Is my 9.25 SCT collimated?

[Dave Smith]

Below are photos showing inside and outside focus images of Arcturus taken with a SPC900C webcam and the second pair is through a x2 Barlow. I have also put an image of Saturn taken with the x2 Barlow last night.

Is my scope collimated correctly and should I not get more detail?

Dave

[beamish]

Looks a tad eccentric to me Dave, needs a tweak I'd say

[dweller25]

It is SLIGHTLY off to the left.

[Paxo]

IMO It does look slightly out Dave (shows up better in the barlowed image). Here is an interesting article on the effects of mis-collimation on the image produced.

Thierry Legault - The collimation

Steve

[Dave Smith]

Thanks for the replies. The problem I have is that now I have processed these images, I can see that the barlowed images are not symmetrical, but not when viewed live. I guess that is down to experience.

Could it be the Barlow itself that causes the asymmetry or is it showing up the need to collimate the scope?

Must try again tonight.

Dave

[ollypenrice]

I just spent some time with a professional and our SCT on the subject of collimation. The first thing he stressed (and demonstrated) is that the star test has to be performed dead centrally because the coma is pretty huge on standard SCTs and being even slightly off axis it is this that you see, so confusing the result. So, first, use a crosshair to centre the star. Bang on axis our scope was, by chance, spot on but even just off axis the coma begins to distort the diffraction rings. I was amazed by how soon this effect appeared.

Olly

[James4]

Dave, I'm also looking to collimate my 9.25 for the first time. Problem is the seeing is bad - at high power the diffraction rings are being distorted all over the place.

I did notice this: I need to collimate with just an eyepiece in teh focuser if I can.

The Diagonal or a reducer or anything else between your eyepiece and the scope can knock you off-axis. And as Olly says - things get eccentric pretty quick.

I had my f/6.3 reducer on - defocussed the Peiades at low power and everything was eccentric except for Merope which was dead centre and showed good symmetry. PS the Merope Nebula was obvious. I'm wating for some steady seeing to get some in focus diffraction rings at high power - per Theirry Legault's instructions. Good luck

[James4]

PS I've had an Explore Scientific 152 Achromat for a few months (that's probably only about 4 outings) and cannot get diffraction rings. Out of fous is just a smooth doughnut .... not sure why this is?

[Freddie]

MetaGuide (free download) has a nice function that stacks images on the fly to allow you to get a better image rather than the constantly changing "live" image.

[Plantins]

If you use a diagonal when viewing always collimate with the diagonal in place.

[Adrian]

I just spent some time with a professional and our SCT on the subject of collimation. The first thing he stressed (and demonstrated) is that the star test has to be performed dead centrally because the coma is pretty huge on standard SCTs and being even slightly off axis it is this that you see, so confusing the result. So, first, use a crosshair to centre the star. Bang on axis our scope was, by chance, spot on but even just off axis the coma begins to distort the diffraction rings. I was amazed by how soon this effect appeared.

Olly

When I change eyepieces, or especially when I change from 2" to the 1.25" adaptor, the object I am viewing is often off centre after the change. That would seem to indicate that they have different optical axes - should I therefore (in theory) recollimate for each eyepiece in order to get the best. [my adaptors/barlows do not have compression rings]

I ask because this will also happen if I am webcam imaging and add the barlow in if the seeing is good enough?

It would also mean that even if the scope is collimated, when I switch to the webcam, or even if I turn the webcam in the holder in order to line up the planet, I would need to recollimate.