Newt Guru's: Offset Secondary woes

[MrB (Simon)]

I recently bought a fast Newtonian astrograph and while I have been able to get collimation pretty close, I could never nail it. I have tried to collimate it so many times over the last couple of months it has almost driven me insane
I just could not work out what I was doing wrong.

Tonight I gave it another go and again failed.
While talking to a friend about it, an image flashed in my head of the spider and it's offset, so I had a good look and I had the sudden realisation that this OTA has been built with the spider's offset working TOWARD the focuser, rather than away!!!
The offset is approx. 5mm (154mm f3.9 with a 66mm secondary), so 10mm from where it should be.

So, before I drill 8 new holes in the nice glossy Carbon Fibre tube to rotate the spider 180 deg, my questions are.....
- How much of an effect will this incorrect offset(10mm) have on my being able to collimate properly?
- Could it also explain the halo's around stars in my images if the collimation is close, but not perfect?

It's a shame it wasn't installed correctly in the first place as the CF tube is now going to look like swiss cheese

[dave brock]

Originally Posted by MrB

So, before I drill 8 new holes in the nice glossy Carbon Fibre tube to rotate the spider 180 deg...

Hi Simon.
You haven't said what make/model scope it is so I can't google to see the set up but I'm not sure why you need to drill 8 more holes. Are there no adjustments at all to move the secondary back? If you turn the spider 180° won't at least 4 of the holes still line up? I'd have thought though that there would be enough flex in the vanes to reuse the existing holes. They may just no longer be exactly 90° anymore.

Dave

[MrB (Simon)]

Originally Posted by dave brock

Hi Simon.
You haven't said what make/model scope it is so I can't google to see the set up but I'm not sure why you need to drill 8 more holes. Are there no adjustments at all to move the secondary back? If you turn the spider 180° won't at least 4 of the holes still line up? I'd have thought though that there would be enough flex in the vanes to reuse the existing holes. They may just no longer be exactly 90° anymore.

Dave

Hi Dave, thanks for the reply. The scope is an ATM job(not by me), and done rather well with the exception of the misplaced spider.

The primary-secondary offset is correct, approx. 5mm toward the primary, so it doesn't need moving back.

Sadly no, none of the holes will line up with the spider when rotated 180 degree's.
The spider is by Protostar http://www.fpi-protostar.com/s4vmnts.htm, and to effect the offset, two vanes are ~65mm long and two are ~60mm long. See quick CAD sketch below.

I would prefer to keep the vanes at 90deg with relation to each other.
I have marked the hole locations on the tube, about to start drilling

EDIT: Updated sketch, black spider is current position, magenta coloured dashed spider is where it needs to be.

[DavidU (Dave)]

Simon, in years gone past I have found that the secondary needs to be off set away from the focuser (as you have said) but also it needs to be moved towards the primary as well. If you draw the light cone to scale within a drawing of the OTA you will easly be able to measure these 2 off sets.
I also found that standard collimation techniques did not work with a very fast astrograph.
If you could give me all the measurements I can have a look for you.
Dave

[Tandum (Robin)]

That's a bugger Simon. I now remember the protostar spider on Peters newt had the offset in the vanes where as this astrosystems spider I just received has the offset in the holder as does the GSO holders I've seen.

I'd be inclined to try just reversing the spider to see what the images look like before drilling new holes.

[MrB (Simon)]

Originally Posted by DavidU

Simon, in years gone past I have found that the secondary needs to be off set away from the focuser (as you have said) but also it needs to be moved towards the primary as well. If you draw the light cone to scale within a drawing of the OTA you will easly be able to measure these 2 off sets.
I also found that standard collimation techniques did not work with a very fast astrograph.
If you could give me all the measurements I can have a look for you.
Dave

Thanks David, the Primary-Secondary offset was done correctly by the original builder, was only the Secondary-Focuser offset that was incorrect.

I have drilled the new holes, which also allowed me to correct one vane that was a bit askew.
Now to reposition the primary so I can achieve focus(there's an extra 8.5mm of light path, the Feathertouch focuser only has ~20mm travel) and then have another go at collimating with the Catseye's.

Fingers Crossed!

[MrB (Simon)]

I will note that I'm not overly impressed with the quality of the Protostar spider. Two of the vanes are not perpendicular, so when looking down the tube from the primary end, they appear to be twice as thick as they should.
Sadly this cannot be fixed

[MrB (Simon)]

While I had the primary+cell removed, thought I would check the centre spot.... yes, It's a long way out

[clive milne]

Originally Posted by MrB

- How much of an effect will this incorrect offset(10mm) have on my being able to collimate properly?

Zero effect on collimation but a significant effect on field illumination.

- Could it also explain the halo's around stars in my images if the collimation is close, but not perfect?

No.

[MrB (Simon)]

OK, we have partial success.
Collimation was not easier but I could get it better collimated. Still not right but I can tweak it some more.

Images below are Before on the left, after on the right.
Before was shot at native focal length(600mm), while after was shot with 0.73x reducer. I have reduced the 'before' crop to match, so there's a definite improvement!
Please note the 'before' shot was 2 minute exposure while the 'after' shot is only 5 seconds.

Also, the halo's are now gone! Happy

[Tandum (Robin)]

shot 1 has eggy stars on the left. I guess it has no correction. In that case I'd expect eggy stars on the right?

Something is bent there ...

[MrB (Simon)]

Originally Posted by Tandum

shot 1 has eggy stars on the left. I guess it has no correction. In that case I'd expect eggy stars on the right?

Something is bent there ...

Hi Robin. That 'before' shot was thru an MPCC!
Yes there are still issues with this scope. In the fullframe version of the 'after' pic there is coma apparent in the left side corners, top and bottom, and also the top right corner, but very little in the bottom right corner.

