Home made flip mirror - Gina style

[Gina]

Rather that continuing to gatecrash smudgeball's thread I'm starting my own. My version is slightly different but uses the same principle of using a cheap 1.25" star diagonal bought from AstroBoot.

I have been having serious problems getting a planetary image onto a webcam imaging sensor - this idea should solve the problem by providing a greater viewing angle in an eyepiece so that the planet can be centred in the eyepiece and should then land on the webcam sensor.

The mirror was held in by a clip in plate on the diagonal. This and it's housing were pared down so that it was no longer held. Also, the ends were cut down to allow it to hinge down from one end and the other end for clearance - more of which later.

The mirror retaining plate was attached with hot melt glue to a 3mm plastic axle with holes as bearings in the star diagonal main body. This enables the plate with mirror attached to flip downwards and allow light to go straight through instead of being reflected. The end of the plate by the axle was cut back to allow the plate to rotate with the axle. Since the mirror was originally loose and held only by the clip in plate, it was glued onto the plate so that mirror and plate formed one flip down unit.

To provide a larger planet image on the webcam sensor I decided to include a Barlow within the box and put the webcam into this. In this way, the image formed by the mirror in an eyepiece covers a much larger angle of view than on the webcam image sensor. Without a very long scope a Barlow is needed to get a reasonable image size. In view of the light path length through the flip mirror, a Barlow between it and the scope is not practical.

I worked out that the Barlow would need to be close to the flip mirror so the mirror plate needed cutting back ( as referred to above) to provide clearance.

Attached are diagrams of the whole unit plus detail of the mirror assembly.

[Gina]

Think I've just about finished my flip mirror Here are some photos of the work in progress. I'll comment each photo separately.

Firstly, these are the component parts.

1. Box with holes cut with hole saw and adjusted with a half-round file.
   
2. Star diagonal already modified to a flip mirror.
   
3. Section of waste water pipe.
   
4. Barlow lens.
   

Here is the modified star diagonal opened up to show the pivoted mirror assembly.

The diagonal unit in the box ready to be slid into place. The "input" hole had to be filed out and the bottom end cut off the diagonal casing to permit the diagonal to be fitted. Turned out to be a bit of a tight fit. I thought the box was bigger than needed but as it turned out, just barely big enough.

The diagonal slid up into position.

Plastic tube and Barlow placed in position to check clearance and line-up.

Plastic tube hot glued into its hole and plastic actuating lever glued onto mirror assembly. This shows the "straight through" position - light going to webcam.

Finally, showing the mirror in the up position for using the eyepiece.

[swamp thing]

Simple, but very neat little gadget.

Regards Steve

[Gina]

Thank you

[Gina]

Been testing my flip mirror this morning on a terrestrial object - distant tree. I was right that the webcam and Barlow needed to be as far in as possible. In fact it won't go in far enough to focus at the same point as an eyepiece in the top so I've had to move the EP out as far as it will go and still be clamped in place. I might see if I can find a suitable size pipe to extend the eyepiece holder. One thing about the unit - it nicely replaces the 80mm extension tube and the focuser is in the middle position.

[Astrokev]

Hi Gina

you may find that the shift in focus between focussing on the tree and infinity enough to preclude using an extension?

[Gina]

I've found the difference in focus between tree and infinity is just a few mm and the drawtube was about 40mm from fully in.

[smudgeball]

Hi (yes stay away from my thread )

Looks good, need to get some clear skies to try mine out.

I'll look to post an image.

Neil

[Gina]

I hope you have more joys with yours than I'm getting with mine!! We are getting some breaks in the cloud tonight and I'm pointed at Jupiter trying to get it to appear in the webcam image. I gave up on the Barlow and put the webcam directly in the plastic tube. It appeared near the edge of the frame after much trying then quietly moved off and I've not seen it since.

After getting the scope roughly pointing at Jupiter with CdC and EDMOD, I aligned Jupiter with the cross hairs in the finder scope using the game controller to slew the mount. That put it in view in the eyepiece on my box so I could then change to a lower slewing rate and move it into the centre. Nice view of Jupiter in that complete with moons. But in spite of checking the alignment with a distant tree in daylight, no white spot appears in the webcam image! Well only fleetingly. Maybe I need a better engineered flip mirror box

[Gina]

Later... Well, I did manage to get Jupiter in the webcam frame - without Barlow, and took some AVI with SharpCap. Seeing was none too good and the planet was wandering about by about a quarter of its diameter. Video was YUY2 @ 10 fps and I took two capture files lasting about 10 mins and 8 mins. In the latter I varied the focus in small steps every few tens of secs through rough focus abd past ind both directions.

