Ok you Dob people....

What's the nature of the "smearing"? Is it directional, or just bloated stars?

If it's directional, is the direction radial from the center or do they all point the same way across the field?

When you were collimating, did the offset of the secondary shadow appear in the same direction on both sides of focus? If it flipped then I think that indicates that your secondary is off-center.

(FWIW, the seeing up here in Louth last night was pretty variable, but not terrible. I recording Pickering 5 to 7.)

-- Jeff.

There are a number of possible reasons you are not getting sharp images...

First off - REGARDLESS of the f/ratio, you should always get tack-sharp images at the centre of the field of view with a Newtonian. If you are not getting pin-point star images in the very centre then you have a problem that is probably not down to collimation. So... point your scope at polaris (it won't run away on you during the test!) and get it into the centre of the field of view. Use a moderately high magnification.

Is the star image sharp at best focus or soft and mushy?

Is the star image perfectly round or is it a strange shape or even spiky?

Using high magnification (the highest you've got) defocus the star slightly by racking the focuser outwards until you can see it as a series of very fine rings. You only need to defocus by a tiny amount to see the rings, but the seeing must be very steady. Is this ring pattern perfectly circular or are they oblong or even slightly triangular? Are the rings perfectly concentric or slightly eccentric?

If the rings are not *perfectly* circular then the mirror may be being pinched in its cell.

If the rings are not *perfectly* concentric then you have a collimation problem... dispite your best efforts!

Next, remembering what the ring pattern looks like outside focus, rack the focuser in until you can see the same pattern of rings just inside focus. Are they *exactly* the same or is the brightness of some of the rings slightly different?

If the ring patterns inside and outside of focus are not *exactly* the same then you may have a problem with the figure (correction) of the mirror, and I can suggest more tests to be sure.

This is called the star test and you should be able to find a whole wealth of info about how to interpret it on the web. Google "telescope star test" or similar.

As someone else already mentioned, going from f/5 to f/4.5 increases the off-axis coma considerably. At f/4 (like my 20") it's horrible! But then I'm looking at faint fuzzies anyway so it doesn't really matter. That said, the 20" f/4 performs very well for planetary and double star work when I keep the object in the exact centre of the field.

Try out these suggestions over the next few clear nights and report back.

Phil.

Try out these suggestions over the next few clear nights and report back.

At this rate it may be this time next year before we hear from Eamonn!

Hi Eamonn,
Sorry to hear of your scope troubles, there is nothing worse then been underwhelmed by a scopes optics.
When I got the 20" last year, I had resigned myself to the fact that its a dedicated deepsky scope and was fully prepared for horrible planetary images, but that scope has consistantly shown me the best planetry images I have ever seen, it completely floors my 12", so I no longer buy the bad seeing argument with >8" aperture scopes.
Phil recommended some excellent tests, you really need to do high power star testing and see what the slightly defocused image looks like. I wonder are the optics pinched? are you using a laser to collimate the optics. Most eyepieces find a F4.5 tougher going then a F5, but you still should be able to get a good star image in the dead center of the FOV. I find a laser collimator is good enough for good collimation.

Phil, the mirror testing system you had at CSP last week would be the bees knees on testing a system like this.

Dave L's comment reminds me that I forgot to mention...

Do NOT trust a laser collimator to align your optics accurately if you are starting from scratch. If you have a truss-tube scope then you *must* get or make yourself a Cheshire collimating eyepiece to really get it right.

I made this mistake when I first got a laser collimator - bunged it in, twiddled knobs until it all looked A-ok... and it gave really bad images. What gives? So I put in the Cheshire eyepiece and was astonished to see that although the mirrors shared *an* optical axis (hence the laser collimation looked ok) it wasn't *the* optical axis of the primary mirror!!!

Lots has been written on this subject too... make with Google for more horror stories!

The good news is that lasers work fine if the scope is already close to perfectly collimated and just needs a tweak on the primary screws.

Phil.

If you don't collimate correctly, you'll get an image looking just like Phil's new avatar....perish the thought!