collimation fast scope

[spillage]

Hi All,

I feel a bit daft asking this, but have gone and over thought things a bit.

I am tying to check the collimation on my 130pds. I know it will be offset due to being a fast scope but after getting it central to the cross hairs on the cheshire the primary centre spot of off, is this right?

cheers

Spill.

I know I am a little out on the image but will sort that out.

[Peter Drew]

Looks to me as though the primary needs a slight tweak. The offset looks about right but the reflection of the whole secondary is displaced towards the upper right . 

[alacant]

1 hour ago, spillage said:

cross hairs on the cheshire the primary centre spot of off, is this right?

Hi. I think the secondary is wrong. As it is, the more vertical of the cross hairs is too far right and the secondary is displaced too far toward the primary -there is displacement, but I believe you have a little too much. Take the secondary away from the primary about 1/2 turn clockwise on the centre retaining screw and redo those cross hairs. A star test will soon confirm whether you're central or not but you can get close with a Cheshire with cross hairs. HTH.

[wimvb]

3 hours ago, spillage said:

Hi All,

I feel a bit daft asking this, but have gone and over thought things a bit.

I am tying to check the collimation on my 130pds. I know it will be offset due to being a fast scope but after getting it central to the cross hairs on the cheshire the primary centre spot of off, is this right?

cheers

Spill.

I know I am a little out on the image but will sort that out.

 

Can someone explain to me why collimation needs to be off center for a "fast" scope? I've centered everything in my 150pds and startest always looks good.

[alacant]

13 minutes ago, wimvb said:

collimation needs to be off center

Not an expert but you can visualise what's happening by cutting through a cone at 45º; the ellipse you get is fatter at the bottom. Maybe the pds has an oversized secondary? HTH and clear skies.

[spillage]

Thanks all, I have redone collimation and ran a star test that looked good. Will take the change tonight to do some lights and see how they come out.

[wimvb]

12 hours ago, alacant said:

Not an expert but you can visualise what's happening by cutting through a cone at 45º; the ellipse you get is fatter at the bottom. Maybe the pds has an oversized secondary? HTH and clear skies.

Thanks. But actually, this "cut" through the light cone or conic section IS an ordinary ellipse, and not an oval (egg shape). But I figured out the answer to my original question, so the argument is moot.

[alacant]

1 hour ago, wimvb said:

I figured out the answer

OK. Well done. I don't understand it. All I know that is to get the Crosshairs on my Cheshire in the correct place, the reflection has to be offset for it to work! 

[wimvb]

1 hour ago, alacant said:

OK. Well done. I don't understand it. All I know that is to get the Crosshairs on my Cheshire in the correct place, the reflection has to be offset for it to work! 

Have a look here for the answer sbout the section being an ellipse and not egg shaped

https://math.stackexchange.com/questions/2184505/why-ellipse-is-a-conic-section

As to the offset, my understanding is this (I hope the exaggerated sketch makes sense. The offset is the difference between the optical axis of the light cone = axis of tube, and the centre of the secondary mirror.)

[alacant]

16 minutes ago, wimvb said:

ellipse and not egg shaped

OMG, yes. I perticularly liked this bit:

Quote

 the axis of symmetry of the ellipse is not on the axis of the cone

Hence the offset? If so, that just about nails it for me. Thanks.

[wimvb]

Exactly.

[Jason D]

To the OP,

In the photo of your first post in this thread, you were trying to align the cheshire/sight-tube tool cross hairs with the pupil reflection of the tool. This is incorrect. You are supposed to align the cross hairs with the reflection of the primary mirror center spot. 

With respect to the secondary mirror silhouette for a well-collimated scope, it is not an elliptical silhouette but rather a shifted circular silhouette which gives the illusion is it elliptical. See attachments below. The secondary mirror silhouette is actually circular in the photos as well as the illustration. The shift is always towards the primary mirror.

 

 

The reason the silhouette appears shifted is because it shows the secondary mirror from a virtual point behind the primary mirror. It is a misconception that only fast scope will show a shifted secondary mirror silhouettes. All Newtonians will show the same; however, it is just more apparent for fast scopes.

 

 

 

The reason the secondary mirror appears as a silhouette is because it reflects the dark area around the focuser opening from inside the OTA. If you were to attach a template around the focuser opening from inside the OTA as shown below, the secondary mirror will no longer appear as a silhouette:

 

Jason