517 replies to this topic

[Dennis Sakva]

150 to 200, so that you don't need to cut exactly 7.34579mm but round to 7 or 8mm (Just a number from my head).

Those are empirically derived numbers. 35-45 will work too, but you'll probably need a bit different cut sizes.

[snowdragonusa]

I was reading back over some of the original posts from Dennis and noticed his red mask had a hole cut out in the center.
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.

[Dennis Sakva]

The hole in the center is just for the secondary mirror. For refractor you don't need it. Just continue cuts where the hole is.

[ayiomamitis]

Dennis,

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!

[Gus_Smedstad]

Post #5 makes it fairly clear that you have the numbers correct. A 1200mm focal length gives you a total width S of 6-8mm, so the slits need to be 3-4mm and spaced by the same amount.

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?

- Gus

[Elliot]

Hi John - I ordered one from your website today http://www.spike-a.com/# but did not see where it will take 2-4 weeks for deleivery. Why so long? Is that accurate?

[John Wunderlin]

Hi- yes, I just added that to the site. The 1st batch of masks I have were prototypes. The 1st production run will be done next week. I'm hoping it will be less than 2 weeks, but I want to manage expectations. If you're interested in a prototype, PM me or email me directly.

[Elliot]

I will anxiously wait.

[John Wunderlin]

One additional suggestion for the DIYers, I experimented with wide ribbon of the type you use for wrapping presents. It showed some promise. You could potentially make a 'dreamcatcher' type mask with perhaps a wood slat down the middle and attach ribbon to it in this pattern. I gave up on this path after Jerry & I hooked up.

[Gus_Smedstad]

As long as you're following this thread, Josh, if you're going to offer these masks, are you going to give any consideration to the slit spacing? I'm unclear how important this is because, not knowing Russian, I haven't read the original thread. But my 8" OTA is an Orion 1000mm, and your site doesn't say whether the slits on an 8" mask are 3mm (for 1000mm f/4.9) or 5-6mm (for a 2000mm f/10).

- Gus

[John Wunderlin]

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.

[Gus_Smedstad]

My apologies for the name mistake. For some inexplicable reason I saw "Josh" every time I read "John."

Arrrgh.

- Gus

[Dennis Sakva]

s=slit+space
So S=10 the cut is 5mm and the uncut space is also 5mm
BUT!
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.

[John Wunderlin]

My apologies for the name mistake.

No problem!

[Snow dog]

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.

Gus

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.
Thanks

[John Wunderlin]

Here is a website that describes the process in quite a bit of detail for the Hartmann style.

http://starryvistas..../FocusMask.aspx

[snowdragonusa]

The thing I don't like about the Hartman Mask is I am taking my 12" scope and reducing the light gathering ability down to perhaps 1/5 or more. The triangles produce small diffraction patterns at best and even placing the toothpicks on I saw little difference (in my case).

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!!!

[Gus_Smedstad]

That article on Starry Vistas is where I got the idea for the diffraction toothpicks in the Hartman mask. The circles I used were a lot larger, and I have no trouble picking out the diffraction spikes, though I do typically zoom in with Live View.

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.

- Gus

[Gus_Smedstad]

BTW, we should probably be calling these "Bahtinov Masks", not Pavel Masks. Oh, sure, Pavel rolls off the tongue, but it's sort of like calling something a "Paul Mask."

- Gus

[xumaxer]

Wow! this sure looks promising!
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?

Thanks!
xumie

[Gus_Smedstad]

I think it's fairly obvious that the final focal length is what matters. Which makes me think that I will need a different mask if I'm using a Powermate to reach 2000mm!

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.

- Gus

[jgraham]

Adam; it took me a while to appreciate the improvement in the light passing through the Bahtinov mask (I like that name) as compared to a Hartman. I also ran into the problem of the classic Hartman loosing sensitivity near the focus and I've been using one of the "+" designs that provides information all the way through the region of best focus like the Bahtinov mask, but it only works well on bright reference stars.

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.

[suburbanskies]

Yes, recognize the inventor and pay a royalty. It is best for you, despite what is "allowable" within the rules.

Mark

[John Wunderlin]

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.

[Nocturnal]

This thing is absolutely AMAZING! I have an F/5.95 110mm refractor with a 0.8x FR. I guestimated the spacing of the mask to be 10mm or 5mm wide slits. Quickly drew one up in the drawing program mentioned above, printed and cut out the slits. I glued the paper mask on a piece of foam board with a hole that turned out to be bigger than the dew shield so the resulting pattern doesn't center. Bam! focus patter is exactly as 'advertised'. Instead of spending 20 minutes with Neb2's HFR focus with numbers jumping all over the place I was focused in about 2 minutes.

My advise: don't worry about the details. Make one from cheap material and try it out.