A 76mm Maksutov

Surplus Shed regularly has a 76mm Maksutov optics set on stock. This set has been discussed widely on the Cloudy Nights ATM forum. As some others did, I also decided to give it a go and log the results here.

Analysis

The approximate design parameters of this all-spherical system are:

Primary: 76mm, RoC=276mm
Corrector: 78mm, R₁=75mm, R₂=82mm, T_c=12.2mm
Secondary: 23mm, RoC=R₂
System: f=810mm, FoV=6mm (0.5°)
Mirror separation: S=108mm, BFL=S+100mm

Note that these values are working approximations, direct measurements in general did not yield satisfactory results in OSLO. The OSLO len file can be downloaded here. Yet the separation of the two elements gravitates around 108mm and it is this value that was taken into account for the OTA design. The assumption is that the elements are well corrected.
Small changes in the separation result in large changes in BFL (about 30x). Since the overall correction is not critically dependent on the separation, it could ultimately be used for focusing like done in popular Schmidt Cassegrain systems from vendors like Meade and Celestron.

Prototype

The size of the system makes it suitable for 3D printing, and this is what I did. In the prototype there are four parts, two for the corrector cell, a primary cell and the OTA backend that also holds the focuser. Everything is glued/screwed into a 100mm PVC pipe segment of about 150mm long.

Alignment is only available for the primary, by means of three spring-loaded screws. The focuser is like TSFOCR2M from TS, and the backend is designed specifically for this. Similar focusers may fit as well, but in the final system I would like to get rid of the focuser and use the primary adjustment to achieve the same.
Below the final system, as tested.

The STL files for 3D printing are in this archive.

Results

A first test showed that the length of the tube was wrong, as it should be 148mm to obtain a reachable focus. The mirror separation indeed is quite touchy! The design allows to easily replace this.
The views on the moon were very good, although the contrast can be better. For this more attention should be given to blocking stray light, and also the central spot on the corrector should be covered on the front of the scope. Star images were pinpoint after proper alignment. The alignment procedure itself is quite easy, and can be done by taking out the coma of a centered and highly magnified star image.

Final solution

The final instrument does no use the Crayford focuser, but uses the mirror separation to this avail. With only a few mm change of separation a large range of focal plane positions can be achieved:

S BFL-S Strehl

105 309 0.95

106 211 0.95

107 147 0.93

108 102 0.91

109 68 0.87

110 42 0.81

For visual use, the sweet spot of the focal plane is approximately on the shoulder of the eyepiece. For use with a camera, the focal plane must be about 50mm farther away. So the design will be for a mirror separation of 108mm and hence a focal plane location of 102mm behind the primary vertex. The separation should then be adjustable for +/- 2mm.

The focuser is based on a snugly fit 'piston' that can slide up and down in the outer PVC tube. The length of this piston makes sure that it does not tilt. This piston focuser tube supports the primary cell, instead of the OTA backend in the prototype. The primary cell in turn is made to precisely fit the focuser tube, in order to further minimize the possible amount of primary lateral shift.
An M5 leadscrew, spring-loaded between focusser piston and OTA back-end, is used to adjust the axial location inside the outer tube.

Considering the dimensions, the length of the PVC tube section is quite critical: a few mm wrong and the focal plane becomes unreachable... So a good calculation based on measured values of the finalized parts is important.

In the drawing above the distances established in my case are shown:

The corrector thickness is what it is, 22mm.
The printed parts of the back-end, the piston and the primary cell have specific thickness (17mm in total).
The primary has a center thickness of 8mm.
The three alignment screws hold a separation of 6-9mm.
The focuser screw hold a separation of 5-10mm.

Hence the focusing range is 5mm, where the primary to secondary separation is allowed to change from 105-110mm. This results in an optimum tube length of 169mm.

The piston that is the center piece of the focusing mechanism. It has to fit snugly in the PVC tube, yet it should slide easily. This is important in order to minimize the wiggle of the image when focusing.
The stainless M5 leadscrew is glued into place with fast curing epoxy. It has to be square, so while curing I secured it with a washer and a nut.
Looking inside the piston, you can see the reinforcement ribs and the three holes for the collimation screws. The primary cell is mounted inside the piston tube.

The back side of the OTA shows the eyepiece tube and locking screw. This is not as fancy as most commercial ones, but it does the job. Here you can see how the focusing works, just like in an SCT with a fixed eyepiece location.
The front side, corrector cell, has an extended baffle tube and a dust lid.

Here the lid is removed, which uncovers the view into the OTA.

The scope is completed with rings and dovetail, made from wood scraps. The whole Surplus Shed Maksutov has cost me less than $50, nevertheless it gives very nice images of the moon and planets. It is a light weight grab and go device, very convenient for occasional observing. For deep sky objects however, the objective diameter really is too small.