I have a 10" imaging newt build just completing and have run into one minor concern (at least I think it's minor but would like some opinions).
A bit of background: It's an open strut tube newt using carbon fibre longitudinal struts with marine ply frame members. It was assembled on a jig with a laser reference on each frame member and the tubes to keep it all in perfect alignment. I did a star test, and imaging test when I first assembled it (prior to final finish) and everything was great.
Now after final finish I have reassembled the components (mirror placement, focuser, guidescope mount, finder mount, etc) and re-collimated everthing. In checking all my measurements (made at the time of the initial test) I found that the secondary four vane spider was not perfectly equal in length in their vane brackets. They are only out by 2.5 mils on one of the four axis, and 1 mil on another.
This is am imaging scope so it has a slightly larger secondary than perhaps a normal visual newt (@ 63mm), and it's in perfect collimation as is.
So the question is, how obsessive do I need to be about the secondary centering when the 'possible' off centre position is out by such a small amount, and my image subs are great with no fall off on the sides.
If I move the secondary I will need to go through the whole collimation again - for what gain? I am tempted to leave it until I have to re-collimate for some reason.
this is actually to your advantage, believe it or not.
With two uneven vanes, you'll be able to give the secondary an off-set that will actually increase the amount of light efficiency to your camera.
As the 'cone' of light comes up to the secondary from the primary, the geometric shape it hits the sec at is not an even one, rather more pear shaped, fatter at the bottom, narrower at the top. The bottom part of the secondary can actually miss some light. This 'off-set' is a setting of the secondary so that it is more efficient. With a visual scope this is not so critical to get right as our eyes won't really register the few % difference. But with photo, this adds up quickly.
The off-set of the secondary is both back away from the focuser, and up from the primay. But if you just set it back, this will give you the majority of the light gain advantage. It is no more than 2mm in either axis.
From your post, I have a couple of questions. The orientation of the vanes, is one of them in line with the focuser? That is, is its anchor point where the focuser is? This will set two other vanes perpendicular to the focuser, and the forth vane is an the far side. If this is the case, it is easier to accommodate the off-set. Next, the length difference in the vanes, are they longer or shorter? As two vanes are the same length, they would be set perpendicular to the focuser. It is then a simple determination of which vane will go where to push the secondary mirror away from the focuser.
There are several sites that are 'off-set calculators'. A quick search will pull them up.
Thanks gents. There is no misalignment in the optical train that I can detect with my laser equipment. I suspect that when I bonded in the front frame member (which was done after the alignment test) that the frame might be just that little bit out of position. That is not a big deal over a 1250mm focal length and where the vanes are adjustable. To Alex's question, the four vane spider is not set with a vane parallel or perpendicular to the focuser. I am familiar with offset and considered that when I first noticed the different vane lengths. The secondary may well be perfectly optically centred as the frame openings are larger than the mirror diametre, and if the placement of the front frame was slightly out alignment it may have no effect. Photo attached. More photos in my Imaging Newt build thread.
Note that the photo is not optically centred, just a view down the tube, hence any appearance (or assumption) that the secondary might not be centred on the holder are not correct. The secondary minor axis is 63mm but when viewed down, most of what you see is the top of the secondary holder which is just 52mm in diametre.
A Cheshire eyepiece will help determine if the secondary is centered. Of course I don't know what collimation tools you have. A stand alone laser won't collimate the secondary. Asking to find out what you have to make the most of your gear,
Guess I should have explained what meant by laser equipment- it isn't the laser collimators but a construction laser system I used in boat buiding. This laser draws horizontal and vertical lines on objects for alignment purposes during construction - that's how my structure was aligned.
Yes, I have a Chesire tube and it was used to centre the secondary and collimate.
I have gone ahead and moved the secondary vanes and allowed for the 3.15mm offset as shown by the calculators. Re-collimated to perfection. I will do a test run tonight if possible.
As the 'cone' of light comes up to the secondary from the primary, the geometric shape it hits the sec at is not an even one, rather more pear shaped, fatter at the bottom, narrower at the top. Alex.
Sorry Alex but that is not the case. It is in fact an ellipse just like the secondary. A cone cut at 45 degrees is an even ellipse.
We offset the secondary to optimise it's size, not to compensate for any "pear shape"
Dave
