Secondary Screws

[Moonshane]

Hi all

My 6" f11 OOUK dob has a small secondary at 25mm minor axis. This is great for increasing the contrast but makes the construction of the spider a little challenging. The issue I have is with the supplied spider screws, i.e. the ones that hold the vanes to the central part of the assembly. They project very slightly and when collimating I can see them (or at least three of them) around the secondary.

I don't think there's any major implications of this although in theory it effectively increases the secondary size a little I suppose. However, I cannot answer a query in my mind as to whether they will create some diffraction effect?

Would I be better with a slightly larger secondary to hide these behind, should I make a new spider, or should I just forget it?

Cheers

Shane

[John]

Probably no worse than the 3 foil spacers that many refractors use between the lens elements Shane.

My old Meade AR6 also had 3 nuts holding the lens cell to the tube that were also in the light path and they were not in the same position as the foil spacers

I think these things can cause issues for imagers at times but I'm not sure they really impact observing noticably.

[Moonshane]

Great stuff. Cheers John. That's what I suspected but good to have confirmation.

You know what it's like though when you can see them every time you look down the Cheshire!

I may eventually have a go at replacing the screws with countersunk versions but for now I'll leave it alone.

[FairviewSteve]

may eventually have a go at replacing the screws with countersunk versions but for now I'll leave it alone.

Is there a way that these could be replaced with somethign that can be easliy turned for colimation? I suppose a cap head bolt would need an alan key so it's not too difficult. I'd like big knobs on the primary of mine though.

Steve

[Moonshane]

the main problem is head size. my secondary is less than 25mm across the short axis. most knurled thumbscrews have a min head size of 10-12mm which sticks out into the optical path. with your 8" you might get away with the ones I used on my 16" see towards the end of the 6" f11 planetary mods thread to which I think you subscribe?

[russ]

Is there a way that these could be replaced with somethign that can be easliy turned for colimation? I suppose a cap head bolt would need an alan key so it's not too difficult. I'd like big knobs on the primary of mine though.

Steve

Steve, it's very easy to add big collimation thumbs screws to the Orion. I bought (very cheaply) some M6 thumbs screws from WDS Ltd. Then replaced the nuts with thumb screws on the back of the Orion.

Here's a link to WDS Ltd:

Plastic Hand Knobs, Thumb Screws & More | WDS Machine Parts

And these look good:

WDS 8225 Hand Knob - open, Plastic Hand Knobs - WDS

This is the back of my Orion:

[russ]

the main problem is head size. my secondary is less than 25mm across the short axis. most knurled thumbscrews have a min head size of 10-12mm which sticks out into the optical path. with your 8" you might get away with the ones I used on my 16" see towards the end of the 6" f11 planetary mods thread to which I think you subscribe?

Shane, could you use three of the thumb screws used to tighten the eyepiece in a focuser? Would you be able to get enough force onto the screw to turn it i wonder?

[Moonshane]

I thought about these but I could not find any that were long enough and I think you'd need a lathe to put a thread into them to use e.g. a thread rod to screw and glue in.

[Moonshane]

Steve, it's very easy to add big collimation thumbs screws to the Orion. I bought (very cheaply) some M6 thumbs screws from WDS Ltd. Then replaced the nuts with thumb screws on the back of the Orion.

Here's a link to WDS Ltd:

Plastic Hand Knobs, Thumb Screws & More | WDS Machine Parts

And these look good:

WDS 8225 Hand Knob - open, Plastic Hand Knobs - WDS

This is the back of my Orion:

sorry, yes, forgot about the primary end. I agree, these work great. use teflon washers though as it makes a big difference.

[russ]

yes the Teflon washers are a must and extremely cheap on Ebay