I've been waiting for someone to produce a quality, affordable, small Newtonian astrograph.
The closest thing I know of is the 130mm Takahashi Epsilon. And though it is a reflector it is not technically a Newtonian.
I got tired of waiting, so I made my own: "The Bellavia Mini".
I took the parabolic, diffraction limited mirror, from a $179 Orion Star Blast II, and made a much stiffer OTA for it, and replaced the small secondary mirror with a larger one to fully illuminate a 16mm circle. (And then found another 4.5 inch mirror, and gave that entire scope, still new in the box, to a 13 year old girl in our club)
It is 114mm aperture, f/4, 450mm focal length. What would that cost in a refractor? The only f/4 refractors I know of are Vixen, Takahashi and Borg (I also have an f/4 Borg).
It is also excellent for EAA.
The build process and some imaging examples here (note, all without a coma corrector):
I could try to make more formal "plans" to give out for free, but it is essentially just a wooden box. The primary support is a little "fancy" in that I used a very stiff and light aluminum-plastic-composite and glued the mirror at an appropriate radius determined using PLOP (no clips - clips cause diffraction flare on stars). I reused the secondary support assembly (but had to extend the screws to reach the corners of the OTA box).
I also made my own custom non-rotating helical focuser (I love helical focusers), but any standard focuser will work. (if attempting this, I suggest to use a rack-and-pinion, not a Crayford, for imaging). I also can swap in a Borg helical focuser, but that makes it more expensive, if cost is an issue.
And like all Newtonians, you should collimate just before every imaging session, and focus at f/4 has to be precise. You will need a good quality coma corrector. I just started using a Baader MPCC, and waiting for an Explore Scientific HRCC02-00 to arrive to see if I like that better.
I also own the 6-inch f/4 astrograph that is branded with many names now, but it is longer and heavier. And every version I see has as issue with the camera placement. It is way too far back from the secondary, causing vignetting for larger sensors. They should have made the tube longer and moved the camera as close in as possible, like a "normal" astrograph.
With the 4.5-inch f/4 and a 2.4 micron pixel camera (like those with the Sony IMX183 or 178 sensors), it is 1.1 arc-sec/pixel. That is fairly ideal for many/most objects.
I may produce a mass-produced version one day, since nobody else seems to want to.
Edited by StevenBellavia, 23 April 2019 - 10:18 AM.