seems to be something to consider http://starlightinstruments.com/store/image/data/PDF/Howies%20updated%20notes[1].pdf

using a paracorr is not any effort at all.

[spaceboy]

If the night at PSP is any thing to go buy I was unaware a paracorr was even there. Shane must have effortlessly adjusted it between EP changes as I only noticed coma the once and I assume that was only due to the demand of every one to look through the moondob.

The SIPS focuser looks amazing Steve but as with Shane I doubt you would really have to make all that many EP changes and surely the money save by choosing a paracorr or similar could be money better spent else where ???

Just my 2p

[swamp thing]

I am sure it's a beautiful focuser but very expensive. Assuming you can afford the cost then I bet it will be superb. Collimation seems to be something to consider http://starlightinstruments.com/store/image/data/PDF/Howies%20updated%20notes[1].pdf

using a paracorr is not any effort at all.

I was looking at a Paracorr anyway Shane and this works out cheaper than a Paracorr + feathertouch focuser.

I can't rely on picking up a second hand Paracorr as they're like hens teeth second hand.

The way I look at the cost is I did save quite a bit with buying a second hand mirror. So I can afford to splash out a bit on the hardware.

This is going to be my last big scope. So going to make it as good as I can.

Don't wanna end up with a scope with the same probs as the Lightbridge. I've used some other people's scopes with good focusers on but never had one myself so this is my treat to me:)

[Moonshane]

then go for it mate! it will be a stunner.

[nytecam]

Thanks.The Black plastic circles that are taped to the support triangles prevent the connecting triangles from rotating. As they cannot rotate without the support triangles moving and the tape prevents this.

The two split bolts at the sides of the mirror (on the centre rung of the tailgate) will accommodate the support strap.

Love the simplicity of your 17.5" Dob. I've often wondered with a design like that how do you keep dew and stray light to a minimum?

Regards Steve

Thanks Steve ---uuuummm tape? I used steel pins beneath each mirror support to ensure no rotation as my pic.

Yes dew a problem with tubeless design:D The primary is deeply recessed but thought I'd use tiny battery powered fans across the primary surface and maybe secondary too but vibration maybe an issue but not perhaps with rubberband fan supports. A large black card shield behind secondary to block any extraneous light within the EP drawtube fov - but sold scope before that happened:hello2:

[swamp thing]

Thanks Steve ---uuuummm tape? I used steel pins beneath each mirror support to ensure no rotation as my pic.

Yeah! Double sided carpet tape is all that's needed in fact on the beacon hill scope the mirror came out of no fixings were used in their cell at all.

The tape only has to hold the triangles in place theres no force acting on them to move them except there own weight. Which is next to nothing.

The only time they will try and move is when they have no weight on them ie when the scope is horizontal. This is almost never. The rest of the time the weight of the primary presses on them and they won't move.

Double sided carpet tape sticks pretty well anyway.

Regards

[nytecam]

Quite right Steve - I'm a bit old-fashioned and never thought of sticky tape as engineering but if it works....

[swamp thing]

Quite right Steve - I'm a bit old-fashioned and never thought of sticky tape as engineering but if it works....

I must admit I was somewhat sceptical at first but it does hold it very well. Looks like we're neighbours, so you'll be able to judge for yourself at some point no doubt.

Where abouts in Surrey are you?

[swamp thing]

Been waiting for parts these past few days, but now things are starting to happen again.

I wanted to wait until the tubing arrived before routing the UTA tube seats, didn't want to guess this.

The tube Inserts arrived first followed by the ally poles for the tubes.

I could then get on with routing the UTA rings for the tube seats.

After this I predrilled them for the UTA truss locaters too.

This done I could start fine sanding and finishing.

The upper tube poles have been cut to length and the tube inserts fitted. This was tricky ( I would search for a better quality insert if I ever build another scope).

I also tried out the truss hardware on the ally tubing and I must say the Websters stuff is exceptionally good well worth buying and grips like a pit bull.

Here's a couple of pics of the UTA it gives some idea on the finished size of the scope.

I will disassemble it again tomorrow to drill the UTA poles for the spider bolts.

Give the rings there top coat of varnish too.

[spaceboy]

I know you know what you are doing Steve but forgive me when I say they look a little reflective / shiny ????

[swamp thing]

They'll be on the outside Nick there is a Matt black plastic light shield to fit inside the rings yet.

All will be revealed soon.

[kookoo_gr]

The truss tube hardware arrived today.

I was going to make my own truss blocks but TBH just couldn't be A&E&£@d. The problems with wooden split blocks needing to be accurate for frequent assembly/disassembly when made of even hardwood is obvious.

I had seen the Webster telescopes hardware and had always been Impressed with the quality of the fixings. Some PM,s exchanged with Umadog (cheers buddy) convinced me this was the way to go.

I must add the service and speed the parts arrived from Detroit was very impressive.

Here's a couple of pics:

The lower mirror box clamp is at the top right. With the UTA clamp below.

The upper clamp locator is just placed on the UTA ring for the photo.

They didn't come with instructions but Eric Webster remembered to email me the fitting instructions. Which are excellent.

Another of the chief reasons I chose these was they leave both ends of the tube open so cables can be run up the truss tube keeping it tidier and safer than cables hanging everywhere.

do you have a link for the clamps of the tube you got?

[spaceboy]

They'll be on the outside Nick there is a Matt black plastic light shield to fit inside the rings yet.

All will be revealed soon.

Ahhh I see. I did say I knew you know what you were doing

[swamp thing]

Ahhh I see. I did say I knew you know what you were doing

Wouldn't go that far.

I'm just following the book, and winging it a bit when they omit details.

Typically thats at the points you need the most help.

[swamp thing]

do you have a link for the clamps of the tube you got?

Do you mean the truss clamps?

If so here:

TELESCOPE TRUSS CLAMPS

[swamp thing]

I took the UTA apart again, gave it a top coat of varnish, marked out then drilled the struts for the spider fixings. Bolted the truss locaters in position.

I've had to resize the spider a little bit and give it another coat of mat black. Placed it into position and it looks ok (the bolts for it will be cut to length then painted mat black before final assembly.

The spider will come back out to fit the plastic light baffle.

I placed one UTA truss clamp on.

And with hound for scale.

Next job focuser and Telrad board. Still debating whether to go with the same ply as the rings or some veneered thinner stuff.

Hopefully get the focuser in the next week or so.

[Daniel-K]

looking good steve

[swamp thing]

Thanks Danny

Hope the 16" is keeping your eyes happy.

[Don Pensack]

You do realize that having the spider vanes arranged as you do means each bright star will have 8 diffraction spikes (in 4 pairs)?

[swamp thing]

Good point Don.

I totally forgot about that

It's out of the scope again at the moment as I'm working on the focuser board.

I'll have to look into it. May have a bit more work to do on the spider after all.

Thanks for that you could well have saved me a lot of head scratching later.

Regards Steve