Are you collimating your scope correctly?

Post by **VinSnr** » Sat Jul 01, 2006 7:18 pm

weixing wrote:Hi,
The focuser position is same for both... What I mean is that I can collimate to either 1) The smaller brightest white "donut" in the center, but the two bigger "donut" (one grey colour and one green colour) is slightly offset or 2) The two bigger "donut" merge to one, but the smaller brightest white "donut" is off center. All this done without touching the focuser, just adjusting the lens cell comillation screws.

Currently, I adjust the collimation to 2 which I think is correct, but just wonder why the brightest smaller "donut" is not in the center... is it because the hole on the reflective surface in cheshire eyepiece is not round or not in the center??? Or should I adjust to collimation 2??

Hope those refractor expert can give some advice.

Thanks and have a nice day.

Does this Cheshire comes with the cross hair?

Post by **weixing** » Sat Jul 01, 2006 7:32 pm

Hi,

Does this Cheshire comes with the cross hair?

No... I think I shouldn't call it a cheshire eyepiece...

It is label as a collimating eyepiece.

Have a nice day.

Post by **VinSnr** » Sat Jul 01, 2006 10:26 pm

weixing wrote:Hi,
Does this Cheshire comes with the cross hair?
No... I think I shouldn't call it a cheshire eyepiece... It is label as a collimating eyepiece.

Have a nice day.

weixing, can you take a photo and let us see how that collimating eyepiece look like. I saw a rubber eyeguard at teh front which is not very common

Canopus Lim · Post by **Canopus Lim** » Sun Jul 02, 2006 6:22 pm

Hi,

I have both laser and cheshire. However, my Orion cheshire eyepiece's crosshair is not centered (offset of about 1mm) and hence I get wrong readings with it all the time. Also, the cheshire is better for people who DO NOT wear spectacles, because whenever I wear my spectacles, I cannot see the edge of the secondary mirror (even if I racked the eyepiece nearest to the secondary mirror) and hence I cannot see if the secondary is 'squared' in the draw tube which actually defeats its purpose of aligning the secondary mirror.

The last point is that the cheshire eyepiece is not good in the dark, as it is difficult to see the crosshair and the 'donut bullseye'. Even if I shine my torch light at the primary mirror, it is really hard to see it. Also the crosshair itself is quite thick and hence when looking at it through the pinhole, it is difficult to tell if the donut is centred as the thickness of the crosshair is already about the size of the donut when seen from the pinhole; and often I see double image of the crosshair.

The laser is good if the laser is good itself. That means the concentricity of the laser beam is parallel to the mechanical axis. Also, the focuser has to have tight tolerance to have good laser readings. Lousy focuser will bounce the beam all over the place and when racking in and out the focuser, the beam will shift. My laser works well and I used it barlowed for the primary, its results is very good; as accurate as the cheshire or collimation eyepiece. I see the donut at the centre of the collimation eyepiece; error is very slight and I doubt it will cause much image issue. Most importantly, it is much faster and convenient to use in the dark.

Post by **weixing** » Sun Jul 02, 2006 8:19 pm

Hi,
Below is the picture of the collimation eyepiece.

Thanks and have a nice day.

PS: Hmm... seem like I should post as a new thread.

Canopus Lim · Post by **Canopus Lim** » Mon Jul 03, 2006 8:55 am

I have read one way of squaring the secondary mirror in Newt. I find it an excellent way. First, it is necessary to centre dot the elliptical secondary mirror. I do this by putting a small dot of paint onto the centre. That small dot will not affect the image as it is like dust. Having centre dot it, using a laser or cheshire, adjust the spider such that the laser beam or the crosshair (in cheshire) strikes that dot or is aligned with it. The secondary mirror will be in the exact centre when viewed from the focuser draw tube. There is no need to put a piece of white paper behind the secondary mirror or use estimation from the cheshire to determine if the secondary mirror is centred. (the cheshire gives the crosshair but it is still necessary to estimate if the secondary mirror is in the centre)

This adjustment is the first thing to do. Do not care if the secondary mirror is tilted as it would not affect the position of the dot since it is along the mechanical axis. After squaring the secondary mirror using this technique, rotate the secondary mirror such that the face of the mirror is perpendicular to the optical axis. Tigthen the spider and it would be good alignment. The remaining steps that will be needed to do often are secondary mirror tilt and primary mirror tilt; which can be done effectively with laser/cheshire.

For Primary mirror tilt alignment, I suggest using the barlowed laser technique. It is as accurate as cheshire and much easier to do in the dark.

chrisyeo · Post by **chrisyeo** » Mon Jul 03, 2006 11:05 am

Dot the secondary? It may be a small dot but will that not affect the light path?

Canopus Lim · Post by **Canopus Lim** » Mon Jul 03, 2006 11:17 am

It won't noticeably as it is just like a dust on the mirror. The purpose of the secondary is just to collect the light from the Primary mirror and reflect it to the eyepiece. A small dot as small as possible but good enough to see it. It will help a lot in judging if the secondary mirror is in the centre when seen from the draw tube. When using the laser, whenever you turn on, you can see the laser spot on the dot and you will be certain that the secondary is squared. The return path of the laser reflected off the primary SHOULD hit that same dot when the scope is collimated. In a way, it can also help check your collimation too.

Canopus Lim · Post by **Canopus Lim** » Mon Jul 03, 2006 1:54 pm

For curiousity sake and reading through cloudynights forum on the debate on whether it is wise to dot the secondary mirror, I did simulation using my office's raytrace software (Zemax). For a dot of 1mm diameter or less (most likely a small dot will be 0.3 or 0.5mm diameter at most), the optical performance is the same. There is no effect on its optical performance. However, if the spot is like 2mm in diameter and above, it does affect optical performance. The 1st diffraction ring intensity will increase due to scatter of light from the small obstruction. Hence, a small dot of 0.5mm diameter will not affect the performance.

It is useful for truss tube newtonian users who use laser to collimate their scopes as the laser can now be used to square the secondary mirror. It is useful for people who use cheshire too as the exact centre of the secondary mirror can be determined instead of using estimation.

Well just an information only. Don't necessary need to follow this method though.

Post by **acc** » Mon Jul 03, 2006 2:40 pm

Interesting discussion... My portaball comes with its secondary already marked by a centre dot to aid in laser-collimation. I seem to recall seeing a faint shadow of the centre dot when looking at the out-of-focus image of the moon (only) but it disappears once the image is in focus. If a reputable maker like Mag1 does this to the secondary, my guess is that the dot has little impact on the image quality. Or so I hope