Perfect focus each time using a Bahtinov Mask

[Dennis]

Originally Posted by h0ughy

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?

Hi Dave

I am inexorably being driven to a laser cut metal solution right now, in order to spend less time focusing and more time imaging. Having experimented now with the following materials and designs:

• Overhead transparency (gaps not cut out)
• Overhead transparency (gaps cut out with Stanley knife and steel rule)
• MDF 3mm thick cut with a router (phew!)
• Black plastic computer binders (gaps cut out with Stanley knife and steel rule)

I am left with the following conclusions:

• Floppy material, such as overhead transparency and black plastic is likely to stretch, sag and not lie in a single plane.
• Materials such as MDF, foam core board, etc may absorb water and over time, become frayed around the edges.
• The process of rapidly converging to and locating precise focus is such a key element that it is worth obtaining and using a well manufactured, stable and rigid mask.
• Ideally, it is best to remove all “easy” variables under your control, such as a sagging mask, furry edges, etc so that you can concentrate on other matters such as eliminating flexure in the focus train, etc.

So, at the moment, I favour either a stainless steel or aluminium thin sheet mask, laser cut for accuracy and cleanness of cuts. I enjoy tinkering so don’t regret the time I have spent just playing around with various designs and implementations, but I am strongly leaning towards a laser cut metal solution right now, in order to spend less time focusing and more time imaging. I’ve had my fun and now its time to get more serious!

Cheers

Dennis

PS- I suspect that those with permanent set ups, temperature controlled focusers that can be indexed, etc. may be scratching their heads at these shenanigans! Oh the joys of setting up and tearing down each night, as well as manually pushing motor focus buttons. LOL!

[h0ughy (David)]

Originally Posted by Dennis

Hi Dave

I am inexorably being driven to a laser cut metal solution right now, in order to spend less time focusing and more time imaging. Having experimented now with the following materials and designs:

• Overhead transparency (gaps not cut out)
• Overhead transparency (gaps cut out with Stanley knife and steel rule)
• MDF 3mm thick cut with a router (phew!)
• Black plastic computer binders (gaps cut out with Stanley knife and steel rule)

I am left with the following conclusions:

• Floppy material, such as overhead transparency and black plastic is likely to stretch, sag and not lie in a single plane.
• Materials such as MDF, foam core board, etc may absorb water and over time, become frayed around the edges.
• The process of rapidly converging to and locating precise focus is such a key element that it is worth obtaining and using a well manufactured, stable and rigid mask.
• Ideally, it is best to remove all “easy” variables under your control, such as a sagging mask, furry edges, etc so that you can concentrate on other matters such as eliminating flexure in the focus train, etc.

So, at the moment, I favour either a stainless steel or aluminium thin sheet mask, laser cut for accuracy and cleanness of cuts. I enjoy tinkering so don’t regret the time I have spent just playing around with various designs and implementations, but I am strongly leaning towards a laser cut metal solution right now, in order to spend less time focusing and more time imaging. I’ve had my fun and now its time to get more serious!

Cheers

Dennis

I was told that aluminium absorbs the particular wavelentgh of light the laser uses and because of this they have to turn up the wick just to get it to cut - which then over heats the aluminium then it does not give you a clean edge. That is why they suggested the Stainless steel. I am leaning more towards this and if we can get a design secured and tested then have the CAD drawings made (i can do that) then send it off for a final quote and get quantities. Probably ask them for a black non reflective finish to be applied to it - if not then spray the bugger with some flat black spray paint

was the design I did worth following up or have you a better design?

what I thought of was to get the part to fit inside the dew shield and be supported by foam spacers to keep it off the optical train. then its flat, easy to install and remove. if we can agree on a design - and get some punters in on it - maybe enough for each standard size for the more popular models you could get an economy of scale for the manufacture - if not then they are $82.50 a pop!!

