Wanted - artificial star

[ballaratdragons (Ken)]

Wow, very professional looking unit David

Keep us informed of the first light, Matt.

[dcnicholls]

Originally Posted by matt

Just got back from picking the unit up from David's (dcnicholls) place.

Nice and compact, and very well made. I was expecting something a lot more "home made", IYKWIM? So, very pleasantly surprised.

Anyway. Nice work, DN.

Can't wait to test it out and let you guys know how it goes

Thanks, Matt. My approach is that if you're going to make something, you may as well put a bit of extra work into making it look nice. However, it's got to work, and I'll be most interested to hear the results of your tests.

DN

[matt]

OK... here's the latest on David's great little artifical star.

A few days ago David was kind enough to get in touch and offer a few modifications to the unit.

He adjusted the resistance to the super bright LED (to make it even brighter) and fitted a much narrow pin-hole (approx 50 microns).

Well, tonight I had a first check of the unit using the 9.25

I'm happy to report it appears to work very nicely

I must admit even with the focus racked all the way in I couldn't quite bring the artifical star to pinpoint focus.

I'm pretty sure I wasn't situated far enough away from the unit.

I measured the distance to be just a fraction over 15 metres. The distance recommended for a 10" scope (by the manufacturers of the PicoStar) is 19 metres.

So I may have to find a little extra distance from somewhere.

Despite these early trials, the focused down image I saw revealed a central airy disc in the darker central obstruction and fresnel rings which were just appearing when I ran out of focus range.

So, looking good...

[Dennis]

Ta for the update Matt. Did you notice any problems with the (unnatural) horizontal orientation of the OTA? The more usual configuration is pointing skywards, so I was wondering if there are any adverse effects due to potential mirror shifting or settling?

Cheers

Dennis

[matt]

G'day Dennis

I did notice a slight shift in the collimation image - the central obstruction and airy disc had shifted ever so slightly, due as you say to the shifting of everything from collimating overhead to pointing at an artifical star at horizontal.

I guess these products are meant to just get you very close, so you can finish the job off on a real star later on

One strange effect I did notice was when I was watching the artifical star image when I'd reached fully racked focus using my StarLight Instruments Feathertouch Focuser.

There appeared a little indentation in the very outer ring of the star image which intruded even further the more tension you applied at the end of focus. I didn't want to try and turn the focus knob any further for fear of damaging something.

But the indentation became quite severe with even the slightest "forcing" of the focus past the end of its travel point.

Any idea what might be causing this strange visual phenomenon?

Cheers

[Dennis]

Hi Matt

Hmm, interesting? From memory, winding the focuser clockwise (CW) is required for close focusing, and C-CW for focusing at infinity?

If that is so, we must be “pulling” the main mirror towards the back of the OTA when focusing on close by terrestrial objects?

I wonder if a physical object some how cuts into the light cone as you approach the point of closest focus? This would appear as an “indentation” in the circular diffraction rings.

I’ve ordered some flocking paper from the US so when it arrives, I’ll be sure to photograph the innards of the C9.25 as I take it apart to flock it. This might reveal something?

Cheers

Dennis

[matt]

Originally Posted by Dennis

I wonder if a physical object some how cuts into the light cone as you approach the point of closest focus? This would appear as an “indentation” in the circular diffraction rings.

I thought that too, Dennis.

However I'm sure I pretty much eliminated that. It was quite easy over such a short distance to identify any potential obstructions.

No, it's definitely something related to being right at the end of the focuser's travel ... and then trying to force it past that point.

I could actually see the indentation grow bigger as I forced the focuser.

[Dennis]

Thanks for that Matt.

When we view a star at infinity, the incoming rays are parallel, and fall on the SCT main mirror with everything (hopefully) optimised.

As we focus on closer and closer objects, the light cone gets steeper and I'm wondering if we are now using the extreme edge of the mirror, which was perhaps masked when viewing objects at infinity?

I'll take a look down the front of my OTA over the next couple of days, just to see how the whole set up looks, racked in and racked out. Can't do much else right now 'cos of the clouds.

Interesting stuff eh - there's always something to learn isn't there?

Cheers

Dennis

[matt]

Originally Posted by Dennis

Interesting stuff eh - there's always something to learn isn't there?

Yep... that's for sure!

I don't reckon it's anything you'll see looking down the tube.

It's something associated with the pressure coming from trying to turn the focuser past the end of its travel. I think its pressing on something which is causing this small aberation?

Anyway... probably best not to do that, eh?!

I'll just have to move the scope further away from the artificial star and get closer to focus

[dcnicholls]

Originally Posted by matt

OK... here's the latest on David's great little artifical star.....

So, looking good...

Thanks for the feedback, Matt. Since I don't have an SC, I can't test it myself.

DN

[dcnicholls]

Originally Posted by Dennis

... Did you notice any problems with the (unnatural) horizontal orientation of the OTA? The more usual configuration is pointing skywards, so I was wondering if there are any adverse effects due to potential mirror shifting or settling?

The star gadget is sufficiently small that you can use blue-tack to stick it to the gutterring on your house or up a tree, if you're sufficiently nimble That would avoid the horizontal orientation.

It also crossed my mind you could toss a string over a high branch of a gum tree and hoist the gadget high above you. Mind you, you'd need a very big gum tree and a good throwing arm!

DN

[Dennis]

Hey Matt

I just read an article on the C14 Yahoo group about a guy who wanted to minimise mirror flop on his C11. He took it apart and inserted 3 custom made “spacer” bars around the edge of the mirror. These spacers were placed longitudinally and were curved to fit snugly against the inside of the OTA. Screws then “pushed” these delrin spacers against the mirror to reduce flop.

Here is an interesting excerpt on how easy it is to deform the mirror:

“I centered on a star and slightly defocused the image to get a series of star-test rings. Then as I brought the mirror locks into contact with the mirror I could easily see distortions in the diffraction rings and visible degradation of the focused image. The bottom line is that you cannot apply much force at all. How much is too much? Imagine getting in your car and turning on the radio. The amount of torque you give the volume knob (at least in my car) is about how tight you can make the screws without distortion”.

Cheers

Dennis