i am with you on this Dennis, i think this method looks far better then the Hartman mask with diffraction gratings. I downloaded the files off cloudy nights but do not have Corel draw. might have a go at a CAD drawing of this and send it to a CAD/Cam cutter to make it up for the 8" vixen, the 127ED and the 8" SCT and the ED80, oh and the 10" lx200R.
Hey Dave,
When you find out how much to get them CAD/CAM cut, can you let me know? I'd be interested in one for my C8 too if it's practical to tack onto your order...
When you find out how much to get them CAD/CAM cut, can you let me know? I'd be interested in one for my C8 too if it's practical to tack onto your order...
What attracted me to the effectiveness of the Bahtinov Mask was the ability to obtain critical focus at long focal lengths and long(ish) exposures.
When using a PowerMate or Barlow to produce an effective focal length of over 5000mm and exposures of over 1 second, this mask seems to be able to dial in the focus very quickly and unambiguously.
Another scenario for a future test is using it with “Remote Live View” on the canon 40D to assist with DSLR focusing as my set up does not have any software support for this task.
Having said all of that, I must say that the nerd in me had the Bahtinov Mask on the ‘scope last night as I went through my range of eyepieces, pumping the motor focusing buttons just to watch the intra-focal, at focus and extra-focal patterns as they danced in front of my eyes.
hey Dennis, RB says can you slow up the animated gif -- its too fast for him
I’ve just slowed it down to 5fps from the original 15fps. The “bouncing around” is a result of the alignment software not coping too well with an image that is changing so much between frames.
Wow this is just fantastic work Dennis, a great find. I have looked at the thread over at CN and the process looks so straight forward and easy to do.
h0ughy i to would like to now what it would cost to make one, this would be great for my C8 and smaller refractors.
Just a thought i had, and it maybe completely impossible. But could something similar to this be used as a filter at the EP? I mean something that could be attached at the focusing end of the telescope. A 1.25" or 2" filter. I am thinking i guess along the lines of the Knife edge and Ronchi type focusing tools.
Here is an “in-line” animated gif of a truncated segment of the original avi file. I haven’t been able to stretch the histogram so the stationary shallow “X” diffraction bars and the moving “central” diffraction bar don’t show up quite as cleanly and clearly as when viewed on screen at the time of capture.
With the naked eye and a Pentax XW14mm eyepiece, the pattern is as bright as a sparkling diamond!
Cheers
Dennis
PS - Press “Refresh” to run or re-run the animated gif.
Last edited by Dennis; 08-09-2008 at 04:39 PM.
Reason: Added post script
Thats why the chap in my post used circles with a strip thru them instead of Rod's method, so he can remove most of the extra diffraction spikes.
I use this method, and its easy and accurate.
Still, Dennis is on a crusade with this one.
Theo.
Sorry, I read your original post as saying that the circles with strip is Rod's method, I just re-read it and still get that impression.
Anyway, the circle/strip method looks like the spikes line up together in the centre.... exactly where the star is, making it hard(for me) to see when they are nicely crossing each other because the star is in the way.
With the three-triangle that I use, the lining up of the spikes is done away from the star, as you get closer to focus, the lines(which are parallel to each other), will move closer eventually overlaping each other at focus.
Imagine holding clenched fists in front of you with each index finger extended(representing two of the parallel spikes, fists as star), now move you hands closer and closer untill the fingers(spikes) overlap.
Even then though, I find this difficult as its hard to tell when they are exactly overlapping each other.
Pavels method looks great, because it is also done away from the centre, but there is no overlapping, making it easier to see minute movements of focus.
Three-triangle example can be seen here: http://www.iceinspace.com.au/63-187-0-0-1-0.html
Strangely, the examples there don't show large diffraction spikes when not in focus, but I do get them. (even with curved spider)
Anyway, I guess whatever works for you, works for you.
Last edited by MrB; 08-09-2008 at 03:49 PM.
Reason: spulling
Just some reminders. If anyone is going to the expense of having one of these made by an external supplier, don’t forget to read the CN post – it continues to grow with new information. The formula I used was as follows, for long focal length ‘scopes:
The range 150-200 is a variable denominator to allow for a calculated result that is a “sensible” value such as 12mm, 6mm, etc rather than say, 10.1732mm. I’m not sure how scalable the design is; my bars and inter-bar gaps are 6mm with the main central bar being 15mm wide.
For short focal length ‘scopes (less than 800mm?) the formula becomes:
(Bar width + inter-Bar gap) = (3)x(Focal length)/(Range 150-200).
This x3 multiplication factor prevents the bars becoming too thin.
Netwolf,
If you have a Ronchi grating you don't need anything else. It will give you pin point focussing everytime!
I think the strength of the objective screen methods is that they provide an image to a webcam/ CCD which can then be focused prior to taking an exposure.
To do this with a Ronchi you really need a flip mirror and the screen position at the same distance as the CCD plane. A bit of quick trial and error usually gets it right.
Here are a couple of screen prints of Fomalhaut using the Bahtinov Mask in conjunction with the Canon 40D “Remote Live View” utility, showing the standard view and the x10 zoom setting view. The best focus was very easy to obtain at the x10 zoom setting.
The star used in this example was Fomalhaut (1.2 mag) and the actual on screen view was slightly better than this screen copy. The two “stationary” shallow X bars and the central “moving” bar were easily visible on the Notebook.
Another good focus indicator is being able to view the pair of 3 distinct diffraction spots either side of the single central diffraction spot.
Also attached is the central, full size crop of a subsequent 5 second exposure of Fomalhaut with the Canon 40D after removing the Bahtinov Mask. Scope was Tak Mewlon 180 at 2160mm F12 prime focus. The 6 diffraction spikes are artefacts of the Mewlon’s 3 vane spider. Focusing life just doesn’t get much better than this!
I wonder how much it would cost to get one lazer cut out of 2mm thick AL sheet..
Oh, I'd need a couple of them wouldnt I... one for prime focus @ 2800mm, one @ F/6.3, and one for planetary imaging @ 6~7m focal length.... could probably do without the one for planetary... but even then, 2 of them for F/10 and F/6.3 respectively would be expensive to get made as one off's
If some enterprising metal (or even plastics) fabricator has a laser cutter or similar, they could probably cut out aluminium (plastic) discs with the mask patterns at a reasonable price locally?
Maybe Ron at Sirius Optics, Underwood has some connections to local workshops?