[Dennis]

Originally Posted by h0ughy

I was told that aluminium absorbs the particular wavelentgh of light the laser uses and because of this they have to turn up the wick just to get it to cut - which then over heats the aluminium then it does not give you a clean edge. That is why they suggested the Stainless steel. I am leaning more towards this and if we can get a design secured and tested then have the CAD drawings made (i can do that) then send it off for a final quote and get quantities. Probably ask them for a black non reflective finish to be applied to it - if not then spray the bugger with some flat black spray paint

was the design I did worth following up or have you a better design?

what I thought of was to get the part to fit inside the dew shield and be supported by foam spacers to keep it off the optical train. then its flat, easy to install and remove. if we can agree on a design - and get some punters in on it - maybe enough for each standard size for the more popular models you could get an economy of scale for the manufacture - if not then they are $82.50 a pop!!

Hi Dave

Thanks for your good work to date.

SS sounds good to me and I would certainly add my name to the list for two masks:

• 1 for the C9.25 and Mewlon 180 (6mm bars).
• 1 for the Vixen 4" refractor and Canon 400mm F5.6 lens (3mm bars).

I would order the larger size for each and then insert a home made foam ring to reduce the mask for the smaller tube in the OTA pairs above.

So far, my testing has led me to believe that I can use one mask per OTA either at prime focus, barlowed or with a reducer/corrector. Also, $82.50 doesn’t sound too exorbitant for such a crucial task as focusing, which we may perform several times per session over the life of the OTA. Worth every penny!

Cheers

Dennis

[h0ughy (David)]

Originally Posted by Dennis

Hi Dave

Thanks for your good work to date.

SS sounds good to me and I would certainly add my name to the list for two masks:

• 1 for the C9.25 and Mewlon 180 (6mm bars).
• 1 for the Vixen 4" refractor and Canon 400mm F5.6 lens (3mm bars).

I would order the larger size for each and then insert a home made foam ring to reduce the mask for the smaller tube in the OTA pairs above.

So far, my testing has led me to believe that I can use one mask per OTA either at prime focus, barlowed or with a reducer/corrector. Also, $82.50 doesn’t sound too exorbitant for such a crucial task as focusing, which we may perform several times per session over the life of the OTA. Worth every penny!

Cheers

Dennis

OK then - send me your dimensions and I will draw up the template in CAD to send off - after i test some more

do you like the template or do you prefer the complete striated version of your own?

[jerry3672 (Jerry Hailey)]

Originally Posted by Dennis

Hi Dave



So far, my testing has led me to believe that I can use one mask per OTA either at prime focus, barlowed or with a reducer/corrector. Also, $82.50 doesn’t sound too exorbitant for such a crucial task as focusing, which we may perform several times per session over the life of the OTA. Worth every penny!

Cheers

Dennis

Dennis, my testing has the same results that you see. One mask per OTA.

What currency is the $82.50?

I used 3003 Aluminum because it less likely to crack over time. Also cutting on a water jet will not over heat the base material like a laser. I was afraid that the small cuts for the 80mm scope would end up breaking if we didn't use a high grade of aluminum. A good quality Bahtinov mask will last more than the life of your OTA.

[Dennis]

Originally Posted by jerry3672

Dennis, my testing has the same results that you see. One mask per OTA.

What currency is the $82.50?

I used 3003 Aluminum because it less likely to crack over time. Also cutting on a water jet will not over heat the base material like a laser. I was afraid that the small cuts for the 80mm scope would end up breaking if we didn't use a high grade of aluminum. A good quality Bahtinov mask will last more than the life of your OTA.

Hi Jerry

The $82.50 quoted by Dave (H0ughy) is in Australian $. However, this would not be for a finished product such as yours, anodised, with fixing tabs, etc.

Until quite recently, we have enjoyed an almost $ for $ parity with the US$ but in the last couple of weeks, the A$ has fallen to around 81c $US making purchases overseas a bit more expensive.

As you have noted, there are hidden matters that begin to emerge when one begins to investigate detailed design, materials selection and fabrication methods.

