Revolutionary new way of focusing! No less!
Posted 04 September 2008 - 06:31 AM
Those are empirically derived numbers. 35-45 will work too, but you'll probably need a bit different cut sizes.
Posted 04 September 2008 - 07:43 AM
While Maks, SCTs, and Newts will see no difference I started thinking about refractors. If we were to cut out the center obstruction hole that would allow more light to enter we would be able to focus on dimmer stars. However, I also wonder if the increased central portion of light would start 'washing out' the defraction patterns.
Any thoughts? I think I might try that experiment out at home tongiht with my papaer cutout for the 80 mm and see if there is a big difference.
Posted 04 September 2008 - 08:10 AM
Posted 04 September 2008 - 09:26 AM
Thanks to both yourself as well as the gentleman who came up with this ingenious creation.
I would like to ask you about the "f/s = 150-200" rule of thumb ... can I assume that it refers to the sum of the slit and the ensuing bar? In other words and using my AP refractor with focal length of 1200 mm, the above rule of thumb yields a value of "s" which is between 6 and 8 mm ... therefore, are the slits one-half of this amount (3-4 mm) and another similar amount for the bar?
As for the angle of the slits for one of the two sides which are not 90 degrees, is this fixed (ex. 20 degrees) or do we have a similar equation for a suggested angle?
Thanks again for your time and attention to this creative tool!
Posted 04 September 2008 - 09:52 AM
I'm not clear at this point whether this is a major improvment over an ordinary Hartman mask which has 6 diffraction spikes. The triangular version gives 18 diffraction spikes, which is really unusable, but if you paint three toothpicks black and attach them to the holes of a 3-circle Hartman mask, you get much the same effect I'm seeing in the animations.
The main difference would seem to be that there are a lot more slits and the specifics of the slit spacing. Perhaps this yields brighter diffraction spikes?
Posted 04 September 2008 - 09:58 AM
Posted 04 September 2008 - 10:30 AM
Posted 04 September 2008 - 10:31 AM
Posted 04 September 2008 - 10:50 AM
As long as you're following this thread, Josh,
I assume you mean me- John?
if you're going to offer these masks, are you going to give any consideration to the slit spacing?
The spacing gets smaller as the diameter of the mask gets smaller. I don't have an 8" with me right now, so Jerry would probably be a better person to ask about that. I suggest PMing him if you want to know the dimensions.
Posted 04 September 2008 - 11:02 AM
Posted 04 September 2008 - 11:02 AM
So S=10 the cut is 5mm and the uncut space is also 5mm
As cuts get too small you can multiply the number by 3 and use 3rd order spectrum (no visual difference) instead of the 1st one. I described all that on 1st page.
Posted 04 September 2008 - 01:25 PM
if you paint three toothpicks black and attach them to the holes of a 3-circle Hartman mask, you get much the same effect I'm seeing in the animations.
I'm interested in how you put the toothpicks in the holes. I just made a hartman mask for my LX-200 and was going to make one of these but if all I need to do is add a few toothpicks it would be great. Could you post a pic or a sketch of how you did it.
Posted 04 September 2008 - 01:53 PM
This Pavel Mask really makes a HUGE distinction on focus by the placement of one spike compared to the other two. I also don't have to worry about zooming in on the star to get a good focus.
Oh, and I get to play with knives and glue to make a new mask!
Bonus all the way around!!!
Posted 04 September 2008 - 03:32 PM
More light, brighter spikes, and a clearer relationship between the spikes would be good, though. Eyeballing the focus is a lot better than a plain Hartman mask, but I still sometimes feel I'm guessing whether they're centered, since the intersection is obscured by the star itself.
Posted 04 September 2008 - 04:34 PM
Posted 04 September 2008 - 04:40 PM
I got one question, though:
- shooting with a WO ZS80FD + 0.8x FR - the focal length on the formula will be the original one (555mm) or with the reducer on (444mm)? will it make a difference?
I also have a 2" extender and DSLR...
BTW, what if I want to get one for my C11?
Posted 04 September 2008 - 05:46 PM
Fortunately for you, the 555mm vs. 444mm difference is well within the margin for error he posted. 3 x FL / (150-200) = 8-11 mm at 555mm, and 7-9mm at 444mm, so 8-9mm will suit either.
Posted 04 September 2008 - 06:17 PM
One question about the Bahtinov mask; is it alignment sensitive? In other words, must it be accurately centered to work well? The reason I ask is centering a mask is easy unless you're using a dew shield on a Cat, then it can get tricky.
This looks like a neat design and I'm looking forward to trying it.
Posted 04 September 2008 - 06:39 PM
Posted 04 September 2008 - 07:25 PM
One question about the Bahtinov mask; is it alignment sensitive?
It does not appear to be. I tested moving the mask's center point around and it did not appear to affect the image significantly. Of course, rotating the mask will rotate the pattern. If it is too far to one side or the other, it may affect the brightness of the affected diffraction spikes, but the center location appears to not change.
Posted 04 September 2008 - 07:44 PM
My advise: don't worry about the details. Make one from cheap material and try it out.