Focus questions

[JOC]

This thread might help:  

 

[JOC]

If the focus is not changing when you move the knobs it may be that the small adjustment screw that locks the focus has been turned by accident (I've done this myself).  If this is moved to the point where it 'bites' no amount of twiddling the knobs will shift the focus, so have a look and see if there is a small screw that doesn't hold one of the focus tubes in place and back this off.  It might help.  I twisted mine by accident in the dark whilst placing the focus tubes and it caused me no end of problems until I got it loose again.

[FarAndBeyond]

Ive managed to focus so that the stars became smaller and brighter, but what i saw was more like small streaks of light, not spherical dots.

im using the tube extension, so it seems that im able to achive focus, the problem is more knowing when im at perfect focus.

[Ricochet]

33 minutes ago, FarAndBeyond said:

Ive managed to focus so that the stars became smaller and brighter, but what i saw was more like small streaks of light, not spherical dots.

im using the tube extension, so it seems that im able to achive focus, the problem is more knowing when im at perfect focus.

Do the streaks of light all point in exactly the same direction and flip through 90° when you go through the point of best focus? If so you have some astigmatism in the system.

You could also be seeing coma, which is inherent in Newtonians with parabolic primary mirrors. When correctly collimated, the central stars will be points and the outer stars will all point to the centre. If the collimation is out then the point like section may be offset. Look around the edges to see is there is a section that is (more) point like.

Finally, the secondary mirror support vanes will create diffraction spikes. On your telescope these should show as a cross, but only the brightest stars should show this.

[JOC]

When properly focussed only planets should resolve as spherical dots.  Stars are more like a white point of light looking very much in the focussed EP rather like you'd see them with the naked eye.  

[FarAndBeyond]

Okay so im back in my old thread here. So haven't had too many times to do further testing, only a couple of times. I've managed to get the stars to smaller pinpricks, but they still does not look like on the photos I see online, the stars I see on photos are more of concentrated pricks of light, i.e. brighter and more concentrated, what I see in the eyepiece is more of diffuse and dimmer dots (that are not perfectly spherical), its a bit hard to explain. Can it have to do with the fact that on photos there is more light collected because of the longer exposure time so that makes them look different from the look in the eyepiece?

And a couple of more questions, the so called airy disc with the rings i have been reading about....i don´t get anything that looks remotely to an airy disc regardless of if im in focus or not, so whats the deal with that? Should it always be visible?

Also, when im using an eyepieces with a high magnification, for example a 4mm eyepiece in the scope i only see completely black when looking through the eyepiece even though i have objects in front of the scope, its like there is something blocking the light from entering the eyepiece. But when looking through it in the daytime I can see through the eyepiece just fine. Surely stars should be bright enough to be visible even through a high magnification eyepiece?

[Stub Mandrel]

Have you read this thread? It will give you a more realistic idea of what to expect to see than astrophotos that combine many minutes or even hours of data.

If stars are not perfectly spherical, it sounds like the scope needs collimation.

With a 4mm eyepiece you are pushing the 200P rather hard and focus will be critical. Field of view will be very small and unless perfectly in focus faint stars won't show up. Find a bright star and focus on that.

[JOC]

I agree with comment above.  I sold a 4mm that I couldn't get to work in a 200P.  I own an excellent 5mm, but even that is only good for a handfull of really bright objects in the UK and then the atmospheric conditions have to be excellent.  I believe the lower the EP mm number the less light it collects, so the brighter the object being looked at has to be.  Try viewing stars at 10mm or higher a single star won't look much different no matter how close you get to it.  

As noted above your eve cannot see what a phograph sees.  So don't try to emulate what you can see in a picture.

[Astro Imp]

10 hours ago, JOC said:

I believe the lower the EP mm number the less light it collects, so the brighter the object being looked at has to be. 

Not quite correct. Your mirror collects light but by increasing the magnification your EP spreads this light over a larger area, the result higher magnification makes the object appear dimmer.

[JOC]

1 hour ago, Astro Imp said:

Not quite correct. Your mirror collects light but by increasing the magnification your EP spreads this light over a larger area, the result higher magnification makes the object appear dimmer.

Many thanks - I knew what the ultimate effect was, just not exactly why - I knew if I put 'I believe' and it was wrong that someone would provide the absolutely correct answer - many thanks for correcting me