Faulty cheshire?

[pipnina]

I noticed that my Skywatcher 250PX (f4.7 newt) was showing some astigmatism at the eyepiece. I knew the secondary mirror was not perfectly collimated when I bought it and I have never touched it until today.

I brought the cheshire out and started twisting the allen keys and got the secondary what looked like absolutely perfect. Took out the cheshire to see and it looked about right, but after puting it back in it seemed off again.

I realized after making sure the cheshire was flat against the focuser, the if I turned the cheshire in the focuser it would move the center of the cross around in a small circle.

I have been looking at replacing it anyway since it's a 1.25" and both of my scopes are now 2" (so the cheshire can't see the edge of the secondary mirror). I can't estimate how large the error is, but it was enough to escape the central dot and ring on the primary and leave a visible gap (with one point on the circle being the point I had collimated to).

Thoughts?

[cjdawson]

Can your Cheshire be collimated itself as well?

[pipnina]

28 minutes ago, cjdawson said:

Can your Cheshire be collimated itself as well?

There don't seem to be any screws to adjust as far as I can see... But this is the one I'm using: https://www.firstlightoptics.com/other-collimation-tools/cheshire-collimating-eyepiece.html

[Charic]

I didn't think you could break a Cheshire tool.

Having said that, your one is fitted with  a wire crosshair. If that's touched in anyway, that could cause an offset, as can a poor sloppy  fitting inside the focuser tube!  That can be fixed with a single wrap of electrical tape or similar, but no overlap, so cut to fit end-to-end of the tape. 

I  think its possible on some units to remove the inner ring that holds the wire in place, and then refit a new wire, applying enough tension to create the final centred cross hair.

Or just buy the new one as planned and see how that fares.

[MarsG76]

Are you sure that it's not a unstable focuser tube?

 

[pipnina]

44 minutes ago, Charic said:

I didn't think you could break a Cheshire tool.

Having said that, your one is fitted with  a wire crosshair. If that's touched in anyway, that could cause an offset, as can a poor sloppy  fitting inside the focuser tube!  That can be fixed with a single wrap of electrical tape or similar, but no overlap, so cut to fit end-to-end of the tape. 

I  think its possible on some units to remove the inner ring that holds the wire in place, and then refit a new wire, applying enough tension to create the final centred cross hair.

Or just buy the new one as planned and see how that fares.

I might have touched those wires a few times... That might have set it off...

23 minutes ago, MarsG76 said:

Are you sure that it's not a unstable focuser tube?

 

I have noticed that tightening the focuser screws to lock it in place does move it a bit. But even when I make sure it is flat against the focuser and unlocked it seems to form a small circle.

I took a photo of the cheshire about 180 degrees from its best. I wonder if the secondary is still poorly collimated as the spider veins do not point to the center spot themselves (the vertical set anyway, middle set seem bang on) and there is a reflection visible on the right side of the secondary shadow.

[MarsG76]

1 minute ago, pipnina said:

spider veins do not point to the center spot

Spider veins do not necessarily cross that the center of the secondary when properly collimated... the photo does look a bit out,,, but to be sure, illuminate the primary center ring and see if it appears inside the chesire donut. If so, than it's most likley very close to collimation.

 

 

[pipnina]

6 minutes ago, MarsG76 said:

Spider veins do not necessarily cross that the center of the secondary when properly collimated... the photo does look a bit out,,, but to be sure, illuminate the primary center ring and see if it appears inside the chesire donut. If so, than it's most likley very close to collimation.

I used my phone torch to illuminate the secondary from in front of the tube. I could see that it created a sort of "halo" effect (circular) which did look offset from the whole dark spot but all sides of this circle were visible. maybe 90% of the shadow's size. I couldn't take a photo because my phone was in my hand though...

Maybe I should simply attempt a star-test at the next opportunity to see if it still produces usable results in the real world? After all that's what matters. I do know the symptom I was hoping to eliminate and imagine I can test for it again.

[MarsG76]

13 minutes ago, pipnina said:

I used my phone torch to illuminate the secondary from in front of the tube. I could see that it created a sort of "halo" effect (circular) which did look offset from the whole dark spot but all sides of this circle were visible. maybe 90% of the shadow's size. I couldn't take a photo because my phone was in my hand though...

Maybe I should simply attempt a star-test at the next opportunity to see if it still produces usable results in the real world? After all that's what matters. I do know the symptom I was hoping to eliminate and imagine I can test for it again.

Star test is good, but first try to get the primary center marker in the middle of the Cheshire dark spot.

 

[Ricochet]

33 minutes ago, pipnina said:

I might have touched those wires a few times... That might have set it off...

I have noticed that tightening the focuser screws to lock it in place does move it a bit. But even when I make sure it is flat against the focuser and unlocked it seems to form a small circle.

I took a photo of the cheshire about 180 degrees from its best. I wonder if the secondary is still poorly collimated as the spider veins do not point to the center spot themselves (the vertical set anyway, middle set seem bang on) and there is a reflection visible on the right side of the secondary shadow.

From your photo it looks like 

1. The "vertical" crosshair wire isn't straight
2. The measurement across the horizontal crosshair from the tube wall to the point where the wires cross is shorter on the left hand side. 

If it really is like that it would explain the effect you are seeing. However, when you rotate the Cheshire don't worry about where it points while you rotate it, only worry about where it points when you are no longer touching it. As you turn it there could easily be movement in the clamp, focuser slop or even flex in the OTA tube that will throw the crosshairs off centre. 

[MarsG76]

Have a look at these:

http://www.astro-baby.com/astrobaby/help/collimation-guide-newtonian-reflector/

https://nimax-img.de/Produktdownloads/13770_6_Collimation.pdf

 

Might help.

[MarsG76]

this too.