In-distinct stars at high mag?

[Bendy]

Hi everyone

Just wondering if it's normal for bright stars to be in-distinct (i.e. non-pin point) at higher magnifications e.g. 200x?

For example, viewing Rigil Kent over the past few weeks at high magnification has revealed it to be almost disc-like but in distinct. However when viewed at lower magnification (e.g. 80x), it appears pin point and crisp.

I presume that this is due to poor seeing conditions as in each case I was unable to conduct a star test (the airy disc was jagged and in constant motion), however, thought I'd check as I read something recently about it potentially being due to imperfections in the human eye??

Any thoughts?

[Atmos]

What sort of telescope are you using? To me it sounds a bit like a collimation issue more than anything.

[Bendy]

Ah, that was another concern I had. It's a 10" dob F4.7. I collimated it using a combination Cheshire eyepiece and sight tube and a number of online guides. As best as I can tell the collimation is sound, the only concern I have is that the secondary is ever so slightly not perfectly centered on the eyepiece, but we are talking fractions of a mm... Sounds like I might need a second opinion on my collimation attempt from someone in the know.

[raymo]

Hi Bendy, firstly, don't get caught up in the collimation trap. It has to be quite a long way out before you will see any significant degradation in optical performance. If you constantly worry about it you won't get much enjoyment from your scope.
Secondly, Alpha Cent,[Rigel Kent] is a double star and will only look a bit pin point like at very low powers. You should clearly see it as double at
anything over about a 100x. Just as the viewing of the moon, or any other sky object deteriorates with high powers, so too are the stars affected. Different types of scopes give more or less sharp images
according to type and quality. A good APO or a Mak will generally give sharper stars at high power than your average Newt.
raymo

[Bendy]

Thanks for your advice guys. I think the collimation is probably ok as I recently joined a local astronomy group and the views through other people's scopes didn't seem appreciably different to mine, at least to my untrained eyes! Although i might get one of the old hands at the club to check it for me just to be sure...

[Blue Skies]

I wouldn't touch that - its probably the offset amount that the secondary needs to be at with a relatively short focal length telescope. A lot people don't realise this. In the old days with the long focal length scopes you did centre everything, but with the modern shorter focal length offset becomes important. I'd do a bit of research on it, there must be some diagrams around to show what it is and why it matters.

I've you've joined up with the Bunbury mob, trust their opinion, some of them have been in the game a long time and know what they're doing. I think what you're describing is perfectly normal, you're just not used to what is 'normal', as you say.

I do a lot of outreach and people often expect that the telescope will magnify the stars into flaming balls of gas, or at least a round disc. Er, no, they're still far away for that...

[mental4astro]

Bendy,

You answered your question yourself!

That the image was jagged and in constant motion means seeing conditions were poor!

Expanding on what Raymo wrote: It does not matter your scope. It does not matter about collimation. When the image is jagged and moving, it means that there is a lot of thermal disturbance in the atmosphere. If the atmosphere is stable, then you can ramp up magnification. When really stable, you can take things to the maximum your scope's aperture will allow. And you've just hit the jackpot!

Collimation (if your scope is a reflector of any type), if not as good as it can be, will show a different set of aberrations.

'Seeing' refers to thermal stability. Typical 'seeing' will only allow upto 150X magnification before the image degrades with a constantly moving mage. Good seeing and you can take things to 250X. Excellent seeing and 400X is possible, and even more, if your scope's aperture is large enough. And it does not matter the size of aperture - seeing affects all scopes the same way as magnification is the same in all instruments.

The rule of thumb for max. magnification is 50X per inch of aperture. So a 4" scope will allow for upto 200X before the image begins to degrade. You can increase the magnification more, but the image quality just degrades. Take, say, my 17.5" dob. I 'should' be able to punch that puppy to 875X. The most I've taken her is 400X, and only a handful of times over 6 years. The 50X rule of thumb is a conservative figure. The better the optics the higher this can be taken. And elevation above sea level also plays a part in this.

Then there is 'transparency'. This means how clear the atmosphere is. Easiest way to think about it is a bushfire. If there is no fire, no smoke, then you can see the horizon clearly. If there is a fire, and there is smoke, how clearly you see the horizon is determined by the density of smoke.

Seeing and transparency are easy to confuse.

Mental.

[Bendy]

Thanks Jacquie and Mental, some great additional advice there. I'm looking forward to the coming summer to see the difference in "seeing", as I imagine that the atmosphere over here will generally be more stable than at present. Although no doubt there'll be a few occasions when I will have to contend with bushfire smoke, literally :)