Forum der Vereinigung der Sternfreunde

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PostPosted: 01. May 2008, 20:17:17 PM 
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Dauernutzer

Joined: 23. February 2008, 19:33:52 PM
Posts: 100
Hello,

The F/15 Cassegrain at MONS (for the WR140 Campaign) still poses some challenges for accurate pointing at an important target, as WR 140.

I have a suggestion for a "helper" that is easy to set up by anyone using "improvisable" means if need be, transportable in luggage, light and somewhat accurate. I would like to discuss it, as it could still be rendered inappropriate by your opinions.

The idea consists of a small wire loop (like those for making soap bubbles, but smaller), and an exchangeable transparent map of the brighter stars near the target, placed at opposite positions on the telescope tube. The loop sight would go to the aperture side of the OTA, and the map would be placed near the observer's head.
Each map would be custom made for each desired target, on a stencil with the positions of some stars (more than 3) marked on it.
The idea would be to point the OTA in such a way that when the observer moved his head to visually align the star on the map with the loop, he would see the star inside the loop. This should happen for more than 3 stars on the map. Extra stars give a sense of redundancy check to make sure the area of the sky is the desired one.

(The following values are merely indicative)
For instance, if these are placed at a distance of 3.44 m, a movement of the star in the map of 1mm would correspond to 1 arc-min of sky. There is also the size of the loop to take into account. Possibly the hole in the loop could be less than 5mm wide, with the loop being thick enough to be visible. This could get the scope to within +/- 3 arc-minutes of the desired position.

If everything is light enough, it may be held against the OTA with some straps tightened around the tube.
The map holder must have a rotation adjustment, just in case. Either the map or the loop should be adjustable in two axis (N/S & E/W, for instance).
The stencil should be kept in a hard border, so that these can be exchangeable between targets without loosing alignment.

The problems I see with this is we are relying on naked eyesight and the stars brighter than WR 140 are 1 degree away. There are some mag 5 stars 2 degrees North of WR 140. A stencil that is centered on WR140, with 1'/mm would need to be larger than 22 cm in N/S direction to reach those mag 5 stars. An A4-sized stencil should do.

If the stencil is physically engraved with the star positions as tiny circles, it may possibly be lit from the side (perpendicularly) by a dim red light.

The frame holding the stencil could be a sandwich of a stencil between two aluminum rectangular frames (so that it can be lit on the side)
The map holder may be trickier to construct.

It would probably be simpler to have a small refractor on a 2-axis mount guide-mount, with an angle scale on each knob, no?...

Cheers,

Fil.


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PostPosted: 02. May 2008, 08:47:46 AM 
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Meister
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Joined: 19. August 2006, 11:45:20 AM
Posts: 2736
Location: 66907 Glan-Münchweiler
Hallo,

I think, this is rather complicated. For the run, I will leave one telradfinder from me. Then we need a clock which shows the "star time" . The rest will be done by the setting circles, I think.

cheers
Berthold


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PostPosted: 02. May 2008, 10:58:33 AM 
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Dauernutzer

Joined: 23. February 2008, 19:33:52 PM
Posts: 100
Quote:
I think, this is rather complicated. For the run, I will leave one telradfinder from me. Then we need a clock which shows the "star time" . The rest will be done by the setting circles, I think.
I can't say I don't agree :)

I have a Vixen GA-4 that I can lend for the run. It is a 1,25" diagonal that projects an illuminated reticle on any 1,25" eye-piece. Reticle has tick marks every 10 micron (I think). It also has a detachable 3x barlow in it.
It was made for guiding, but works wonders for placing a cross-hair on any eyepice, with any focal length.

Does anyone know the resolution of the setting circles?

Cheers,

Fil.

PS - Thank you for moving this topic to the right place. It was my first post, and I did not notice until after posting it.

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