Got to love Monty Python! ... and so much of their stuff is applicable to all sorts of situations.
Poking around in the shed last night I found some lengths of 20x20x1 angle aluminium and some 12x3 aluminium bar. It might take me a while to fit it in around all my other projects, but I'm going to try fabricating a mask to fit the C8 based on an aluminium frame and fishing line for the "bars".
I figure since fishing line works to make diffraction spikes when applied across the objective, it should work OK for this job as well. It is after all the edges that produce the diffraction, not the width of the bars. That being the case a fishing line mask should allow maximum light into the scope which just might make it usable on fainter stars or with less changes to camera settings.
I just had a browse of the Russian Astronomy Forum (from a link in the CN post) and noted that there appear to be several variants of the mask.
The one that I have drawn has a full width (solid) horizontal bar.
One variant has a gap, not a bar as the central horizontal feature.
Other variants have a solid isosceles triangle on the angled side.
Hmm, I wonder which is the optimal design? All of them?
Cheers
Dennis
Intuitively I would expect the longer the total length of edges in one direction the stronger the diffraction spike in that direction... sound feasible?
Welcome to Ice In Space and thanks for the details provided.
I do recollect reading one of your posts on CN along the lines of “what a surprise Pavel will get when the Royalties start flowing in” so I figured you were pursuing some kind of arrangement to recognise his contribution!
Are you considering setting up a distribution outlet in Australia?
Cheers
Dennis
I am a member of the Russian forum hosted by Pavel. I asked for permision to call the mask by his name.
The only plans to sell the spike-a mask in Australia is by shipping from the US. Info can be found on the web www.spike-a.com
I am a member of the Russian forum hosted by Pavel. I asked for permision to call the mask by his name.
The only plans to sell the spike-a mask in Australia is by shipping from the US. Info can be found on the web www.spike-a.com
Jerry, i have a few questions:-
- according to the formulae the grid pattern widths change with the focal lengths of the scope - the product you seem to be selling does not look as though you can take that into account but rather have to accept a predetermined grid width that you have setup for manufacturing - IE with the 10" Meade it is 13mm, but for my R200SS vixen f4 it is 5mm grid spacings? Your web site has no options for choosing a grid sizing - just the outer diameter of the grid mask?
Thats a good point David, I hadnt noticed that... Also, if you have an F10 8" with a 6.3 F/R.. according to the formula it should require different width bars, but no options for such spacings.
Jerry, i have a few questions:-
- according to the formulae the grid pattern widths change with the focal lengths of the scope - the product you seem to be selling does not look as though you can take that into account but rather have to accept a predetermined grid width that you have setup for manufacturing - IE with the 10" Meade it is 13mm, but for my R200SS vixen f4 it is 5mm grid spacings? Your web site has no options for choosing a grid sizing - just the outer diameter of the grid mask?
My math is not the same as the Russian site. I use the clear aperture and not focal length to determine the cutout and spacing. I am not concerned about hitting the 3rd, 5th, and 7th order spectrum as much as getting the maximum light gathering power of the scope. Very simple math- For the horizontal cutouts just take the clear aperture and divide by 35. This will give you a center cutout and 8 more in each direction. So for a mask that will fit an 8 inch SN type scope the math is 8/35= .228 inches. By the way this is the 10 inch mask. For the angled side- multiply the cutout on the horizontal side by 1.05. so the example is .228 X 1.05 = .240 inches.
I think too many people are getting caught up in the math formula that hit the spectrum order for a particular focal length. My test shows that is is not as critical as good straight lines and a mirror image top to bottom and left to right.
For those that just want to hit the spectrum and split the color band, then by all means, take the time to calculate your focal length. But if you just want an easy way to get the best focus fast-- Then keep it simple.
I have posted this to Pavel and his group as a suggestion. And now on to you nice people.
Hi Jerry and welcome to IIS. I have read the happenings on CN. No one i know speaks russian, but would a translator like bablefish help?
I did get translating software and have contacted Pavel. He said we could call the focus device a "Bahtinov Mask". I made my intentions to share any profits with him.
I hope nothing gets lost in the translation. i don't want to offend someone that has offered such a great tool to amateur astronomers all over the world.
I did get translating software and have contacted Pavel. He said we could call the focus device a "Bahtinov Mask". I made my intentions to share any profits with him.
I hope nothing gets lost in the translation. i don't want to offend someone that has offered such a great tool to amateur astronomers all over the world.
Yep the last thing you want is a mistranslated word
DId you trial many variants of this design before you settled on your version?
The seeing was quite poor last night, hovering around 4-5/10, so I thought I’d have a play with my home made Bahtinov Mask, as the detail on Jupiter and even the Moon at prime focus was washed out and watery.
Here is an image of a grossly over exposed Jupiter and 3 of his Galilean moons at the F12 prime focus (2160mm) of the Mewlon 180. With the correct exposure for Jupiter’s disc, there were 3 Jovian discs – the “real” one and a pair fainter facsimiles either side from the mask.
You can just make out the diffraction pattern from Jupiter’s washed out discs (all 3 of them due to the mask!)
Yep the last thing you want is a mistranslated word
DId you trial many variants of this design before you settled on your version?
I did my share of testing. They all worked except for one. I sent a design to John for his 80mm. We had the bright idea that you could print the pattern on a clear transparency and it would work. The transparency changes the focus just like adding a filter to the eyepiece. The spikes were dull and not in the center when the ccd was in focus. My last design is what i posted here. It is very forgiving to the focal ratio. Still the bigger scopes put up a brighter pattern. I can focus my dsi with my sn-8 down to a mag 6 star with no problems. This is nice when your target is not near a bright star.
ok just drew up a couple of templates - one for Alan Meehan's ED100 and the other is a test for the C8 and the Vixen R200SS - just to see if it will work on both. Warning - the ED100 is on a4 paper and is at true size - the other is A3 paper size for the 8" scopes. I havent measured the outer diameter of the scopes yet so a little bit of poetic license was used for the outer area of the pattern. See what you think?
this version of the design took components from dennis's, a few of CN and one on the russian site, of which I included the image of here. I am not sure as to the benefit of the central mask circle, but i am sure there is an explaination as to why it might be needed?
