Secret of the Hartman mask

[[1ponders]]

Here's a trivial question for everyone:D

You've setup your system and you're just getting ready to image Saturn with a web cam and you want to make sure your focus is spot on. You decide to use a hartman mask to assist you. As luck would have it Gemini is right next door to Saturn with Castor and Pollux shining nice and bright just below Saturn. Now, we all know that Castor (Alpha Gemini) is brighter than Pollux (Beta Gemini, well not really but we'll leave that bit of trivia for another night) so it would be much easier to see Castor using the Hartman and webcam. Right?

Well maybe:)

You will have difficulty using it. You should use Pollux.

Why?

Lets see who get this right first.:D

Good luck

Answer next week if noone gets it:whistle:

Paul

[rumples riot]

I'll take a punt and say that Pollux is very near the same BV colour. Pollux has a BV colour of 1.00 and Saturn is 1.04. And on top of this pollux is nearer in Magnitude to Saturn.

Anyway am even close?

Paul

[gbeal]

Not eluding to the double star snag are you.
BTW, I have never used a mask.
If the seeing won't allow a decent image to focus on, with the Cassini being used for Saturn, then I do something else.
Gary

[[1ponders]]

Boom Boom :P

Good on you Gary. :D

Yes its a double/multiple star problem. When you stop down the aperture with the mask your reduce your telescopes ability to clearly resolve many double stars. Its not such a problem with VERY close multiples, but with stars like Castor and Alpha Centauri for example, the separation is such that when you stop down the telescope and try to focus with the mask you can not get a single star image.

There are a number of factors that contribute to the problem, resolving power of telescope with mask (the smaller the holes in relation to the telescope aperture, the less resolving power) seeing conditions, quality of optics, turbulance in the OTA for reflectors, internal/external temp dif in refractors/sct.

So how do we deal with the problem? Easy make sure that the star being used is not a multiple (in the case of Castor a sextuple, 3 pairs).

And if you are having trouble focusing with the mask, just check your star chart or atlas, maybe your using a double star and don't know it. But make sure you come back later after imaging and check it out. It could be well worth looking at in its own right.

Quick work Gary:cool2: :party2: :bowdown:

[[1ponders]]

Heres an image for those who don't know what a hartman mask is.

I like to tape up the holes so they are triangular. This gives me diffraction spikes to line up. I find it helps heaps when trying to decide "Is it in focus? Or is it just out?"

:cheers:
Paul

[rumples riot]

Now that is an interesting idea, had not thought of doing that with my Hartman mask. Perhaps I will give it a try.

[[1ponders]]

I freely admit that it wasn't my idea (I've seen it on a few websites). I need to make the original circles larger and closer to the edge to try to increase the amount of light getting to the eyepiece to make the star images brighter.

I'll let you know if it improves the accuracy once completed.

Paul

[iceman]

Excellent work Paul, I made a cardboard hartman mask myself and use it for making sure focus is spot on.

And good ides about what star to choose, I think I may have fallen for that at least once.. I'll try and use a moon of saturn or jupiter to focus on if I can, it means I don't have to move too far from the planet with my dob.

[[1ponders]]

Give it a go and let us know how it goes Ice.

I use a star simply because thats what it was suggested to me to use. Using the tape across the holes with a star gives good long diffractions spikes to line up with. Would you get the same using the moon or jupiter?

[iceman]

It depends how bright it is.. I made my cardboard mask a few months ago and I have round holes.. i'll have to try making them triangles..

[[1ponders]]

Hint: in masking in the triangles keep the adjacent sides of the triangles as parallel to each other as possible.

The closer to parallel the sharper the diffraction spikes when focus is achieved (depending on seeing that is). My bottom triangle looks slightly out of alignment with the top two but that is only due to the angle I took the photo. All the triangles are equilateral triangles and of the same size. I have read that the shape and size of the holes isn't critical. However after using unequal sized and shaped holes and equal sized and shaped, I've found this arrangement much easier to use, especially if the seeing is not so good.

Notice I've used three long pieces of tape to form a large equilateral triangle on the outside then filled in the final side of each triangle where the outer tape crosses the perimeter of each circle. I used a compass with the radius set from the centre of the mask to the outer edge of one of the circles and scribed a circle round all three circles. Where where it touched each circle was where the long pieces of tape ran from/to.

Do both sides of the mask to stop sticky tape from touching the corrector plate for SCTs or picking up bits of dirt that can become dislodged and fall into your OTA for newts, and to strenthen the mask.

Next fine night I'll image Sirius to give an idea of how the spikes look/work

Good luck

ps I cut out the lid of a twenty litre plastic mayonaise tub for the mask and cut a Brocolli box lid into a circular collar to fix the mask to, to slide over the top of the OTA.

[iceman]

But Sirius is a double! :)

[[1ponders]]

Yes. but the companion is so small and close it shouldn't make any difference. Its when the stars are fairly wide that they tend to cause problems. (See points above about effecting influences)

If it does (here I go experimenting again :) ) I'll use Rigel or Betelgeuse

[iceman]

I know Sirius' companion is too close, was just having a little joke.. :P :lol:

[[1ponders]]

:doh: :lol2: Ya got me Tex:P

[[1ponders]]

Finally got a clear night to have a shot at Sirius. The first image is just out of focus. A dozen or so image were stacked just to improve clarity.

2X Omni barlow
ToUcam

The colour would normally be Sirius' normal blue but I was mucking round with some of the ToUcam settings at the same time

[[1ponders]]

And in focus.

The seeing was terrible last night. Windy and turbulent to buggery, apart from dodging clouds.

Even though these images were stacked, they are a pretty good representation of what you would see. At least through an 8" SCT

[[1ponders]]

And redone later with its normal blue

[iceman]

Nice one.. what gain level do you use when achieving focus?

[[1ponders]]

I don't have anything preferred Ice. Its usually what the ToUcam sets as default for whatever settings I have selected at the time. Thats why one of the focussed images is brownish and the other blue.

Once I've achieved focus, I lock the mirror (Meade SCT) and then slew to my viewing object. Then i muck around with exposure, white balance etc.

[[1ponders]]

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]

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? :confused:

[[1ponders]]

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]]

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.