Having read Gary Seronik's article in AS&T this month, I decided to have a crack at building one. My handyman skills are dodgy at best but nothing ventured!!
Have put the basics of the OTA together so the mirror cell is made, side struts cut and prepared and the upper ring is cut and varnished. Assembled it looks as per photos attached.
The changes I have made from Gary's design are
1. I went with a f4 mirror rather than f4.2 as the f4 was size that Bintel sold.
2. The struts are 25mm square aluminium tubing. Gary used 1/4 T nuts to add to 4 wooden blocks that inserted in the ends of these. I could only find 5/16 T nuts and they are too big to fit inside the 1" tube so I went with 6mm threaded inserts instead.
Next step is installing the mirror, focuser and secondary spider, holder and mirror. I have ordered a nice new Moonlite and Astrosystems secondary holder and spider. The other major difference with my scope and Garys is I have gone with a tradition 4 vane spider rather than a curved one. I am used to setting up a 4 vane but if it turns out too weighty I can change it over later. The 8" mirror and the 2" secondary have arrived so I will probably install the primary tonight.
What does the round hole in the bottom under the primary mean ?
And are two instead of three trusses not a bit unstable ?
Do you have a design of the rockerbox (I assume it will get a Dob ) ?
The round hole is to mount a fan. My hope is that 2 trusses in a small scope will prove sufficiently stable. Certainly Gary seemed to find it OK.
An online version of building it is here. http://garyseronik.com/?q=node/206
I have fitted the primary in the cell. Just 3 very substantial blobs of silicone adhesive above each of the collimation screw heads. used some spacers so ther is about a 1mm gap between the ply and the mirror to allow for air movement.
The mirror came without a centre spot so I have added that. Not a major issue but had to pay $4.95 for a whole packet of white rings, and only used 1!!
Next step is to await the bits to enable me to complete the OTA, namely, focuser, spider and secondary holder. That will enable me to work out the balance point and thus the depth of the rocker. So probably no further action until next week.
. used some spacers so ther is about a 1mm gap between the ply and the mirror to allow for air movement.
I wouldn't let a silicone blob be any thinner than 3mm myself - it can have some pretty strong warping power - though you may get away with it on 3 points if the plywood warps.
Mark
The gap is actually closer to 2.5mm (I just measured it!). Also the collimation bolts are countersunk into the mirror plate so they are flush with the top of the plate. As they are coach bolts there is quite a lot of room within the holes. I did this to give the silcone more area to grip onto to make it more secure.
Looking good Malcolm .
I had the early version of Gary's travel scope and it worked great even survived being posted home when my Landy died in WA , not as airline friendly as your scope though.
Good job, Malcolm. I'm having a go at the same project. What focusser are you using? I was planning on using a SW Crayford that I have, but ray tracing for an F4 showed that I'd need a 65mm minimum secondary. By using a low-profile focusser, I can get it down to 50mm, a decrease of 40% in the light loss from the secondary shadow. An F5 would reduce it even more, but the struts would have to be longer.
Thanks Graeme
I have ordered a Moonlite, bit costly but I really like Moonlites, so what the heck, it's only money!!
It is quite a low profile so I have grabbed a 50mm secondary & I think it should work, if not it's all a learning experience!
Alastair
It is a GSO mirror from Bintel. The threaded inserts are from the local hardware store. Easy to install in wood, just drill a 9mm hole and use an allen key to screw in. They take a 6mm thread.
I have cut 4 21x21mm squares of 18mm marine ply and then put a threaded insert into the middle. Then glued in place with 2part JB Weld epoxy (also from Home Hardware!)
Further update.
The moonlite has arrived and I have a note from the post office taht another packege is there, either a pack of Ebony Star or a secondary holder and spider!
was the ebony star from overseas?
how do you plan on keeping the focuser plate stable as its fixed only on one side? the weight of the focuser with your eyepieces would be substantial.
are you using dowels and glue?