Cheers

Dennis

[Merlin66 (Ken)]

Looking like a great idea!
My 2c
The central "disk" is there I think because it can be... this areas is shaded by the secondary ( in an SCT); when placed on a refractor it would help by brightening the first diffraction ring.
A couple of velcro tabs would be very easy and versatile.

[Dennis]

Originally Posted by h0ughy

Dennis,
>snip
What did you think of the alternate design?

Hi Dave

I just downloaded the zipped CorelDraw files from the uba-post on CN which appear to be some of Pavel’s original designs.

The zipped file contained x3 CorelDraw files and when I opened each one, the centred, middle horizontal element was always a solid bar (not a gap) consistently across all 3 files. I also note that John & Jerry’s Spike-a™ has a centred gap, not a centred bar.

Correction:
The x3 CorelDraw files in the zipped package do indeed show a blank or gap rather than a solid bar, as the central horizontal element on the left hand side of the mask. Incorrect observation due to reading a thumbnail size image. Upon opening the full size drawings in Corel Draw, it was clear that the central element is a bar.

I suspect that in real world terms, there will not be a difference. However, as we can faithfully draw the mask and have it cut to Pavel’s original design I see no reason not to follow it. That is, keep the central bar a solid bar.
What do you think?

Cheers

Dennis

[jerry3672 (Jerry Hailey)]

Originally Posted by Dennis

Hi Dave

I just downloaded the zipped CorelDraw files from the uba-post on CN which appear to be some of Pavel’s original designs.

The zipped file contained x3 CorelDraw files and when I opened each one, the centred, middle horizontal element was always a solid bar (not a gap) consistently across all 3 files. I also note that John & Jerry’s Spike-a™ has a centred gap, not a centred bar.

I suspect that in real world terms, there will not be a difference. However, as we can faithfully draw the mask and have it cut to Pavel’s original design I see no reason not to follow it. That is, keep the central bar a solid bar.

What do you think?




Dennis

When in doubt go to the original. Here is a link to Pavel Bahtinov's forum. it is in Russian.

http://www.astronomy.ru/forum/index....,10421.40.html

You see Pavel has a center Cutout just like my variation. The most important fact is that the slots are a mirror image of each other in respect to the centerline of the OTA.

[Dennis]

Hi Jerry

Thanks for that. I’ve just finished reading all 16 pages on Pavel’s Russian Forum in the post dedicated to what has now become the Bahtinov Mask. There are some bright boys over there for sure.

The Babel Fish translation wasn’t too painful to follow!

Cheers

Dennis

[h0ughy (David)]

http://astrojargon.net/maskgen.aspx this was posted on the russian ste

[TrevorW]

So guys where can I buy one at a reasonable price to fit a 80ED with a dewshield OD of 110mm

Cheers

[Dennis]

Originally Posted by h0ughy

http://astrojargon.net/maskgen.aspx this was posted on the russian ste

Hi Dave

I’ve e-mailed you the x3 Corel Draw files that I found on Pavel’s (Russian) Forum from the link on CN.

I also tried the “Generator” from the http link on CN but my installation of CorelDraw V12 would not open the .svg file generated by that cool application, although I have been able to open other .svg files?

Cheers

Dennis

[Dennis]

Originally Posted by TrevorW

So guys where can I buy one at a reasonable price to fit a 80ED with a dewshield OD of 110mm

Cheers

Hi Trevor

Unless you want to fabricate your own, the only commercial source of these that I know of is from John and Jerry at Spike-a™.

They have a nice description on how to order for the correct size.

Cheers

Dennis

[multiweb (Marc)]

Actually I made one for my ED80 and also for my 5" celestron. I have a template that is easily scaleable. Here's a focus run I did tonight around M17 :
http://www.multiweb.com.au/astro/focus_run.gif

and one unprocessed sub at http://www.multiweb.com.au/astro/one_sub.jpg. This was taken next to a 3/4 moon and a thin haze of clouds. (FWHM was approx 6.8" - 7") so far from the best seeing conditions. Nevertheless, considering, the result was pretty spot on.

