collimation question

[geordie85]

Hi all

I'm hoping someone can shed some light on my problem. I've recently taken apart, flocked and put everything back on my skywatcher 200p. I then collimated my telescope and went out on the first clear night.

Now I'm having trouble achieving perfect focus and when I check my collimation with an out of focus star on the live view of my dslr, when it's in the center it looks perfectly collimated but if it's on the edge of the field of view it looks like the collimation is way off.

Could this be because the focuser is not aligned properly? Or maybe it's because I'm also using a coma corrector? Possibly it's because I've never checked collimation with an out of focus star and this is normal? Or maybe it's something else that I haven't thought of?

Any help, thoughts or ideas are greatly appreciated.

Thanks

[-Aldebaran-]

Have you checked all collimation points with your own eyes and they're all centred and lined up? We're you using a Cheshire tube and cap or laser collimation tool?

What exactly dis you take apart?

[geordie85]

I used a Cheshire collimator and a collimator cap And checked everything. It all seems to be perfectly aligned. When I look through the Cheshire I've got a nice round circle with the target looking bit centered.

The only parts I didn't take apart was the mirror out of its cell and I left the focuser in tact but I did remove it from the OTA.

[-Aldebaran-]

What was the purpose of removing the focuser from the OTA? The focuser has 4 screws so I shouldn't think it would be misaligned, the only other things I can think of is that maybe you may have overtightened the screws on the primary or secondary mirror and caused a slight "pinching" which could cause astigmatism in the outer fov or its the eyepiece itself, but I think it's onlyore noticeable in wider field eyepieces, had you not noticed it before? And what eyepieces do you use?

I'm sure someone with a bit more knowledge than I will come to your aid soon.

[Stu]

What exactly do you mean by 'having trouble achieving perfect focus' ?

If the collimation is correct when the star is centered then it should be ok, it will always look off centre towards the edge.

[faulksy]

do you mean you cant achieve focus due to not enough travel in the focuser. maybe when you colimated you sent the mirror down the tube to far. this is a easy mistake, we all colimate by tightening the mirror. also your not trying to colimate with your coma corector in situ ?

[NGC 1502]

 when it's in the center it looks perfectly collimated but if it's on the edge of the field of view it looks like the collimation is way off.

That's as it should be and is perfectly ok.   When I'm checking collimation with an out of focus star, once it all looks good with the star in the centre, I often deliberately put the star towards each edge to see if the diffraction rings look skewed by the same amount in all directions - just to make that clear, I put the defocused star to the left edge, the right edge, top and bottom edges, all is fine if the rings are skewed by a similar amount.  Then back to the centre as a final check for concentric rings.

But not sure what you mean by "having trouble achieving perfect focus".

Regards, Ed.

[Scooot]

If your scope is collimated correctly, when you view through the coma corrector the scope should look collimated near the edges. Without the coma corrector it won't because of coma. If I understand you correctly and you're not seeing a round defraction pattern near the edges with the coma corrector in place, but you are when the image is centred, I'd say your coma corrector isn't working properly. What one are you using? Is its spacing correct?

[geordie85]

@aldebaran I removed the focuser to flock the while scope. I also didn't touch the screws on the mirror cell.

@bbigsumorian. The first chance I got I wanted to test my scope so pointed it at jupiter. I was only viewing via the live view of my camera. Usually I can clearly see the bands but they seemed rather fuzzy. It may actually be due to bad seeing and jupiter being rather low at the time. I've also never noticed that at the edge of view it would look out of collimation on an unfocused star.

@Faulksy. No I have plenty of travel in both directions. I was only using the coma corrector whilst testing looking at an unfocused star.

It's a clear night tonight so will be able to get a proper test on how things go. Hopefully I'll just have been too premature and everything will be fine when imaging.

[Stu]

@bbigsumorian. The first chance I got I wanted to test my scope so pointed it at jupiter. I was only viewing via the live view of my camera. Usually I can clearly see the bands but they seemed rather fuzzy. It may actually be due to bad seeing and jupiter being rather low at the time. I've also never noticed that at the edge of view it would look out of collimation on an unfocused star.

Hi,

That was the reason I asked what exactly you meant by not able to get perfect focus.

I've been observing Jupiter tonight and the seeing conditions are pretty bad. I had a period of around 10 mins later on when there was decent detail visible but outside that it has been poor.

I would not leap to any conclusions, and see how things are when conditions improve

[jetstream]

If you watch a defocused star travel across the FOV in a fast Newt the center "donut" (shadow) will move to the side as a result of coma, the coma free field in a fastish newt is fairly small-my f4.8 shows like this. This can be the folly of using stars to test collimation if unaware of the effect, on a planet (in focus) the result is a "smeared" image ( a liitle) as it moves out of the coma free area.

The real way to star test is at very high mag, which will open up the Airy pattern showing the disk. Cooling is also a huge part of the telescope picture and can cause all sorts of issues.

[laser_jock99]

Hi all

I'm hoping someone can shed some light on my problem. I've recently taken apart, flocked and put everything back on my skywatcher 200p.

Thanks

You broke Rule No. 1 and removed the spider/secondary mirror! A lot of 'collimation woe threads' seem to start with the removal of the secondary mirror. The secondary is best left where it is and only given minor tweaks IMHO.