Thanks Mike, my mask arrived at the QLD Astrofest before the weekend. Good quality, well packed and priced. Hope you are able to turn a quid for the work you are doing.
My 2mm acrylic mask arrived safely the other day. Super quick service and a nice product at a great price. It even has the scope details etched into the surface. Thanks so much, Mike.
Sorry guys these plastic boards from ALDI don't work for my laser at all. They are catching fire and starting to melt when cut with the laser. I will keep looking for something thin that will work for carey masks.
You can get rubberised cutting boards from Spotlight and the like. I hand cut a focusing mask from one a few years back. I wonder how they would go with a laser?
They are made of a few layers of rubber or similar material laminated together.
I have tried a previous one of these, but there is this big plastic cover (over the central obstruction) in the centre of the lens that makes it difficult to place the mask flat.
Do you think it may be possible to cut out a central hole to let it lay flat against the lens.
Or do you think it would be better to oversize the mask to sit on the outer tube rim.
The mask would be about an inch from the lens in that case.
I am unsure how this would effect true focus.
Any ideas?
Last edited by Allan_L; 22-09-2009 at 10:02 AM.
Reason: image added
Sorry guys these plastic boards from ALDI don't work for my laser at all. They are catching fire and starting to melt when cut with the laser. I will keep looking for something thin that will work for carey masks.
After a lot of experimentation I have failed to find a thin plastic sheet that can be cut by a laser. They all buckle under the heat.
My best results are with paper card such as used for mounting photographs. I have had excellent results with 1.4mm card (my original prototype!) and recently 1.0mm card.
By the way, Jim Kendrick of Kendrick Astro Instruments recently did an assessment of the Carey mask. With a short 530mm scope he found that it could achieve an accuracy comparable to and in some cases slightly better than a Bahtinov. His recommendation is that the Bahtinov is more suitable than the Carey with short focal length scopes (I agree).
He also commented that the Carey probably would appeal more to DSLR users.
If any of you have tried a Carey mask I would be most grateful for feedback.
I have tried a previous one of these, but there is this big plastic cover (over the central obstruction) in the centre of the lens that makes it difficult to place the mask flat.
Any ideas?
I’m not sure of how the C9.25 corrector cell and secondary holder compare to those of the C11, but here are a couple of photos showing my C9.25 Bahtinov Mask.
I glued a thin strip of closed cell foam around the circumference of the mask so it keeps the mask proud of the secondary holder, with the foam strip sitting nicely in the corrector cell. The side walls of the corrector cell prevent the mask from flopping about.
I’m not sure of how the C9.25 corrector cell and secondary holder compare to those of the C11, but here are a couple of photos showing my C9.25 Bahtinov Mask.
I glued a thin strip of closed cell foam around the circumference of the mask so it keeps the mask proud of the secondary holder, with the foam strip sitting nicely in the corrector cell. The side walls of the corrector cell prevent the mask from flopping about.
Cheers
Dennis
Thanks Dennis,
That looks great.
Can you please tell me what is closed cell foam and where would I get it.
Also how did you attach it? I presume it needs to be very exact to get a good fit?
Thanks again for the heads up Dennis.
Great job
The normal (open cell) foam used in chairs, cushions, etc is the spongy stuff that absorbs water and is quite soft, limp and squashable – not suited to our needs.
Closed cell foam is the stuff used in camping mats (between your sleeping bag and the tent floor) or in those inter-locking square mats sold in camping stores for tent or caravan entrances. It is generally more rigid, quite tough and does not absorb water as it has been chemically blown.
I had an old camping mat so I just cut a thin strip approximately 10mm wide and then used a contact adhesive to glue it to my stainless steel mask (from H0ughy) which had been spray painted with a matt black paint.
You spread a layer of glue on the edge of the foam strip and a layer of glue on the circumference of the mask. Wait approx 10-15 mins (read the instructions) and the glue dries to become slightly tacky and then you carefully mate the foam strip with the mask, curving it as you work your way around. The main downside with contact adhesive, apart from the fumes it gives off, is that once the two surfaces have been pressed into contact, that’s it – you cannot unpeel the foam strip and mask to re-seat the strip, so you need to get it right 1st time.
You can get rubberised cutting boards from Spotlight and the like. I hand cut a focusing mask from one a few years back. I wonder how they would go with a laser?
They are made of a few layers of rubber or similar material laminated together.
Regards,
Eric
I am afraid they won't work too. I might be able to cut them without burning but they are thick and very expensive. May be their price isn't an issue as I have seen some of them cheap in the Reject shops but they are still thick(>3mm I guess) It may be worth doing Carey masks from 2mm acrylic in this case.
After a lot of experimentation I have failed to find a thin plastic sheet that can be cut by a laser. They all buckle under the heat.
My best results are with paper card such as used for mounting photographs. I have had excellent results with 1.4mm card (my original prototype!) and recently 1.0mm card.
I thought about the same solution but not with acrylic support. It would waste a lot of acrylic and it's quite expensive. MDF will be much cheaper. The only problem is that the whole mask will suck in water from air and it will bend eventually and loose its shape.
Yes, MDF is cheaper, but I bought a big supply of perspex!
The absorbtion of moisture is a problem. My 1.0mm mask starts to bow a little after taking it out into the night air. I keep it indoors during the day and it flattens out perfectly. I then take it out and focus while it is still straight. The bowing does not actually do much to the diffraction spikes because the slots remain parallel.
I might devise a support that does not require gluing the card into it.
If the card is free to expand and contract then it may stay flat.
If only someone would produce black perspex 1.5mm thick!
I have tried a previous one of these, but there is this big plastic cover (over the central obstruction) in the centre of the lens that makes it difficult to place the mask flat.
Do you think it may be possible to cut out a central hole to let it lay flat against the lens.
Or do you think it would be better to oversize the mask to sit on the outer tube rim.
The mask would be about an inch from the lens in that case.
I am unsure how this would effect true focus.
Any ideas?
I don't think it would affect focus if you put it one inch from the lens but it might be harder to mount as the mask will be one inch away from the scope and will hang on the trim only.
I can cut out the centre from the mask so it would fit on the plastic cover but it would probably reduce your scope aperture. There should be a solid ring at least 5-6mm thick around the hole. And this would be taken from your scope aperture unless plastic cover diameter is less than the central obstruction diameter.
We can do also what Dennis suggested and I will leave the solid centre on your mask but there might be another problem. Your mask aperture is already very close to the maximum I can cut. If you add too much for you mask OD to mount it on the trim it will exceed 290mm OD I can cut.
I just wanted to reply that at this point I cannot do Carey mask. Unless you are happy to get one made from 250gsm paper and it isn't bigger than 200m in diameter :-)
card wouldn't be a option, too light and flimsy plus with Dew, that would be the end of its life! looking for something a little more durable! I can cut out a cary mask to see how it goes anyhow, ive got some nice thin plastic so ill give it a go
Hi Mike I sent you a PM a few days ago ordering 2 masks, on checking for replies it shows my message was not sent???. 1 mask was for a Celestron 9.25 seconfd was for a meade 80mm APO. If you did not recieve this PM let me know and I will resend the details.
send away
