Skyliner 250px dob collimation

[knobby]

Help !

please ...

following Astro babys excellent guide

quote :

' Collimation Step 1 - The Spider Vanes and Secondary Mirror Holder Before attempting to collimate the secondary mirror it's essential to get the mirrors holder central to the tube and the primary mirror. Some scopes allow adjustment of the spider vanes. If your scope has this facility you need to check whether the secondary mirror holder is held perfectly central by the spider vanes. Note: Some scopes require the secondary mirror to be offset from the centre of the tube. This in itself raises questions. Some telescopes will have the secondary offset on its holder in which case the centre of the vanes should be centred to the tube while other manufacturers will opt for the vanes centre point being off centre. The offset on most scopes will be in millimetres and wont affect collimation by much. You should check whether your telescope requires offsetting with the manufacturer or supplier. If you cannot get an answer assume that the mirror holder is to be central to the telescope tube. There are two methods to accomplish getting the mirror holder central to the tube. The first is relatively simply which is to measure the distance between the tube and the secondary mirror holder along each of the vanes. The distance should be identical for each vane. If the vanes are not equal than adjust them using the vanes attachment / adjustment screws. The cross head on the central screw makes an ideal reference point.

is the skyliner offset or not ?

Any ideas

Cheers

[skyhigh]

Hi Knobby,

Having recently collimated my 250PX using Astro babys collimation guide, I measured the distance from the centre screw to the sides by laying a ruler on each of the vanes as per the guide and found the distance to be the same for all four.My assumption based on my findings is that "offset" is not a factor.

Having looked on the skywatcher website, I cannot find anything suggesting that offset does or does not apply.

[rory]

i dont believe so. my 200 is central.

[skyhigh]

Just to add..

The instruction manual covers the collimation process in 2 pages and talks of secondary and primary mirror adjustment.

There is no mention of the spider adjustment or offset.

[Moonshane]

this article explains it quite well. http://www.catseyecollimation.com/mccluney.html the offset will be built into the position of the secondary mirror when it was glued to the diagonal holder. therefore, just ensure your holder is central (i.e. the spider vanes the same length) and you will automatically get the offset correct when centering in the focuser.

it's not something to worry about, especially for visual as it makes a very small difference and one I'd be surprised if you can detect with your eye.

[Paulb3098]

Hi Knobby,

My spider vanes are all the same and the secondary is central. I collimate with a cheshire or hotech laser and it's spot on every time. As I have a flexitube I collimate every time I go out and found a set of “bob knobs” are a real help, make’s it a 2 minute job. Good luck.

Paul

[knobby]

Cheers guys

My spider is off centre by close to 3 mm

Gonna be tweaking this weekend!

Sent from my HD2Droid

[knobby]

Cheers for all the replies ...

Spider now centered, secondary looks round !, primary spot in the middle ... iffy lazer collimator (collimated of course) points to primary dot ... reflects back smack bang in the middle of the 45 degree window ... just need to star test ... OH, look its cloudy again !

sorry guys, must be my fault this time.

'to Paul' the 250px seems to have thumb screws already, similar to Bobs knobs.

Thanks again for all your help

[pat]

My spider vane is off by 2mm in my Orion f4.7. Collimation checks out ok. Stars are pinpoint. The secondary is centered in the sight tube. I believe as long as the secondary is correctly centered, you can ignore the small spider length differance. Pat

[knobby]

My spider vane is off by 2mm in my Orion f4.7. Collimation checks out ok. Stars are pinpoint. The secondary is centered in the sight tube. I believe as long as the secondary is correctly centered, you can ignore the small spider length differance. Pat

You're probably right, I work with optics and all this collimation talk can get you a bit paranoid ...

Just want to get the best out of my baby!

And its cloudy, what else would I be doing?

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[Moonshane]

everything is adjustable and as you say it would be possible to adjust the misalignment of the spider via the secondary adjustment. but for what it's worth I just adjust it so it's closer to 'perfect' if possible.

[knobby]

everything is adjustable and as you say it would be possible to adjust the misalignment of the spider via the secondary adjustment. but for what it's worth I just adjust it so it's closer to 'perfect' if possible.

So its not just me then

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[Moonshane]

at least if your collimation is good then you know that it's something else (like bad seeing) that's affecting the view (if anything is).