Interesting collimation technique

[Starflyer]

17 hours ago, BrendanC said:

@Starflyer I'm very interested in this bit: SharpCap using the built in collimation circles. 

Where is this in Sharpcap? 

Hi Brendan,

That bit is a quote from @Avocette post further up. I haven't tried this yet, I need to get the hardware together and then I'll probably try it with Mire de Collimation, I don't have much experience with SharpCap.

 

Cheers,

Ian

 

[BrendanC]

On 19/08/2020 at 23:35, johninderby said:

Using a Concentre is more precise as it allows for perfectly centering the secondary under the focuser and making sure it appears a perfect circle as well. If the secondary is perfectly positioned then simply adjusting the primary so it is dead level with the mirror support will mean the scope is very close to being collimated and only a minor tweek is needed to reach perfect collimation.

The seconday is not supposed to be physically centered in the tube but rather optically aligned with the primary mirror. 

I know this is an old thread, but I was just reading it again and realised I don't understand this bit: 'allows for perfectly centering the secondary under the focuser' vs 'the seconday is not supposed to be physically centered in the tube'.

Aren't they contradictory? Should the secondary be centered in the tube or not? I've been thinking about this recently. There's supposed to be an offset in fast (above F5)  Newts, but I always thought that meant the secondary was centered, but the image from the Cheshire relative to the primary showed an offset. But, I've been wondering recently whether  the secondary should also be physically offset, in which case, would that produce a central image? Diagram below showing what I mean.

 

 

[jetstream]

@BrendanC there are 2 offsets - away from the focuser and down the tube. No big deal visually- but the non offset away from the focuser can cause DSC to be less accurate. Full offset-in both directions- allow us to use a bit smaller secondary - my 24" is fully offset- and has very low central obstruction.

For an imager, full offset will allow for the smallest central obstruction with the needed high fully illuminated field.

[jetstream]

@Jason D Can explain all this better than anyone else here.

[Spile]

On 09/08/2020 at 19:36, BrendanC said:

I'd be very interested to know if anyone uses any similar technique, or whether anyone tries this method out and with what degree of success.

Not for myself but I will often overlay circles on other images taken through a cap from users requesting a check.

[BrendanC]

On 13/05/2023 at 14:16, jetstream said:

@BrendanC there are 2 offsets - away from the focuser and down the tube. No big deal visually- but the non offset away from the focuser can cause DSC to be less accurate. Full offset-in both directions- allow us to use a bit smaller secondary - my 24" is fully offset- and has very low central obstruction.

For an imager, full offset will allow for the smallest central obstruction with the needed high fully illuminated field.

Thanks for this (and I was sure I was following this topic otherwise I'd have replied sooner!)

So, of the diagram I included, which should I be looking at? Option 1, 2, or something else? 

[Spile]

If the secondary is centred then you will see the dark bulge “pointing” towards the primary. That was how my Chinese reflector was set up and how I align it. It’s model E in Vic Menard’s secondary alignment method. About halfway down the page and final diagram by Jason Khadder. https://vicmenard.com/addendum-to-perspectives/

 

[jetstream]

On 15/05/2023 at 07:37, BrendanC said:

Thanks for this (and I was sure I was following this topic otherwise I'd have replied sooner!)

So, of the diagram I included, which should I be looking at? Option 1, 2, or something else? 

All you have to do is use a sight tube and a cheshire- everything is then set up automatically. If you really want to collimate for a closed light path buy the Catseye autocollimator.... I have one of those too lol

[jetstream]

On 15/05/2023 at 16:00, Spile said:

If the secondary is centred then you will see the dark bulge “pointing” towards the primary. That was how my Chinese reflector was set up and how I align it. It’s model E in Vic Menard’s secondary alignment method. About halfway down the page and final diagram by Jason Khadder. https://vicmenard.com/addendum-to-perspectives/

 

Glad you posted the link, they are both great helpful guys 👍 Ive seen some try to argue about collimation with them... one better be well versed in collimation so as not to look silly at the end of the conversation lol.

[BrendanC]

Thanks for the links but I'm afraid I started reading that page and just got lost.

I guess I'll just keep collimating with the secondary centre aligned to produce the offset pattern as before.

[jetstream]

57 minutes ago, BrendanC said:

I guess I'll just keep collimating with the secondary centre aligned to produce the offset pattern as before.

Its not something that you choose really.... offset away from the focuser is fixed from mounting and using a sight tube provides the offset down the tube. I highly recommend a sight tube cheshire combo or much better yet, the Catseye separate sight tube and cheshire.

For me, I use the sight tube, then a Glatter laser to line up the sec on the primary and then the Catseye cheshire for primary collimation. Doing so allows extremely high mag lunar/planetary viewing.I dont autocollimate anymore- 700x+ on the moon is good enough for me with my 15" f4.8 dob.

A collimation cap does not provide adequate collimation IMHO. I also have a cheap but excellent Celestron sight tube /cheshire combination tool that works great.

I wish Mr Khadder was around here still to run through things for you.

Gerry

[jetstream]

Actually I just remembered a member here who knows collimation extremely well- @Piero can run through it for us possibly.

[Louis D]

50 minutes ago, jetstream said:

For me, I use the sight tube, then a Glatter laser to line up the sec on the primary and then the Catseye cheshire for primary collimation.

Pretty much the same for me except that I use an AstroSystems laser instead of a Glatter laser and a Rigel Aline for the primary collimation.

For the big truss Dob, I use the laser while working at the back of the scope to get the primary return spot coincident with the outbound spot on the face of the secondary to get it pretty close before checking with the Aline.  I don't have 6 foot long arms to adjust the primary while peering into the Aline at the secondary cage mounted focuser.

[Jason D]

 

From visual observation perspective, there is no difference between a fully offset secondary mirror and a partially offset secondary mirror after the proper collimation steps have been followed.

The following diagram shows the difference between a fully offset secondary mirror mount (right diagram) and a partially offset secondary mirror mount (left diagram). In the partially offset secondary mirror mount, the secondary mirror is mounted centrally with respect to its stalk.

For a telescope with a secondary mirror mounted centrally on its stalk (partially offset), the primary mirror will end up getting tilted slightly towards the focuser to compensate as shown in the following diagram. The tilting will be taken care of automatically/implicitly once the proper collimation steps are followed.

The only cases where a fully offset secondary mirror mount would be needed:

1- The OTA opening is too restrictive and tilting the primary mirror towards the focuser will introduce obstructions into the light path as shown below (the obstruction is the vane clip in this case)

2- There is a corrective lens at the OTA opening. In this case, it is imperative to have the primary mirror axis being coincident with the corrective lens axis.

3- You are using DSC to get little more accuracy though the accuracy will be small -- not a major reason for the fully offset case.

Jason