The disc will not be covered by many pixels. Pixel size 3.2 microns - scope fl 600mm. So if I can find the angular size of Jupiter I should be able to work out the disc size in pixels.

Now I shall have to think how I can improve the mechanical accuracy of my device!

[AstroMadBloke]

That is just fantastic Gina, May have to make one for myself.

[Astrokev]

I've found the difference in focus between tree and infinity is just a few mm and the drawtube was about 40mm from fully in.

Fair enough. Just a thought! Good luck in getting your device completed. I would imagine the biggest challenge is getting accurate alignment of the axes to prevent distortion.

[Gina]

Yes, alignment is a problem. And something seems to have gone wrong as an object centred in the eyepiece is not appearing in the webcam frame. I'll have to check it again on the faraway trees in daylight. Must have lost alignment somewhere.

[Gina]

That is just fantastic Gina, May have to make one for myself.

Thank you Hopefully it will be when I get it right

[Gina]

Been out to the obsy this morning checking this out and it's definitely out of alignment. One thing I need to improve is the seating of the mirror in the up/active position. I'll be working on it and report progress.

[Gina]

Found a little lump of plastic in the diagonal that was preventing the mirror seating properly on its ledge, which I've removed. I'm changing the hinge arrangement, which was not accurate enough and using a small compression spring to ensure the mirror seats properly and is not pulled off alignment by the hinging arrangement. I'll post photos later.

[Gina]

I'm considering other changes too. A interesting fact is that the Celestron 2x Barlow I have been using has a lens cell that screws into a tube with a thread the same size as a 1.25" filter. This means that the lens can be mounted separately from the tube.

Now I have been collecting various colour filters for planetary use as well as a dark green one for solar (sunspots) used in conjunction with the Baader solar film on the front of the scope. Now it seems sensible to mount these in a filter wheel for cleanliness and convenience and this could go between Barlow lens cell and webcam. So I'm thinking of combining flip mirror and filter wheel. I am already building a bigger filter wheel for larger filters for use with a DSLR and could use a similar design for these 1.25" filters. The point is that I could make a combined unit thereby saving several optical connections.

Anyway, for now I will probably continue with the present box and make a new pivot for the mirror plate, so that I have something I can use fairly soon.

[michael.h.f.wilkinson]

Looks really nice. I do have one question about the design. Most flip mirrors I have seen pull the mirror up, so a non-reflective underside of the mirror lines the optical path, rather than the reflective side. If you use your design on large targets like the moon, don't you get spurious reflections off the mirror?

[Gina]

Looks really nice. I do have one question about the design. Most flip mirrors I have seen pull the mirror up, so a non-reflective underside of the mirror lines the optical path, rather than the reflective side. If you use your design on large targets like the moon, don't you get spurious reflections off the mirror?

Yes, I've been working on the box this afternoon and realised that the mirror reflective surface is only just outside the light path. In fact light from the area surrounding the image will reflect back onto the image sensor. That's no good! . Time for a rethink!

[Gina]

Just been playing about with my design diagram and I now see what you mean by pulling the mirror up. Thank you New design in progress...

[Gina]

I think I've worked out a design to flip the mirror upwards out of the light path and present the black plastic plate to the light path surroundings. This could be roughened or painted matt black. It will require the diagonal to be completely butchered and the parts mounted onto plates at right angles or a square/rectangular box. Die cast aluminium or plastic boxes do NOT have sides exactly at a right angle but are sloped so that they can be removed from the mould. I shall probably construct my own box.

[Gina]

Another update :-

[michael.h.f.wilkinson]

Looking better

[Gina]

Another new design - yet more complicated but has it's points

1. Moved the Barlow lens cell to before the flip mirror but with mechanism to move it out of the way for viewing with the EP. This gives higher magnification.
   
2. Included a filter wheel for coloured filters to bring out various features of planets etc.
   
3. The webcam fits in it's own tube and slides in/out to change magnification of the Barlow.
   

[Viv]

nice job Gina !

Think i'll have to give that one a go.