I think it's the focuser now as I still can't nail collimation. I can get it very close, much closer than before, but no amount of fiddling will nail it. Still got some sleuthing to do.

[Jason D (Jason)]

Originally Posted by MrB

While I had the primary+cell removed, thought I would check the centre spot.... yes, It's a long way out

How far off? This could definitely impact the quality of your view.

Originally Posted by clive milne

Zero effect on collimation but a significant effect on field illumination.

Clive, I disagree. Moving the secondary mirror by 5mm towards or away from the focuser has little to no impact on the quality of the image including field illumination. This can’t be the source of the OP’s issue. There are three potential impacts when moving the secondary mirror towards or away from the focuser:
1- Moving the focal plane within the focuser. Some EPs might not reach focus.
2- Accuracy of DSC (if present) will be impacted as the angle between the optical and OTA axes change
3- If the OTA opening is tight or if the OTA has baffles then vignetting might be a possibility

Originally Posted by MrB

I think it's the focuser now as I still can't nail collimation. I can get it very close, much closer than before, but no amount of fiddling will nail it. Still got some sleuthing to do.

What makes you believe you have a collimation issue? It is the quality of your image or is it based on the reflections you see via your Catseye collimation tools? Which Catseye tools do you have?

Jason

[MrB (Simon)]

Hi Ghassan, many thanks for posting, your contributions to newtonian collimation are much appreciated! I have read your threads on CN and posts here and on Stargazerslounge. I have to admit I am still digesting your postings on CN!

How far off? This could definitely impact the quality of your view.

I measured the placement of the original spot(a dodgy hand-cut lopsided(not equilateral) triangle) with a steel rule with 0.5mm graduations gently placed over the mirror. The rule is double sided and I used the reflection of the scale on the backside of the rule(lining them up with the scale on the topside) to eliminate parallax error due to the mirrors sagitta.
The rule was randomly placed (though it was checked to be on diameter) and I measured the spot as being out ~0.5mm, I then rotated the cell(incl. mirror) 90 degrees and measured again, this time it was 1.5mm out. I could've continued around the mirror to find the maximum but that would've been pointless.
On a 154mm mirror, this (and the wonky triangle) was enough for me to remove the old spot and place a new 'hotspot'.

2- Accuracy of DSC (if present) will be impacted as the angle between the optical and OTA axes change
3- If the OTA opening is tight or if the OTA has baffles then vignetting might be a possibility.

Pardon my ignorance, what is DSC? (EDIT:Nevermind, found it in Catseye/Pensack's guide, guess I must have ignored it because I don't have Digital Setting Circles )
Yes there was vignetting by the tube when the offset was toward the focuser.
The tube is 190mm inside diameter with a 154mm mirror, the focuser centre is 180mm from the front of the tube.

What makes you believe you have a collimation issue? It is the quality of your image or is it based on the reflections you see via your Catseye collimation tools? Which Catseye tools do you have?

Based on not being able to get the reflections lined up no matter how much I fiddle with the mirror. Also because some corners of my images show more coma than other corners, so the focuser is possibly not perpendicular to the tube, tho maybe this could be due to collimation not being spot-on yet?
If I remove the autocollimator from the focuser and hold it by hand above the focuser drawtube and carefully moving the autocollimator around, I can get all reflections to line up perfectly thru the on and off-axis pupil holes.
I think some of my issues are due to the autocollimator being about 85mm inside the focal plane(reflection '2' is grossly enlarged), I am machining a spacer today to move the autocollimator out to the focal plane.
My tools are home-made on my lathe, they are equivalent to a Blackcat XL and an Infinity XLK with your offset viewing pupil.
Their accuracy is fine, I have tested the autocollimator as you described in a post on Stargazers Lounge(Here). There is zero image shift of the hotspot reflections when I rotate the autocollimator in the focuser.

[Jason D (Jason)]

Originally Posted by MrB

Hi Ghassan, many thanks for posting, your contributions to newtonian collimation are much appreciated! I have read your threads on CN and posts here and on Stargazerslounge. I have to admit I am still digesting your postings on CN!

Wow… Thanks…

…and I measured the spot as being out ~0.5mm, I then rotated the cell(incl. mirror) 90 degrees and measured again, this time it was 1.5mm out. I could've continued around the mirror to find the maximum but that would've been pointless.

On a 154mm mirror, this (and the wonky triangle) was enough for me to remove the old spot and place a new 'hotspot'.

Glad that you fixed that issue. I would not have felt comfortable with that much error in the original placement.

Yes there was vignetting by the tube when the offset was toward the focuser.

The tube is 190mm inside diameter with a 154mm mirror, the focuser centre is 180mm from the front of the tube.

Vegetating does not seem to be a possible issue.

If I remove the autocollimator from the focuser and hold it by hand above the focuser drawtube and carefully moving the autocollimator around, I can get all reflections to line up perfectly thru the on and off-axis pupil holes.

I think some of my issues are due to the autocollimator being about 85mm inside the focal plane(reflection '2' is grossly enlarged), I am machining a spacer today to move the autocollimator out to the focal plane.

Sounds you have a good understanding of the dual-pupil autocollimator functionality. Yep, when you are below the focal plane by that much then reflection 2 will become much larger and aligning reflection will be more challenging. Adding an extension tube as you are in the processing of doing will ease aligning reflections and add accuracy.

My tools are home-made on my lathe, they are equivalent to a Blackcat XL and an Infinity XLK with your offset viewing pupil.

Their accuracy is fine, I have tested the autocollimator as you described in a post on Stargazers Lounge(Here). There is zero image shift of the hotspot reflections when I rotate the autocollimator in the focuser.

If your home-made autocollimator passes that stringent criteria then you have build a quality autocollimator.

Jason