I think the mask is working great. Saves me heaps of time.

[citivolus (Ric)]

I seem to have come rather late to this party, but will be interested to compare this mask to one of my own Hartmann/diffraction mask designs that I've been working on periodically for the past year or so. I had investigate laser cutting from ABS and other materials as well, but had mostly given up on developing it commercially due to the large number of aperture sizes that would need to be created and high initial production costs. I had also considered just targeting the refractor market, due to the lower number of sizes it would take to reach a large percentage of the market.

I've kept the design to myself so far to keep someone from exploiting it commercially before I was ready for market, but non-commercial use would have been encouraged. I've built prototypes of my current generation for a C9.25 and a Megrez 90.

Dennis, is there any chance I could stop by some time and see your mask in action? I'm quite anxious to see if this trumps my designs, and if I should toss mine in the bin I could throw a basic version of mine together for your Mewlon if I could get the OTA inside diameter, mirror diameter, and central obstruction size.

I'm particularly interested in limiting magnitude, overall brightness, critical focus zone, and ease of use, especially when used with live view. I'm not saying mine is better than this design, just that I'd love to compare the two side by side.

I may just print my own later today to try out, if the kids will leave me alone for long enough

Regards,
Eric

[Dennis]

Originally Posted by multiweb

Actually I made one for my ED80 and also for my 5" celestron. I have a template that is easily scaleable. Here's a focus run I did tonight around M17 :
http://www.multiweb.com.au/astro/focus_run.gif

and one unprocessed sub at http://www.multiweb.com.au/astro/one_sub.jpg. This was taken next to a 3/4 moon and a thin haze of clouds. (FWHM was approx 6.8" - 7") so far from the best seeing conditions. Nevertheless, considering, the result was pretty spot on.

I think the mask is working great. Saves me heaps of time.

Hi Marc

Glad to hear of your success, my home made Bahtinov Mask has certainly delivered the goods so far.

I had a look at the sub frame of M17 and noticed that on the brightest star with the 4 diffraction spikes, the spikes seemed off-centre to the star disc? These spikes can’t be from the ED80 or C5?

Great image by the way!

Cheers

Dennis

[Dennis]

Hi Eric

I’ve PM’d my contact details so we can hook up some time for a test. It would be very interesting to see your design too. I’ve just built another prototype as I found some 6mm black tape in my wife’s sewing box.

It was somewhat easier laying down strips of 6mm tape and gluing them than cutting 6mm gaps in various materials.

Cheers

Dennis

Originally Posted by citivolus

>snip
Dennis, is there any chance I could stop by some time and see your mask in action? I'm quite anxious to see if this trumps my designs, and if I should toss mine in the bin I could throw a basic version of mine together for your Mewlon if I could get the OTA inside diameter, mirror diameter, and central obstruction size.

I'm particularly interested in limiting magnitude, overall brightness, critical focus zone, and ease of use, especially when used with live view. I'm not saying mine is better than this design, just that I'd love to compare the two side by side.
>snip
Regards,
Eric

[multiweb (Marc)]

Originally Posted by Dennis

Hi Marc

Glad to hear of your success, my home made Bahtinov Mask has certainly delivered the goods so far.

I had a look at the sub frame of M17 and noticed that on the brightest star with the 4 diffraction spikes, the spikes seemed off-centre to the star disc? These spikes can’t be from the ED80 or C5?

Great image by the way!

Cheers

Dennis

Correct. Forgot to mention it. This was taken at prime focus on a celestron 5" newtonian . QHY8 1min exposure. I did an 80mm mask for the ED80 and one 160mm for the celestron.

[Dennis]

Originally Posted by multiweb

Correct. Forgot to mention it. This was taken at prime focus on a celestron 5" newtonian . QHY8 1min exposure. I did an 80mm mask for the ED80 and one 160mm for the celestron.

Thanks Marc – I assumed the C5 was an SCT!

Cheers

Dennis