Partial offset collimation

[Stonius (Markus)]

Hi there,

If you're going for a partial offset collimation as explained here in diagram C, my understanding is that the mechanical and optical axes of the telescope are different.

So if you pop in a laser collimator, should you get *two dots on the secondary? One outgoing, the other incoming? I'm only getting the one. Everythings lining up perfectly, but I want to make sure I havent just done a perfect collimation without the offset as in diagram A.

Markus

[sharptrack2 (Kevin)]

My thoughts on your question...

What might be misconstrued here is that you would see the offset at the secondary. As mentioned in the article the difference is fractions of a degree variation in the tilt of each mirror, and the drawings are exaggerated.

I suspect that if you look at the secondary reflection, as closely, as safely possible, the laser dot will be just slightly elongated, the beam width will not be tight enough to see 2 distinct dots.

At this point a star test would be the next best verification of how well the optics are collimated.

[Stonius (Markus)]

Hmm. So if they look the same visually, is it even possible to verify that your scope is set up one way or the other?

[AstroJunk (Jonathan)]

Offsetting the mirror is in diag B. There will only be one dot and it will be a few mill away from the centre along the long axis, higher up towards the open end of the scope.

C is, er; unique advice! Tilting the primary mirror to get full illumination is will introduce rather unwelcome aberrations at the camera/ocular!!!

Attached is the distance I have offset my 20" f5 scopes 3.5 inch mirror. Its not much

[Shiraz (Ray)]

you should get one beam path and one dot. Offset just aligns the optical axis with mechanical axis, but otherwise the collimation produces identical results and it is moderately difficult to verify how your scope is set up.

[Stonius (Markus)]

Okay, thanks for the input, fellas. So the result looks the same in terms of a laser, but you will notice the difference in the sight tube because the secondary will be a little closer to the primary, and that difference should be the difference as given in the equation.

Thanks!!

Markus

[ausastronomer (John Bambury)]

Originally Posted by Stonius

Hi there,

If you're going for a partial offset collimation as explained here in diagram C, my understanding is that the mechanical and optical axes of the telescope are different.

So if you pop in a laser collimator, should you get *two dots on the secondary? One outgoing, the other incoming? I'm only getting the one. Everythings lining up perfectly, but I want to make sure I havent just done a perfect collimation without the offset as in diagram A.

Markus

Hi Markus,

You are really over complicating your own learning curve by worrying about this. Secondary offset really only becomes important under 2 scenarios.

1: For visual observers who are using a minimalist sized secondary mirror by design and regularly using eyepieces with a large field stop. ie 31mm Nagler, 35mm or 41mm Panoptic, 40ml Pentax XW and a few of the new longer focal length ES eyepieces. That in itself is counterproductive because you wouldn't design the telescope with a minimalist sized secondary mirror if you intended a lot of use with such eyepieces.

2: For imagers who are intending very long exposures and who need super accurate tracking.

Secondary offset is designed to place the axis of the light path at the optical centre of the secondary mirror and it is also intended to make the optical and mechanical axes of the telescope coincidental. Even with either partial or full offsets on any telescope under 18" aperture, the observable differences are pretty small.

You will also notice if you run some numbers through the formula that the amount of offset required away from the focuser and towards the primary mirror increases as the aperture of the telescope increases and its F ratio decreases. With your 16"/F4.5 the offset would be about 4mm and with your 8"/F6 about 1.5mm. However, both of these scopes (as are all mass produced scopes) are supplied with generously sized secondary mirrors. I don't think you are going to achieve a single thing (other than confusing yourself) worrying about full or partial offsets with either of these telescopes.

If you position your secondary mirror using your sight tube you will achieve a partial offset (toward the primary mirror) in any case.

I don't worry about secondary offset in any of my telescopes which all have marginally sized secondary mirrors by design and they all work pretty well. I used to offset the secondary in my 18"/F4.5 Obsession which uses a borderline sized secondary @ 3.1" (probably smaller than the secondary in your 16"), but ultimately I figured it was a good bit of extra work for no observable gain.

Cheers,
John B

[AstroJunk (Jonathan)]

You also miss out on on-axis light too, it's not just the wide angles. This can be quite a lot if your secondary is 'traditionally' sized. I upped my 3.1 secondary to a 3.5 and offset it. The view was much better!

Originally Posted by ausastronomer

Hi Markus,

You are really over complicating your own learning curve by worrying about this. Secondary offset really only becomes important under 2 scenarios.

1: For visual observers who are using a minimalist sized secondary mirror by design and regularly using eyepieces with a large field stop. ie 31mm Nagler, 35mm or 41mm Panoptic, 40ml Pentax XW and a few of the new longer focal length ES eyepieces. That in itself is counterproductive because you wouldn't design the telescope with a minimalist sized secondary mirror if you intended a lot of use with such eyepieces.

2: For imagers who are intending very long exposures and who need super accurate tracking.

Secondary offset is designed to place the axis of the light path at the optical centre of the secondary mirror and it is also intended to make the optical and mechanical axes of the telescope coincidental. Even with either partial or full offsets on any telescope under 18" aperture, the observable differences are pretty small.

You will also notice if you run some numbers through the formula that the amount of offset required away from the focuser and towards the primary mirror increases as the aperture of the telescope increases and its F ratio decreases. With your 16"/F4.5 the offset would be about 4mm and with your 8"/F6 about 1.5mm. However, both of these scopes (as are all mass produced scopes) are supplied with generously sized secondary mirrors. I don't think you are going to achieve a single thing (other than confusing yourself) worrying about full or partial offsets with either of these telescopes.

If you position your secondary mirror using your sight tube you will achieve a partial offset (toward the primary mirror) in any case.

I don't worry about secondary offset in any of my telescopes which all have marginally sized secondary mirrors by design and they all work pretty well. I used to offset the secondary in my 18"/F4.5 Obsession which uses a borderline sized secondary @ 3.1" (probably smaller than the secondary in your 16"), but ultimately I figured it was a good bit of extra work for no observable gain.

Cheers,
John B

[AstroJunk (Jonathan)]

I might add to that - 16" scopes are probably too small to notice the difference unless f4 or below.

[ausastronomer (John Bambury)]

Originally Posted by AstroJunk

You also miss out on on-axis light too, it's not just the wide angles. This can be quite a lot if your secondary is 'traditionally' sized. I upped my 3.1 secondary to a 3.5 and offset it. The view was much better!

Hi Jonathan,

If that was on your 20"/F5 the secondary really was undersized (15.5% CO), not traditionally sized and I think most of any gain you noticed would be attributed to up sizing the secondary to 3.5" and not so much by off setting it. Without running the numbers I am guessing the FIF with the 3.1" secondary to have been just less than 6mm. That all having been said I think the offset is important with today's trend at larger and faster newtonians. I would agree that it is probably worthwhile with scopes larger than 16" or 18" and faster than F4. It is also less important if you tend to not use long focal length eyepieces very often, which I don't. Normally the first eyepiece I start with in my 18"/F4.5 is a 17mm Nagler, or a 14mm Pentax XW.

Cheers,
John B