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#1 Atl

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Posted 10 January 2013 - 10:23 PM

I have a telescope I bought that originally had an older astrosystems spider but the secondary holder had been damaged. I decided to replace the whole spider. Last summer I installed a straight vane 1800Destiny spider on a small scope, and I liked the positive pressure collimation screws so I decided to stay with that brand. This time I got a 4 vane curved spider with holder. I specified a 14" internal diameter. The spider was actually a 13.5" across. It took some playing with it to get it in... little stretching and wiggling. It ended up very tensioned and tight. I am not sure if this is how a curved spider should be so I included a pic. Is this proper? I know the 4 vane has more diffraction but I wanted to re use the holes.

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#2 killdabuddha

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Posted 11 January 2013 - 12:21 AM

No. Sorry. Gotta send it back and get the right size. The proper curve depends upon it, if you want the effect of a curved vane.

#3 careysub

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Posted 11 January 2013 - 09:13 AM

No. Sorry. Gotta send it back and get the right size. The proper curve depends upon it, if you want the effect of a curved vane.


'Buddha is right, the vanes must be part of a circular arc to get the even distriution of diffracted light.

However it would seem that an alternative to sending them back might be to use mounting spacer blocks at the attachment points (just pieces of wood) inside the tube.

#4 jg3

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Posted 11 January 2013 - 11:04 AM

Some tension is good for keeping the attachments well-seated. But not if they pinch the tube, or permanently bend beyond elastic bending.

Spacer blocks are a fine solution if they don't obstruct. Depends (approximately) on the mirror's clear aperture plus the largest usable field size (i.e. about 1.25 or 2 inches).

#5 Atl

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Posted 11 January 2013 - 08:42 PM

I corresponded with the owner of 1800destiny today. He acknowledged the curve was not as much on the 4 vane but that the 4 vane had to be designed that way to avoid diffraction overlap. The tension isn't a big concern as the tube is not deforming any. I did purchase some slightly longer screws and released just a hair of the tension on the spider. It is now collimated and cooling off for a test. I have to say the 1800Destiny secondary holder collimated very simply and quickly.

#6 Atl

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Posted 12 January 2013 - 02:35 PM

I took some of the advice in the responses to good effect. I added nylon washers cut to square as a spacer under the spider arms. These were 1/16" thick recovering the original full curve of the spider. So if youre ever considering a 4 vane curved this is how it sets. I re worked the light sheild to accomodate the spider. I also reflocked the sheild. This is the finished project. I tested it last night. There is noticeable diffraction, but it spreads it out to where I can easily spot the trapezium a-f stars in M42 with no serious effort. Contrary to what some have written elsewhere...the effect is distinct and noticeable. I am very happy with the massive adjustability and stability of the equipment.

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