Last Updated March 22, 2003

Here is my conceptual view of the 116 degree folded design. I've chosen to microflex this scope as a test to see how well the technology works and hopefully I'll end up with a high resolution and portable instrument. This design gives me a 16" f5.2 scope with an eyepiece height of 66". This mirror incorporates a spherical curved back mirror design. Both the front and rear surfaces have the same radius of curvature of 166". The mirror looks like a meniscus lens but polished only on the reflective frontal surface. I'd like to thank Bill Kelly for his guidance in this endeavor. The basic design follows the article as written in the November 2000 Sky & Telescope magazine.

Design Goals:

1)    High resolution instrument good for planetary, lunar and deep sky work.

2)    Highly portable and minimalist in design. My goal is a smaller packing size than my current 12.5" so it fits nicely in the trunk of the 2000 Maxima.

3)    Eyepiece height low enough to not require a ladder or step (~68").

4)    Cost control

    My Conceptual Design

Project Cost Breakdown:

Item:	Planned Cost (CDN)	Actual Cost (future)
Cost of the Primary Glass	$150	$150
Cost of coating the primary mirror	$200
Cost of 3.0" Diagonal	$275
Cost of focuser (home made helical Crayford)	$50
Flex Mirror Cell (aluminum puller, outer push frame, foam, adhesive)	$100
AL Truss tubes	$90
Aluminum Frame (no wood anywhere)	$110
Secondary Cage (frame, spider, clamps)	$80
Hardware (screws, bolts, washers, clamps)	$100
Misc. Items	$75	$25
Total Damage	$1340 Canadian

Hogging Out Process:

Both sides need to be hogged out! To save time I've chosen to use my diamond grinding wheel approach on both the front and back surface of this mirror. I do not recommend this approach unless you take extreme precautions in safety. My total diamond grinding time was 1.5 hrs.

    The photo on the left shows how I smoothed out the curve using a cast iron 4" Pipe tool. 

                         The 4" tool cuts extremely fast!! Total working time front and back was about 2 hours with #36 grit.

 

    Photo of the Spherical Curved Back 0.192" sagitta.

    Spherical front 0.192" sagitta

    Back side tool. I cast this tool right against the back curve left immediately following the 4" pipe procedure.

    Another view of the back side tool. Notice the tool did not grind out to the edge. 

                        Measuring the back side sagitta was tricky because I could not simply measure the dig depth of the tool.

                        I ended up using a height gage to measure the center to edge height of the back curve.

    Rough Grinding the outside edge of the rear curve using the tool on top using and the Chordal stroke.

                         This is the reverse to what we're used to doing. Used #36 grit then 60 and 80. 

                         Total time both sides was 2.5 hrs.

 

    Edge on view of the concave tool and curved back mirror mounted back side down.

    How I measured the mirror to get the wedge close. At this stage the wedge is within 0.002" and I'm still using #60 grit.

                         De-wedging at this stage was all done on the back side of the tool.

    This is how I inspect for pits. I look for uniform pit size. An eyepiece can also be used with good results.

 

Preparation for Fine Grinding

The first step in fine grinding is to get find a way to fully support the mirror from deforming during fine grinding and polishing. I used a back side tool with lots and lots of silicone blobs - about 150 to provide the flexible yet still support needed for this work. The photos below show how I accomplished this. During my 1st attempt I oiled the back of the mirror to prevent the silicone from sticking. Well it ended up sticking anyways and what a MESS! I ended up stripping the silicone and re-griding the mirror to get the silicone off of the glass.

    Lots and lots of small blobs. Notice the tooth picks at the edge used to space the mirror off of the tool 

                        thereby determining the silicone thickness.

    Drying the silicone took 24 hrs with a small fan blowing air in between the air space. Rotated every 1/2 day.

    View of the 1mm thick support blobs of silicone.

    Another view. These support points worked really well!

 

Fine Grinding and De-wedging the Mirror

With a Micro-flex mirror it is essential to have a very uniform thickness mirror. The goal was to obtain less than 0.001" wedge. I used a dial indicator, and flat board and two edge supports to accomplish this. The indicator is mounted about 1/8" from the edge of the mirror (back side up) and adjusted to zero. Rotating the mirror gave an indicator reading of the amount of wedge. The measurement repeatability is less than 1/4 thou 0.00025". It is important to check repeatability before using this measurement as valid data. I spent about 5 hours find grinding and de-wedging the mirror. De-wedging the front surface was complete by the time I reached #320 grit.

    Wedge measurement.

    Top view of wedge measurement.

 

Polishing the Mirror

The plan is to use aggressive polishing pads to quickly polish out the glass then make a thin full size pitch lap to get the accurate sphere. The finale will be slow smooth strokes using rouge.

    Photo of the polishing tool and the pads I used.

 

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