Star test

[Jimtheslim]

As some of you may have read I have been having a bit of a to-do with collimating my new 250p. Yesterday I was quite happy with it so when I returned home from work at 1am and it was clear I couldn't help trying it out. I know you shouldn't rush and let the scope cool etc but I can't help myself.

Anyway, I found stars to be sharp pin points and when going into and passed focus they formed a fat, fuzzy almost perfectly circular donut in which I could make out the spider vains. There were no multiple concentric rings that I could see (or remember seeing). Can I assume that I am in a half decent collimation? Everything looks in collimation when using the cheshire, although the eyepiece hole is slightly off centre which according to astrobaby's guide is quite normal for fast scopes such as my f/7.

Couldn't help looking at Jupiter. This looked far better than the oval blob I got first time before it was collimated. However there was a distinct cross of light running through the centre. Not sure what this was, possibly light spreading across the spider veins? Would this be normal for a relfector? The image was also fuzzier and less contrasted than I'd hoped at higher mag. Could this simply be that I hadn't let the scope cool sufficiently (or at all to be precise)? I also wondered whether this is where faster scopes become more demanding on eyepieces. Mine are all Meade plossls. Image was sharp at 96x using the 12.4 but suffered from the cross of light. At 187x using the 6.4. it was very fuzzy. I know seeing conditions will also play there part however.

[Paul73]

Sounds about right!

If the stars are sharp. Then you've got it right.

Jupiter doesn't take high magnification that well. Normally, I find the view sharper at x150 than x200. That is down to atmospherics. Could be either non-cooled scope or the sky quality wasn't up to hi mag views last night. Were the bright stars a bit fuzzy at the higher mag?

Paul

[acey]

24 minutes ago, Jimtheslim said:

 

Anyway, I found stars to be sharp pin points and when going into and passed focus they formed a fat, fuzzy almost perfectly circular donut in which I could make out the spider vains. There were no multiple concentric rings that I could see (or remember seeing). Can I assume that I am in a half decent collimation? Everything looks in collimation when using the cheshire, although the eyepiece hole is slightly off centre which according to astrobaby's guide is quite normal for fast scopes such as my f/7.

If you could see the shadow of the spider vanes you were going too far out of focus. For a star test you want highest power and just inside and outside focus. In any case it sounds like you're adequately collimated: a Cheshire is a very accurate way of assessing it. Collimation has no effect on image contrast, if you're unhappy with that there are a dozen possibilities. Poor contrast means scattered light and there are lots of things that scatter light (spider vanes, secondary mirrors, internal reflections in optics or telescope tube/focuser... the list goes on and on, but doesn't include collimation). One could also include inappropriate expectation. Magnification in itself doesn't alter contrast, but it alters contrast perception and image sharpness, with the image having lower surface brightness and exagerrated atmospheric turbulence. So if you over-magnify you're going to get a poorer image.

[Starwiz]

Just to be certain, I would have another go, but with the mirror properly cooled, 50 minutes minimum for the 250.  As the mirror cools, different parts of it will be at different temperatures, so it may have an effect.  Have you tried using a dew shield in case you're getting unwanted light entering the telescope?  

John

[Jimtheslim]

39 minutes ago, Paul73 said:

Sounds about right!

If the stars are sharp. Then you've got it right.

Jupiter doesn't take high magnification that well. Normally, I find the view sharper at x150 than x200. That is down to atmospherics. Could be either non-cooled scope or the sky quality wasn't up to hi mag views last night. Were the bright stars a bit fuzzy at the higher mag?

Paul

To be honest I didn't try the stars at high mag as I was just out for 20 minutes or so and rushing to see what Jupiter had to offer. I know star tests should e done with high mag, but having seen neat donut shapes with low mag that gave me a bit of confidence.

From using my refractor I'm aware that as you go up in mag you're also going to go up in atmospheric haze and turbulence. I just wondered if the cross of light should be anything to worry about. Jupiter did seem to have glimpses of good focus but was in and out. I guess a properly cooled scope will give me a better indication. As for the dew shield, no I don't have one. Guess I should add it to my wish list!