Trouble with pin point stars

[johnkirkpatrick]

I was out last night to a slightly darker site than my garden, and the view through my finder was showing stars as pin points but through my scope they were not, Is it time to collimate? of have I got a prob with the mirrors?? I've looked at my mirrors through a coll cap and all seems well, I just packed scope into car and gave up.

[Chris H]

Due to the secondary mirror spider a dob won't quite bring stars to a nice crisp point.

[Doc]

As Chris has said it's the design of the newt/dob, due to the spider vanes you never get pin point stars, you always see slight spikes due to the vanes.

Also take into account some mirrors suffer astigmatism and then some have coma.

If you want pin point light sources then a refractor is the way to go.

[johnkirkpatrick]

yeh that's true, but its not what I used to see, It just didn't look as good as usual. even Jupiter looked out of focus. I wonder if I knocked the secondery while blackening the edges with a sharpie? Everything looked out of focus really..

[Doc]

In that case I would say you have knocked your collimation out. Or it was a terrible night for seeing and this made the scope feel worse.

[Uplooker]

You should still see stars as points of light, whether it be a refractor or reflector. You need to collimate.

Ian

[pvaz]

Bad seeing often causes that. I wouldn't fiddle too much without testing it again on a decent night.

[brianb]

it was a terrible night for seeing

This can happen, I've known it to be near impossible to focus a 80mm refractor at x24.

A medium / large scope will almost always be like this until it's cooled down close to ambient, but jet stream smearing can still take its toll even when the scope is perfectly in tune with its environment.

A collimation issue will cause the enlarged star images near focus to have a "V" shape. If they're symmetrical, your collimation is reasonable.

it's the design of the newt/dob ... If you want pin point light sources then a refractor is the way to go.

Sorry, but this is rubbish. A well designed, properly collimated Newtonian cooled to ambient temperature has pretty much the same "pinpoint" star images as a well designed, properly collimated refractor of the same aperture and the same optical quality, also cooled to ambient temperature. Diffraction spikes caused by the spider are not seen except on the brightest stars and in images, and the central obstruction's effect on the structure of the Airy disk is hardly noticeable with typical Newtonian secondary sizes being not more than 20% of the diameter of the primary. In fact Newtonians, being absolutely free of chromatic aberration, usually give a better image than refractors of the same aperture, especially at the same price point - a good refractor tube will cost 5 to 10 times as much as a good Newtonian tube of the same aperture and focal ratio.

Refractors, like reflectors, have off-axis aberrations, principally astigmatism and field curvature; but are not immune to coma, especially in short focus scopes using aspherical elements to eliminate spherical aberration.

One thing that can give Newtonians a bad reputation is making them too short (less than about f/6). Not only does this increase the sensitivity to poor collimation, it also means that they cannot cope with simple, cheap eyepieces - if you're getting bloated star images in a f/5 Newt even when it's properly cooled and the seeing is steady, then try a decent eyepiece - a f/5 Newt really requires the sort of eyepieces that have price tags running well into three figures to work properly. So does a f/5 refractor, but you see far less of those around.

[SiriusB]

Due to the secondary mirror spider a dob won't quite bring stars to a nice crisp point.

Brighter stars maybe, fainter ones should be pretty pinpoint as the difraction spikes are too faint to show up.

[Doc]

I agree dimmer stars are perfect pinpoints, brighter ones will exhibit spikes. My collimation is very good indeed and I struggle with perfect pinpoint stars on the brighter ones. My refractor gives much better points of light, and hence easier to split doubles etc.

Where a dob excels is deep space then pin point lights are irrelevant.

I have very good eyepieces but still suffer astigmatism, very hard to see and most people wouldn't even bother with it, but I know it's there.

[brianb]

I have very good eyepieces but still suffer astigmatism

Noticeable astigmatism in a Newtonian is almost always either induced by the eyepiece or stress in the secondary from improper mounting - glue should be the sort that doesn't contract when dry (rubber solution is good); clips should be just barely tight enough to prevent the optics from rattling. Overtightening the rings that hold the eyepiece together can also induce astigmatism.

[johnkirkpatrick]

I had my scope out cooling for over an hour last night an guess what pinpoint stars lol, BUT after a few mins at the ep blurry stars and thats when i knew it was dew causing the problem lol, Now how do i prevent it happening again, I thing my body heat and breath was making it worse as the dew came and went, then came again. I do think I need to collimate as I still have never done this not having and coll tools and a star test always looks ok to me. Getting a hotech laser from my wife this year so interesting to see how far out it is lol. Also I notice as I move around the ep I get comet tails unless I keep my eye dead centre which is hard when its freezing.

[gedmac]

one very good tip i was given is keep the ep,s in a nice warm pocket until needed,helps to stave off dew a little longer:p