The Importance of Being Collimated

Apologies to all Oscar Wilde fans for the cheesy title, but i wanted to show how important collimation of a scope is.

As some of you may be aware from discussions in the astrophotography thread, I was having some issues with my focal reducer recently - or at least I thought it was the focal reducer.

I downloaded the demo of CCDInspector - a program that analyses your image and provides information on the entire optical-camera system. One of the things it can do is create a 3D graphical representation of the curvature of the image.

Here's a screengrab of the results from the CCDInspector program of an image of M35 through my 12" with focal reducer from a few weeks back.



To put it in context, the "perfect" result would be a completely flat plane. In this case, it's distinctly curved.

Following some discussions with Dave Lillis, I collimated the scope as best I could with a 6mm eyepiece. The collimation was not out by much anyway - I only needed to tweak the collimation by fractions of a single turn. I then took another image of M35. This image was then run through CCD Inspector and here's the result:



Whilst the collimation is still out by a tiny fraction, the results really surprised me. I always knew collimation was important, especially for imaging planets, but never realised just how important it really is until now. I hope this lesson I learnt the hard way is useful to others here and shows that what appears to be a small error in collimation can have a significant effect on image quality.

The question now remains: are the errors that are left still serious?

Regards,

Thats pretty neat. You should put the link up here to for the CCD Inspector Mike.

The difference between the 2 graphs for such a tiny turn is remarkable.

Thats pretty neat. You should put the link up here to for the CCD Inspector Mike.

The difference between the 2 graphs for such a tiny turn is remarkable.

Here ya go: www.ccdware.com/products/ccdinspector/

Well Michael, as you know, I had a similar problem with the canon a few years back where the stars would flare badly towards one edge of the field of view, I found at the time it was a collimation issue, so I'm glad it was the same solvable issue here.
BTW, I collimate with a 3x barlow and toucam and a star like regulus or vega, so its bang on perfect, my particular barlow does not mislead the collimation as it is still perfect without it.

One thing about the second curve, my reading of that is that you will not get an evenly illuminated field, peaking at the center when perfectly collimated, so taking flats will be particularly important to compensate for this.
I'd bet a lumicon giant easy guiderwith its 3" diameter would help reduce this somewhat, the question is how bad is it really, how much dimming does that graph represent in the real image.

If I get a few hours free and a clear sky, I'll try what you did with the canon and GEG and since I have exactly the same scope, its a fair comparison.

Here's a 3d graphical representation of an image of M35 from this evening with the camera and the 12" at f10.



As you can see here, it's pretty flat. Would be nice to get the scope working at f/6 with a similarly flat field.

The short f/ratio of f/4 that I use on a large Wynne Newtonian has to be checked out each time for collimation. I disagree with a lot of users that laser collimation will collimate your telescope. It does not do it precisely. I use a Cheshire eyepiece each time that is incredibly precise and this results in excellent star images. I also have been using CCD Inspector which is also incredibly precise. Although you might think you have collimated and then find that CCD Inspector shows unexpected discrepancies.

Longer f/ratios of around f/8 to f/11 are much easier to collimate.

I have found that excellent collimation will provide a increase in limiting magnitude, higher resolution, etc. It really is important to check it out on a routine basis. Everytime I go to the telescope, I may have to tweak it with a Cheshire eyepiece.

Most astronomers underestimate collimation or avoid the work to put into it or are ignorant about the effects of misalignment of there scopes.

I know from my experience using this large Wynne Newtonian that misalignment carries a toll, for example, an off axis mixture of two pure abberrations, coma and astigmatism. Telescopes have such a constricted field of view that coma caused by the objective is generally not visible unless the optics are misaligned and imaged at high magnification. Unfortunately, coma and astigmatism in composite form are common errors in poorly collimated reflectors and catadioptrics. Alignment is probably the single greatest contributor to the underserved shabby reputation of relectors. Most owners of a 10 inch reflectors are actually deluded into thinking that it effectively works at full aperture. This is not true, is not effectively using it at full aperture, in particular with a severe misalignment reduces the low spatial frequency contrast to that of a 2.5 inch telescope from 10 inches. Even correcting the misalignment until the axis is tilted by twice the tolerance only improves the contrast so this 10 inch mirror acts like one of half its aperture. This is like an instrument as if it were for a 50% or 60% obstructed aperture. No one would stand for such huge obstructions, but many telescope owners casually accept misalignments of this magnitude.

The filtration of even a slightly misaligned telescope is enough to severely degrade images. Collimation is important to carry out. You can hardly do worse than an unaligned instrument, and the potential improvements obtained with only a small effort can be profound.

Eamonn A