I did my share of testing. They all worked except for one. I sent a design to John for his 80mm. We had the bright idea that you could print the pattern on a clear transparency and it would work. The transparency changes the focus just like adding a filter to the eyepiece. The spikes were dull and not in the center when the ccd was in focus. My last design is what i posted here. It is very forgiving to the focal ratio. Still the bigger scopes put up a brighter pattern. I can focus my dsi with my sn-8 down to a mag 6 star with no problems. This is nice when your target is not near a bright star.
You can just make out the diffraction pattern from Jupiter’s washed out discs (all 3 of them due to the mask!)
Hi all- I had to jump in on this one. I'm Jerry's business partner, John Wunderlin.
You can't use Jupiter very well to focus- I saw the same thing in my testing- it has to be a pinpoint of light, though on the Russian forum I see someone was able to use lights on a distant building in daylight that worked pretty well.
The other item related to this is that collimation affects the pattern. If your collimation is off, the pattern is distorted and my not provide proper focus, though if your collimation is off, you'll have additional imaging issues
With the seeing so poor last night, 4-5/10 at best, I decided to just have a play around with my home made mask and whacked a x4 PowerMate into the Mewlon 180 F12 to give me an efl of a whopping 8640mm at F48.
Using the SBIG ST7 CCD camera I picked up a mag 5.16 star and was still able to focus using the mask, although I had to push the exposure out to 2 seconds. What I did notice, is that the focusing assist function in CCDSoft (the ST7 camera control software) gave me a slightly different “best focus” indication compared to the mask.
So far, I can only think of 2 reasons for this:
My mask is made from a cut out overhead transparency and is therefore floppy so all the diffraction elements do not lie in a single plane.
The weight of the ST7 CCD camera and CFW8 filter wheel dangling on the end of the 2” x4 PowerMate was causing the motorised focuser (Moonlite) to flex slightly so the plane of the CCD chip was slightly angled.
The effect is more noticeable in the right image of this pair.
Incidentally, I was still able to use the CCDSoft focusing assist function with the mask fitted, to find the best point of focus due to the sensitivity of the ST7 even at an fl of 8640mm at F48. Also, having the mask fitted made the overall focusing routine so much more efficient in terms of converging quickly to the optimum focus point.
So, even despite some quite poor seeing conditions, the mask was able to perform very effectively with 2 sec exposures at F48 – an impressive result for a piece of cut-out overhead transparency!
>snip
You can just make out the diffraction pattern from Jupiter’s washed out discs (all 3 of them due to the mask!)
Cheers
Dennis
Quote:
Originally Posted by wunder
Hi all- I had to jump in on this one. I'm Jerry's business partner, John Wunderlin.
You can't use Jupiter very well to focus- I saw the same thing in my testing- it has to be a pinpoint of light,
>snip
Hi John
Welcome to Ice In Space – it’s nice to have the business owners providing their insight into this device.
The comment about the diffraction pattern seen from Jupiter’s washed out disc was posted as an observation rather than a recommendation for focusing, as I suspect most amateur astronomers are aware of the requirement for a point source of light, although the Galilean moons (extended discs) do serve as very useful indicators to confirm focus.
With the seeing so poor last night, 4-5/10 at best, I decided to just have a play around with my home made mask and whacked a x4 PowerMate into the Mewlon 180 F12 to give me an efl of a whopping 8640mm at F48.
Using the SBIG ST7 CCD camera I picked up a mag 5.16 star and was still able to focus using the mask, although I had to push the exposure out to 2 seconds. What I did notice, is that the focusing assist function in CCDSoft (the ST7 camera control software) gave me a slightly different “best focus” indication compared to the mask.
So far, I can only think of 2 reasons for this:
My mask is made from a cut out overhead transparency and is therefore floppy so all the diffraction elements do not lie in a single plane.
The weight of the ST7 CCD camera and CFW8 filter wheel dangling on the end of the 2” x4 PowerMate was causing the motorised focuser (Moonlite) to flex slightly so the plane of the CCD chip was slightly angled.
The effect is more noticeable in the right image of this pair.
Incidentally, I was still able to use the CCDSoft focusing assist function with the mask fitted, to find the best point of focus due to the sensitivity of the ST7 even at an fl of 8640mm at F48. Also, having the mask fitted made the overall focusing routine so much more efficient in terms of converging quickly to the optimum focus point.
So, even despite some quite poor seeing conditions, the mask was able to perform very effectively with 2 sec exposures at F48 – an impressive result for a piece of cut-out overhead transparency!
Cheers
Dennis
Dennis,
I don't know why I didn't think of it earlier but I went to an art supply place this afternoon and purchased some black 5mm foam core board. two A3 size sheets were about 8 dollars. it is light, black and you can easily cut it with a scalpel. I have asked Alan if he would be able to make a screen print up for the larger sizes to print directly on the larger (8" and above) masks - then cut it out. Hopefully this will work. that way the cost for the baoard is under $10, easily made with some TLC and a bit of time and hopefully specifically designed to the scopes requirements. What did you think of the alternate design?
Got to love Monty Python! ... and so much of their stuff is applicable to all sorts of situations.
or with less changes to camera settings.
that way the cost for the baoard is under $10, easily made with some TLC and a bit of time and hopefully specifically designed to the scopes requirements. What did you think of the alternate design?
