is this ok ?

[seahawk]

HELP!
Finally got myself a collimation tool, and was shocked when i saw how far out my 2 week old "new" 200mm newt was. After several hours reading & playing with adjustments if got it almost spot on, except for the scondary mirror/focusser alignment, looking down the focusser, the secondary sits too far down in the tube to give a perfect collimation "after" shot. I tried loosening the the new crayford focuser i also fitted but there was not enough sideways movement to align everything. In the end i had to remove the secondary mirror, removed the tension spring from the centre screw so as i could wind the adjustment screws RIGHT in to bring the mirror up the tube to align it in the focusser tube, it now looks 100% using the collimator tool, but after having to move the secondary an inch or so up the tube, will this affect the focal length or cause any issues, i have not tested it outdoors as yet due to poor conditions. ( The secondary mirror position fault was evident with both focussers, not just after fitting the crayford)

My other concern also now, is after fittting the new 2" crayford focusser, i no longer have enough adjustment to focus correctly with my eyepieces, i need to use a short 2x barlow to achieve focus, will i need some kind of 'extension' tube to gain the extra lenght ? ( my old focusser did have longer travel, but a ngihtmare to use)
Thanks in advance.

[erick]

Sort of sounds OK? An inch - wow! After retracting the three secondary collimation screws, all you had to do was pull the Secondary mirror a bit up the tube with the central screw, then push the collimation screws back in. Is the compression spring still in place?

You need a 2" 35mm extension tube. Bintel sell a GSO one for $39. I seem to recall that they have an internal "lip" that may prevent some 2" eyepieces/barlows from going fully in.

It's a bit unusual that the new focusser needs the extension tube. Is is a "low profile" focusser?

[seahawk]

Hi Eric, i had to leave the spring out, otherwise would not pull in far enough to align, its secure enough without it. I was thinking the focusing problem may have been caused by the need to move the secondary mirror up so much, may have done something with the focul lengths ???
Seems like you start playing with things & create more problems once you start modifying....
Alex

[erick]

Well, here is my thinking. The focal length of the primary mirror does not change. The distance from the surface of the primary to where the image forms is unchanged, regardless of where the converging rays bounce off the 45 deg diagonal secondary mirror. However, the distance from the central axis out to the image formation point will change, depending on how far from the primary the rays deflect 90 degs off the secondary. The effect is that the image is formed higher or lower in the focusser. Thinking again about what you said - if you had to move the secondary an inch up the tube, the focal point should have moved an inch towards the tube axis, ie. deeper in the focusser. Hence I don't understand why you find that the eyepieces need to be higher in the focusser. Just check again without the barlow. You can always loosen the clamp screw and lift an eyepiece up (and even out!) to see where the image comes into focus.

By the way, does the new focusser look "square" to the tube - 90 deg? Obviously if the focusser is pointing up or down the tube. the secondary will appear to not be directly beneath the focusser.

[DavidU]

This sounds like an odd problem. if the secondary was 1" too low there is no chance at all the scope would have passed any quallity control in any factory. You may be able to wind in your 3 primary collimation screws some 10mm or so placing the mirror further up the tube to aid in your lost focus travel.
What brand is the scope? If it is a used one perhaps it had been modified as a photographic scope, thus the lack of focus travel

[seahawk]

Definitly square to the tube, i can achieve focus by pulling the eyepiece out a little further, with the focusser at it longest travel out, so i need more "travel" or an extension piece, have only tried it with a 25PL so far, not my other ep's. Having just measured both focussers, my old one from its mounting base is to the business end 190mm and the new one 155mm, so 35mm shorter. I am also wondering if i will have a bigger problem if i try 2" ep's which will sit even further in after remove the 1.25 extension ??

[erick]

That is why I was wondering if the change of focusser was an issue with respect to the perceived position of the Secondary.

Agree with the try to move Primary up the tube on its collimation screws.

But a 35mm extension tube may an easy solution. I'm sure someone will sell you a used one if you ask?