Spider upgrade - orientation of vanes

[mbalkham]

Hi all,

I'm upgrading my spider vanes to the TS Optics double carbon spider (Teleskop-Express: TS-Optics Carbon Double-Spider for 295-310 mm inside tube diameter (teleskop-express.de)). The aim here is to be a bit stiffer and hopefully hold collimation better plus to tighten my diffraction spikes. I will also us copper foil on the vanes for the secondary heater rather than wires.

I've been trying to find advice on the orientation of the vanes relative to the focuser. Everything I see online points to one of the vanes being inline with the centerre of the focuser. However on my OOUK CT10 the stock spider is offset at 45 degrees to this. That makes collimation more tricky as the adjustments tilt/twist the secondary not relative to the optical axis (the two left thumbscrews are not parallel to the optical axis).

Question am I overthinking this just install in the existing holes or should I redrill the fixings in a more usual pattern relative to the focuser?

Thanks in advance

Matt   

Edited November 24, 2020 by mbalkham

[Pixies]

If you turned the secondary assembly and spiders 90deg in that photo, wouldn't that align the adjusters as you want them?

[andrew s]

Optically I don't think it will make any difference. Mechanically it might be better to avoid the focuser where the tube is already under some stress.

Regards Andrew 

[mbalkham]

In the current arrangement, thumbscrews aren't parallel to the vanes 

Edited November 24, 2020 by mbalkham

[mbalkham]

In the new one I'm not sure if 45 degree rotation works either because of the offset of the two thumbscrews? Will wait to see when it arrives! I've also reached out to TS Optics for advice.

 

Edited November 24, 2020 by mbalkham

[mbalkham]

I went with changing the orientation. Not collimated yet but I'm sure it will be easier if I'm the same plane. Looks and feels much stiffer which must be good. Hopefully test it this weekend. 

[mbalkham]

So testing tonight produced a funny set of diffraction spikes. Does this mean I haven't managed to get it square? or is this some other issue (don't think it is collimation or focus but could be tilt, focuser tube alignment, something else?). Also screenshot of ASTAP CCD Inspector for same image.

 

Edited December 10, 2020 by mbalkham

[CraigT82]

I think the split spikes are caused when the secondary isn't centered in the tube, i.e. opposing pairs of vanes don't form a straight line.

 

Looking at your images, it barely looks out... but maybe something worth checking?

 

 

Edited December 11, 2020 by CraigT82

[mbalkham]

I think you might be right. I remeasured the spacing and it looks like my 4 holes are not equally spaced around the circumference (not by much). I was not far off centre but obviously not close enough and that was pulling vanes offline a little. 

I've filled the holes and will redrill tomorrow above the last lot. Will triple check all the measurements and accuracy of the pilot holes before redrilling.

I think I'm parallel to the primary. I'll check again. Will also check the focuser alignment again. 

[Peter Drew]

I think it might have something to do with effectively having 8 vanes.    🤔

[han59]

Even with uneven spaced holes, you still should be able to center the secondary.

[Starflyer]

On 11/12/2020 at 09:41, mbalkham said:

I think you might be right. I remeasured the spacing and it looks like my 4 holes are not equally spaced around the circumference (not by much). I was not far off centre but obviously not close enough and that was pulling vanes offline a little. 

I've filled the holes and will redrill tomorrow above the last lot. Will triple check all the measurements and accuracy of the pilot holes before redrilling.

I think I'm parallel to the primary. I'll check again. Will also check the focuser alignment again. 

Did you ever get the spikes tighter / to single spikes?

I'm considering getting one of these for my CT10.

[mbalkham]

@Starflyer

I'm getting closer to getting it dialled in. My current iteration uses a stainless steel band (see photos) but I've tried just marking and drilling and couldn't get it accurate enough. Any difference in distance to the primary twists the spider (and the spikes don't align) and any slight difference in spacing around the tube misaligns the vanes with a similar result. I considered trying to spring load the attachment to ensure my bolt tightening was equal and not distorting the spider but adopted adjustment under the stars to try and dial in as best I could.

What I've learnt so far:

1. The hole alignment is critical and drilling them right is tricky! (See below for my latest thoughts). Not only to they need to be equidistant from each other but also from the primary. Any error means that the vanes don't end up in the right alignment and the diffraction spikes never perfectly align.
2. Changing the rotation by 45degrees has made collimation slightly easier as I suspected above with alignment of the adjustment screws with the optical arrangement.
3. My mirror isn't equidistant from the end of my tube) or more likely the end of my tube isn't square) so I can't measure from the end of the tube for anything critical
4. A wide band of paper/thin card is prob the best way to mark 4 equidistant holes. Take the band off and fold in half twice and mark those lines. The width of the band means it will be parallel to the tube
5. My current solution was to clamp a (actually 2) stainless steel hose clamps over the top of the marked paper band and mark the 4 equidistant lines and the end of the hose clamps. Then to remove the hose clamps, remeasure/check that the holes are equidistant and punch and drill the 4 spider holes in the centre of the band. When you tighten the clamp it will square itself to the tube was my theory
6. I aligned one of the holes placing the bolt through the clamp and OTA and then filing out the remaining 3 holes to perfectly align.
7. I have filled the old holes with thickened epoxy with black dye. They look ok (from a distance!)
8. Measure the spider to check it is roughly centred. Tighten the for bolts of the spider finger tight. 

