I started off with a Meade LX200 10" SCT and used this for imaging for 5 years. As with most pursuits there is a desire to improve and better equipment is eventually needed.
After acquiring a AP1200 mount in 2002 , I bought a 12" SCT to go on it but the focal length was just too long and the use of focal reducers resulted in a "soft' image. A better quality ota was sought.
The Newtonian design is often overlooked as an imaging platform, I think mainly due to the physical size of the ota in apertures of 10" and above.
If an adequate mount is used the Newt can provide excellent results, cost effectively.
After searching the internet there were very few commercially available Newtonian ota's available that were optimised for imaging. So 'home brew' it was.
First I had to decide on what size and focal ratio mirror was needed.
My ST7 camera has 9 micron pixels so a focal length of 1600mm yields an image scale of just over 1 arc sec per pixel.which gives a reasonable sampling in uk skies.
My observatory also limits the size of ota I could build. A 10" f6.3 mirror was chosen.
The Ideal Design.
The ota has to hold the optics very accurately and not move from precise collimation.
A substantial focuser and focuser mounting is needed to support the weight of the ST7 CCD camera .
Aluminium has quite high thermal expansion coefficient. An alternative material for the ota ,with improved thermal properties, would be desirable.
The overall weight of the ota is of secondary importance.
Doesn't break the bank !
Initial Layout Drawing.
Orion Optics (Crewe. UK) supplied a 10" f6.3 optical set with 1/8 wavefront, enhanced Hi-Lux coatings and a focal length of 1583mm and a 63mm (25% OB) secondary.
The secondary is over-sized for the ST7 chip. The fully illuminated focal plane is18mm in diameter so will cope with a larger chip, if I upgrade.
I considered a truss design with carbon fibre poles but this required many machined parts increasing the cost.
The tube is 1500mm overall length, OD 320mm and 4mm wall section in fibreglass. There is 30mm from the inner wall to the primary for thermal clearance.
The thermal expansion coefficient for this tube is 1/2 of aluminium. I did get a price for a carbon fibre tube, but Hubble was starting to look cheap !
The round fibreglass tube construction means a rotatable ota to allow comfortable eyepiece position when used visually ( just now and then !)
Most mirror cell designs have an intermediate mirror holder which in turn is fixed to the tube end plate with jacking screws. I felt that this 2 piece design is a source of flexure / movement which would in turn give collimation problems. I chose to mount the triangular 3 point supports directly to the end plate but this means having adjustable mirror retainers. It makes adjusting collimation a little more complicated but hopefully once adjusted will remain so for longer.
<Update 1> I checked collimation with the ota in the West side of the mount and checked with it again on the East side. I thought it had changed ever so slightly but the seeing was preventing an accurate assessment. My Meade sct couldn't do this due to mirror flop. I had to collimate the Meade on the side of the mount I was to image on which was a pain and it would change during the night as the scope approached the meridian. The Newt. looks free from these problems.
<Update 2> The mirror is moving slightly when resuming from a horizontal parked position, although collimation does remain constant for an observing session. There is a slight clearance from the mirror edge to the edge retainers. I will close this gap slightly.
