Secret of the Hartman mask - Page 2

[1ponders] (Paul) · #21 09-01-2005, 05:43 PM

Oh and BTW I did make another couple of masks to try to get some idea as to what might be the optimal size for the holes.

While I found the larger the hole size greatly improves the brightness of the multiple images, the greater the distance the holes are from the center and therefore each other, made the easiest for focusing, (Images seemed to "snap" together, if you can follow that).

Even in very turbulent seeing, using large holes at the edge of the mask mad focusing a relatively easy activity. Especially if you have a electric or microfocuser to remove OTA shake.

The next mask I make will have all the points of the triangles pointing towards the center, instead of the bases of the triangle. This will then allow me to get the largest part of the triangles away from the center, which will then (hopefully) make the focusing more precise.

silvinator · #22 16-01-2005, 04:48 PM

Hey Paul, do you think the circles have to be exact and pretty looking or can you just make them rough? Just asking because in the review of the DSI by Suk Lee that Aragorn posted shows his hartman mask to be a not so nice looking thing, yet he still managed to focus pretty well.
By the way, you make using and constructing one of these things look easy. I'm glad you posted your tips as it doesn't look so daunting a task now!
Do you think you could also get diffraction spikes by placing a thin wire of some sort in front of the circular holes, or am I just talking nonsense?

[1ponders] (Paul) · #23 17-01-2005, 03:08 PM

I'm not sure about the wire Silvie. You may end up with a view like the photos taken through newtonians, you know with the spikey stars, which would serve the same purpose.

I know the review you are talking about and that was how I did my very first mask. It worked fine, but I've found these changes really make it easier to decide when your focus is spot on, especially with the spikes caused by the triangles.

In fact the star images themselves are triangular as well when its out of focus. As you near focus the triangular images merge to become a normal star images and the diffraction spikes stick out the side.

One thing I learnt last night was that the closer you can get your mask to the corrector lense of an SCT the sharper the spikes are, much sharper than sitting on the end of the OTA.

I was using a Celestron C11 last night but didn't have a mask specifically for it. So I used my 8" mask and placed it inside the OTA up against the secondary mirror mounting. As luck would have it the mask fitted perfectly snug up against the inside walls of the OTA and rested on the outer corrector lense mounting ring so didn't touch the lense at all, but was about 15 mm away.

This just happens to be dumb luck that when I cut the styrene for mask the outer diameter just happened to be almost exactly the same as the inside diameter of the OTA of the C11

Any hoo...what I found was that with the mask this close to the corrector the spikes were razor sharp when focused. In fact as I was focusing in each spike was in fact two lines as in a reticle, and the closer I got to focus the closer the lines came together. It was considerable easier to determine when focus has been reached.

Now the C11 has a more OTA protuding past the secondary mounting than in the meade so my next exercise it to take to one of my masks, cut a hole in it so the secondary mounting fits through it and see if the same thing will happen with my 8".

I'll keep you "posted" "P

[1ponders] (Paul) · #24 17-01-2005, 03:17 PM

Ice, regarding focusing on a planet.

The theory (apparently, according to an old amatuer astonomer friend of mine up here) behind using a star rather than a planet is that a star is a point light source and is easier to achieve correct focus. A planet being an extended object will be harder to achieve simply because turbulance in the air may cause one part of the image to be infocus while the other is out of focus.

That's the theory at least, though I did it with saturn last night and didn't have any problems, though my difractions spikes weren't any where near as nice and sharp as in a star.