Niels Noordhoek’s weblog

Given typical dutch weather, it is nice to have a star of your own.

It is very helpful during collimation or playing with a Bahtinov mask.

I used a D-cell Maglite, single LED version.Place some aluminium foil over the LED and use a very sharp needle to puncture a tiny hole in the aluminium. Make sure you do not cover the LED in alumnium foil, the LED wil overheat and die.

When you place the artificial star at some distance (telescope still should be able to focus), the diameter of the hole should be smaller than the resolving power of your telescope.

Example for a Celestron C8:

- Resolving power: about 1.5 arc seconds

- Distance to the artificial star: 12 m.

- 1.5 arc seconds at 12m = tan(1.5/(60×60)  degrees)  x 12 m= 90 microns (about the diameter of a thick human hair)

An indication that the hole must be  really small. That can be achieved by using a very sharp needle and by placing the aluminium foil on a hard and flat (glass) surface when puncturing it.

The rings are much more closed than last year.

A Bahtinov mask is a mask on a telescope that produces a line pattern that shows whether you are focussed well, and even shows if you are front or back focussed if not properly focussed. It requires a quite complicated mask, but almost the same effect can be achieved with 2 wires stuck to the aperture:

This results in a line pattern (Bahtinov results in dotted lines of 1st, 2nd etc order diffraction patterns):

But it still tells you where your current focus is (front, back or in focus):

An unexpectedly bright comet, already heading away from the sun close to Mel20 in Perseus:

piggyback on the C8/LXD75

And a bit closer: C8, f/5.6 focal reducer and 2x barlow allowed for prime focus photography (bit awkward, maybe should heve left all the extra glass out but there was little time to try between the clouds passing over head). The comet fully filled the 10 megapixel frame

First time I managed to find and image Uranus.

August 10th, 23h28 UT, 3.7 arc seconds small (= like inspecting a human iris from 1000 m away)  and 26 deg above the horizon with a remarkably good seeing.

After all the calculations it occurred to me that I checked the clock just when a GRS transit was occurring. After the clouds presented a hole to image through I got this: quite nice considering the horrible seeing.

July 17th 2007, 21h28 UTC. The transit was predicted 21h01 so GRS must have moved 16 degrees past the central meridian assuming it is at 119 degrees longitude. The tangent of 16 degrees=16/60=0.25 so the GRS must have moved about 0.25 times the radius of Jupiter, which seems to agree more or less with the image.
In addition a Europa and its shadow are visible!

A simulation/prediction of the GRS position at 21h28:

Horrible image quality because of the poor seeing, short movies, warm telescope, low altitude etc, but the idea is there.

For the believers: the Great Red Spot is on the lower right.

Fist time I tried my UV/IR blockign filter and Televue 2.4x Powermate (very heavy quality).

Images taken on July 15th 2007, at 20h59 and 21h19 UT. The 20 minutes between the recordings should be good for 12 degrees of rotation.

As animated GIF:

Simulation of the GRS position at that time: