I'll be trying gout this mas next week
Knocked one of these over today for the C14,
6 mm foam board (with black cardboard on both surfaces), might spray it with something to plastify it? but then that might cause some ondulations...not sure. Anyways, worth a try i guess
Overall diameter 16 inch inside circle diameter 14"
slits of 12mm (same width for closed/open/straight and angled sections)
will let you know how I go
The two small tabs will be bent, the large one is for handling it and has a hole in it to hang up the mask when not in use
cheers
frank
PS thanks to everyone and Mr Pavel Bahtinov for their contributions to the amateur astrophotography world!
I have also made 2 of these things out of foam board, one for my 127mm and one for the VC200L.
They both work like a dream. I didn't bother painting mine and they are white. I can't see what difference it will make as it is only for focussing and doesn't matter if it gives a bit of dispersion etc.
Total price $16 for the sheet of foam board with a large amount left over for the kids to use on "craft"
I'll be trying gout this mas next week
Knocked one of these over today for the C14,
6 mm foam board (with black cardboard on both surfaces), might spray it with something to plastify it? but then that might cause some ondulations...not sure. Anyways, worth a try i guess
Overall diameter 16 inch inside circle diameter 14"
slits of 12mm (same width for closed/open/straight and angled sections)
will let you know how I go
The two small tabs will be bent, the large one is for handling it and has a hole in it to hang up the mask when not in use
cheers
frank
PS thanks to everyone and Mr Pavel Bahtinov for their contributions to the amateur astrophotography world!
Nice work Frank. Let us know how you get on using it.
I have also made 2 of these things out of foam board, one for my 127mm and one for the VC200L.
They both work like a dream. I didn't bother painting mine and they are white. I can't see what difference it will make as it is only for focussing and doesn't matter if it gives a bit of dispersion etc.
Total price $16 for the sheet of foam board with a large amount left over for the kids to use on "craft"
Hi Terry
I used a sheet of those square, interlocking foam mats that you buy in packs of 6 at camping shops, to use as a walkway at the entrance of tents.
A packet of 6 cost me $22.00 and the remaining 5 will be put to use for their original design intention – as tent entrance mats!
Here is an image of how my home made Bahtinov Mask performed when focusing on Io, one of the Galilean moons of Jupiter. You can just see the glow from Jupiter’s overexposed disc on the right edge of the frame. This really is a very, very neat and tidy way to focus when operating at such long focal lengths.
Celestron C9.25 with TeleVue x4 PowerMate.
DBK21AF04.AS, F40 at 9400mm efl.
Frames exposed for 0.5 secs.
I was playing around visually with my C9.25 mask last night and noted that depending on where I placed my eye in the exit pupil of the eyepiece, the centre “moving” line could move up or down ever so slightly within the “stationary” shallow cross.
With a CCD camera at prime focus or using the TeleVue x2.5 and x4 PowerMates I was able to nail focus despite the poor seeing.
Going back over some items that Jerry has mentioned, it is important to ensure that the mask is drawn and cut well, is mirrored around the centre, has clean edges, etc.
I’ve had heaps of fun and stimulation so far. I have enjoyed learning more about optics and diffraction during the drawing, fabrication and use of several prototype masks, but it is likely that eventually, I will purchase a rigid, professionally cut model. I consider it a small investment for a life times focusing.
my home cut one made from foam core board was a bit of a miss. My patience ran a bit thin towards the end of the 11 inch mask and the cutting got very average. I bit the bullet and bought one for my c11 and the refractors.
Here are some examples of focusing with a single mask on the C9.25 F10, native focal length of 2350m.
At prime focus (2350mm).
With TeleVue x2.5 PowerMate (5875mm).
With TeleVue x4 PowerMate (9400mm).
The Bahtinov Mask was printed on two A4 overhead transparencies and then the gaps were cut out using a Stanley knife and steel rule, before gluing the mask to a camping mat foam ring.
Forgot to mention my image of the mask was focused on Venus, so as you know fairly low on the horizon and obviously some atmospherics came into play. Also haven't tried it for visual focus yet
Forgot to mention my image of the mask was focused on Venus, so as you know fairly low on the horizon and obviously some atmospherics came into play. Also haven't tried it for visual focus yet
Cheers
I was wondering how you managed to obtain such a bright diffraction pattern – now I know why!
It’s best not to focus on an extended object such as the disc of a planet; the mask really requires a point source of light to be at its most effective. However, I do focus on the Galilean moons of Jupiter if I can’t find a nearby bright star. I’ve found that their discs are so small that they approximate to a point source for the range of focal lengths I’ve used so far.
I forgot to add that an extended disc can be considered to be a collection of point sources dotted around the circumference of the disc, so the mask can be fooled by these multiple point sources.
I forgot to add that an extended disc can be considered to be a collection of point sources dotted around the circumference of the disc, so the mask can be fooled by these multiple point sources.
Cheers
Dennis
Read: do not attempt to focus on the trapezium in Orion...
I tested mine out focusing on Canopus then taking a 45 second sub of Canopus and it was still just that little bit soft... but some of my scalpel cuts were a bit haggard... Thank god I'm not a surgeon.
A 45 sec exposure, that surprises me - what was your optical system set up?
With the mask on either the C9.25 or Mewlon 180, with the TeleVue x4 PowerMate plugged in, I was focusing the DBK21AF04 using exposures of between ½ and 2 seconds on mag 1 stars?
I might try making one of these up?
How do they work on fast f/ scopes? Also where can you buy foam board?
cheers Gary
Foam board can be purchased at art stores or someone like Jacksons Drawing supplies. They work just as well on fast scopes mines an 80 f/7 and my others a 120 f/5.
I might try making one of these up?
How do they work on fast f/ scopes? Also where can you buy foam board?
cheers Gary
Hi Gary
I’ve not yet tried foam board, but what I can say, is that the overhead transparency material seems to make a very good mask.
H0ughy has posted some standard sizes and if you can print one of these on A4 overhead transparency, then the slots are reasonably easy to cut out with a sharp Stanley knife and steel rule. The transparency material is surprisingly tough and doesn’t seem to tear easily. I always cut with the steel rule lying on the black bar, leaving the clear slot visible – it seems easier to see the edge that way. I then cut all the slots along one long edge and turn the work around and do the other long edge. Finally, I cut the narrow edge at the middle bar and at the inner circumference.
I bonded the mask (once cut out) onto a closed cell foam ring made from those camping tent door mats that interlock together. I used contact adhesive.
Here are examples of my C9.25 mask and Mewlon 180 mask. The C9.25 has a central aluminium bar to help with stiffness and a nylon bolt to allow me to place the mask on and remove it with the dew shield fitted. When fitted, the C9.25 mask sits on the metal frame surrounding the corrector plate. The Mewlon mask just sits over the end of the OTA. The last file is a printed but uncut A4 transparency.
Cheers
Dennis
EDIT:
The C9.25 mask required 2 sheets of A4 transparency paper. I printed the central bar + left hand horizontal bars on 1 sheet, then the central bar + right hand angled bars on the 2nd sheet. After cutting out the slots, I overlapped the central bar and glued them together.