I did the collimation after installation as per usual (catseye and glatter combo for me). I did also check that the focuser was square and centred using a hole opposite the focuser drilled with he paper trick. After that I used the laser to check and adjust focuser alignment.

Under the sky I honed my centring of the spider. If it's not dead centre it distorts the vanes and the diffraction spikes aren't right (they diverge away from the star). My Rho image below shows some residual diffraction spike divergence (and was before I installed my steel band). I've improved it a little more since I took that image (see the other test image of Rigel I think).

My process for adjusting under the stars was:

1. Point at a bright star, focus and take an image
2. Adjust the spider screws towards the diverging diffraction spike. If your sensor is aligned to the spider this should be easier. If at 45 degrees I suspect you'll need to adjust all four screws simultaneously (and refocus between adjustments)
3. In my case i could adjust left and right without affecting the focus too much but up and down requires a refocus (as it moves the secondary towards/away from the sensor.
4. After a bit of fiddling I got to a point I was relatively happy (I do still want to make some final tweaks). This adjustment is likely to have affected your collimation so recollimate at this point.

All in all a lot of hassle! I'm getting there. I recon one more night under the stars and I'll have it pretty dialled in.

Good luck if you do decide to make the mod! I recon sticking with the factory spider holes might be a lot easier. Then again you might highlight some issues you never thought you had when you put the new spider in! Def holds collimation better and I think my diffraction spikes will be better (eventually!).

 

 

Edited June 7, 2021 by mbalkham

[nfotis]

Maybe curved vanes could mitigate the intense diffraction effect?

Like some RASA users doing this trick:

https://www.cloudynights.com/topic/691031-very-simple-rasa-cable-management/

N.F.

 

[Starflyer]

On 06/06/2021 at 23:21, mbalkham said:

@Starflyer

I'm getting closer to getting it dialled in. My current iteration uses a stainless steel band (see photos) but I've tried just marking and drilling and couldn't get it accurate enough. Any difference in distance to the primary twists the spider (and the spikes don't align) and any slight difference in spacing around the tube misaligns the vanes with a similar result. I considered trying to spring load the attachment to ensure my bolt tightening was equal and not distorting the spider but adopted adjustment under the stars to try and dial in as best I could.

What I've learnt so far:

1. The hole alignment is critical and drilling them right is tricky! (See below for my latest thoughts). Not only to they need to be equidistant from each other but also from the primary. Any error means that the vanes don't end up in the right alignment and the diffraction spikes never perfectly align.
2. Changing the rotation by 45degrees has made collimation slightly easier as I suspected above with alignment of the adjustment screws with the optical arrangement.
3. My mirror isn't equidistant from the end of my tube) or more likely the end of my tube isn't square) so I can't measure from the end of the tube for anything critical
4. A wide band of paper/thin card is prob the best way to mark 4 equidistant holes. Take the band off and fold in half twice and mark those lines. The width of the band means it will be parallel to the tube
5. My current solution was to clamp a (actually 2) stainless steel hose clamps over the top of the marked paper band and mark the 4 equidistant lines and the end of the hose clamps. Then to remove the hose clamps, remeasure/check that the holes are equidistant and punch and drill the 4 spider holes in the centre of the band. When you tighten the clamp it will square itself to the tube was my theory
6. I aligned one of the holes placing the bolt through the clamp and OTA and then filing out the remaining 3 holes to perfectly align.
7. I have filled the old holes with thickened epoxy with black dye. They look ok (from a distance!)
8. Measure the spider to check it is roughly centred. Tighten the for bolts of the spider finger tight. 

I did the collimation after installation as per usual (catseye and glatter combo for me). I did also check that the focuser was square and centred using a hole opposite the focuser drilled with he paper trick. After that I used the laser to check and adjust focuser alignment.

Under the sky I honed my centring of the spider. If it's not dead centre it distorts the vanes and the diffraction spikes aren't right (they diverge away from the star). My Rho image below shows some residual diffraction spike divergence (and was before I installed my steel band). I've improved it a little more since I took that image (see the other test image of Rigel I think).

My process for adjusting under the stars was:

1. Point at a bright star, focus and take an image
2. Adjust the spider screws towards the diverging diffraction spike. If your sensor is aligned to the spider this should be easier. If at 45 degrees I suspect you'll need to adjust all four screws simultaneously (and refocus between adjustments)
3. In my case i could adjust left and right without affecting the focus too much but up and down requires a refocus (as it moves the secondary towards/away from the sensor.
4. After a bit of fiddling I got to a point I was relatively happy (I do still want to make some final tweaks). This adjustment is likely to have affected your collimation so recollimate at this point.

All in all a lot of hassle! I'm getting there. I recon one more night under the stars and I'll have it pretty dialled in.

Good luck if you do decide to make the mod! I recon sticking with the factory spider holes might be a lot easier. Then again you might highlight some issues you never thought you had when you put the new spider in! Def holds collimation better and I think my diffraction spikes will be better (eventually!).

 

 

Thanks for the very in-depth reply.  I was planning to use the existing holes, I prefer the spikes at 45° to horizontal anyway.

You seem to have it very nearly dialled in, I'd be happy with that.

I've seen CNC machined spiders and thought about one of these but can't find one to fit the CT10. Zero adjustment, so it should be exactly centered, but if you need to adjust you're out of luck.

Thanks again,

Ian