DIY Super Newt

[laser_jock99]

There are a lot of budget F4 Newtonians out there, typically the optics are fine but they are let down by poor tube mechanics. If you want to pay top dollar for Carbon Fibre 'tuned' OTA's that's fine - but the optics are the same. My starting point is a £390 GSO/Revelation Astro 200mm F4 steel tube Newtonian.

To keep weight down the OTA tube wall is made of 0.7mm thickness steel tube. The focuser assembly is bolted to this flimsy tube by four M3 screws. Problems begin when you hang a heavy camera off the focuser assembly. The OTA tube bends and critical collimation and/or CCD orthogonality is lost. Since the main aim of telescope collimation is to line up the optics with the optical axis of the focuser assembly- if the focuser optical axis is movable then you will never succeed!

This mod is therefore mainly about boosting the stability of the focuser assembly in steel tube Newts by adding an internal tube strengthening plate.

I began with spare piece of 1.5mm thickness aluminium sheet (7” x 19” an old 4U rack blank panel). I gave this to my local sheet metal company and they roller bent it (FOC!) into a 230mm diameter curve- slightly under the inside diameter of the OTA.

Next step was to cut a hole with a coping saw slightly larger than the existing hole.

My rough cut hole was then trued up with a flap wheel.

The finished bare strengthening plate.

Next step is to remove the focuser assembly from the OTA- this is made easier by taking off the Crayford Roller first.

At this point it is wise to cover the secondary mirror with several bags to protect it from the next operations. The primary mirror cell has also been removed for it's safety and for access.

[laser_jock99]

Next the bare plate is laid over the OTA and the outline marked with tape. This will tell where the plate is inside when I'm drilling the rivet holes. Note at this point the plate has been cleaned and prepped for painting.

The marked up OTA- the positions of the rivet can be now be added (I used a 7cm spacing)

The strengthening plate is now sprayed matt black & ready to go inside the OTA via the primary mirror end.

With the OTA firmly held on a work bench drilling can start. Clamp the strengthening plate to the OTA and open out one focuser to 6mm and bolt together with an M5 bolt. I did the holes one at a time and bolted them to avoid any shifting after releasing the G-clamp.

Once the focuser holes are temporarily secure, I drilled out the finder bracket holes and bolted them up. Next I worked outwards away from the focuser, one rivet at a time to ensure the plates came together properly, clamping where possible. A handy tip is to drill the rivet holes through insulating tape as it avoids drill slippage and marking of the OTA paintwork.

The finished OTA outside, fairly neat riveting job considering it's the first time I've tried this! Note also the much stronger M5 stainless bolts holding the focuser to the OTA.

[laser_jock99]

A look inside the OTA now shows an awful lot of potentially light scattering ironmongery!

The next task is to paint all the screws & rivets matt black. I used a long handled brush to get at the back!

At this point I felt a light baffle behind the strengthening would be a good idea. I measured all our pie baking tins but to no avail. Eventually a found a plastic bucket the right size and cut the base off it!

I offered the bucket base up to the primary mirror and noted that a 10-15mm baffle edge was about right. I used one of the earlier rejected pie tins to mark a cutting circle.

The plastic baffle ring was roughened up to help the paint stick and sprayed matt black. Being slightly over diameter helped since I could split it, the overlap made it self holding inside the OTA.

A view down the finished OTA showing the strengthening plate and new baffle.

The focuser assembly is now more solid with the load spread over a large 19"x7" plate and held on with M5 instead of loose M3 bolts. Hopefully it should not shift anymore. I will be testing this scope as the nights get darker and maybe incorporating the same mod to my 12" F4 scope.

So- with my other mods (six spring mirror cell, long Losmandy bars/top bar etc.) and the ASA Keller 0.73x reducer corrector I'm now a step closer to a Super Newt!

[Moonshane]

excellent job! the baffle is particularly innovative. this strengthening plate idea could also be used for those fitting a new focuser to the skywatcher tubes with massive holes in the OTA.

[101nut]

It's an interesting piece of work but why did you not make the 'strengthening plate' the full tube circumference, bond it directly to the tube and avoid all the rivet problems?

AndyG

[laser_jock99]

It's an interesting piece of work but why did you not make the 'strengthening plate' the full tube circumference, bond it directly to the tube and avoid all the rivet problems?

AndyG

The plate was that size mainly because it was the size of the piece of metal I had to hand. When I mod the 12" F4 I might calculate the plate to be a full circumference and maybe wider. I was going to epoxy & rivet- but last minute I decided epoxy would be hard to apply without making a mess up the rest of the OTA. Six bolts and 19 rivets are sufficient anyway.

[dazza1639]

Did you put a separating layer between the aluminium plate and the steel tube? If not you may suffer from bi-metallic corrosion between the aluminium & steel. Especially in the damp dewy conditions the scope will be subjected to.

[laser_jock99]

Did you put a separating layer between the aluminium plate and the steel tube? If not you may suffer from bi-metallic corrosion between the aluminium & steel. Especially in the damp dewy conditions the scope will be subjected to.

There is the layer of black tube paint- but I'm not expecting any galvactic corrosion - it's a telescope not a Landrover! I can get the aluminuim sheet pre-anodised with protective film. I could leave the film on the non painted side next time if it withstands the roller bending process?

[dazza1639]

If not PVC barrier tape is quite cheap or electrical tape