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.
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.
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
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.
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?"
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.
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.
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?
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.
Last edited by [1ponders]; 06-01-2005 at 04:34 PM.
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
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.
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Secret of the Hartman mask
Ya got me Tex
