2 minute artificial star? Enough to get by?

[vapeinspace]

Hi guys,

I purchased a second-hand Celestron 8SE the other day and it's not in great condition. At some point someone has dropped the secondary mirror onto the primary mirror. This has resulted in a slightly scratched primary mirror and a tiny crack (very small) on the side of the secondary mirror.

(if anyone knows somewhere to find a secondhand secondary mirror plz let me know)

Just for the sake of it, I tried it out on Saturn the other night and it was obvious that at a minimum... it required collimation. I've since ordered the cheap artificial star made by Hubble Optics but as I wait for it to arrive is there anything majorly wrong with this 2 minute dodgy job?

I've never attempted to collimate any type of scope before. But I get the general idea so I though I'd give it a shot of making a (very) simple artificial star.

Nothing special here, just some thick cardboard (I know I should have used a larger piece) which I pushed a staple through (without punching the staple) using a stapler, a frosted piece of plastic to even out the light (SD memory card case cut in half), some sticky tape and a LED torch.

Attached is some photo's, I put the torch about 15-20 meters down a long hall. I think I've got it roughly collimated so I'll give Saturn a go again tonight.


In the pictures I made two holes using the staple, one brighter.

My question however is this.

What is so special about the expensive artificial stars? Will something this dodgy get the job done? or am I missing something?

Thanks,
Ryan
:thumbsup:

[julianh72]

A 10 mm chrome ball bearing makes a great artificial star - put it out in the sun for daytime use (position it in the sun against a dark background), or put it down the far end of a dark hallway and shine a bright halogen light on it for indoor / night testing.

When you do the maths, the image of the sun or a halogen light on a 10 mm sphere at a distance of 20 metres or so is small enough to be a pinpoint for practical optical testing purposes.

[Merlin66]

Ryan,
Usually a pinhole, as small as possible in aluminium foil is used.
I have a couple of the Hubble torches which work 200% - the accuracy of the pinhole is what's its all about.
The pinhole should be around x10 the focal length from the scope - for the C8 that would mean around 20m.

The image you show looks Ok BUT what's causing the secondary fainter image? I assume you only have one hole in the card?

[vapeinspace]

Great. Thanks for the tip Julian, it's always good to have other options.

And for the maths too!

:thumbsup:

[vapeinspace]

Thanks Ken, makes sense. The reason for the two holes was I just did a quick dodgy job and used a stapler. One of the holes I made larger with a pin so I could try the different brightness. Yup, foil would have made much more sense. But I checked out Saturn again last night and it looks like, for what it's worth... I have a working telescope.

I did have 3 holes and in the end (after the shown photo) I did block the other hole and only used one.

:thanx:

[Merlin66]

The full maths for the glitter point is in Suiter's "Star Testing Astronomical Telescopes", Appendix F, "Glitter in a shiny sphere"
Summary:
The glitter point in sunlight will be <1/300 the diameter of the sphere - a 25mm sphere gives a 0.076mm (76micron) glitter point.