Half- Moon shaped guide stars with OAG

[kirkster501]

Hi all,

I'm getting "half moon" guide stars with my Moravian OAG and Lodestar.  When I look at the pick-off prism it is just less than half moon as visible in the picture below and my out of focus guide stars look exactly like the piece of mirror showing.  Is this an issue with PHD?  Will it cause an issue?  I have been slightly out of focus with my guide stars and having a few guiding issues because I cannot get enough in travel to get better focus so I have done some tinkering and think I have that component sorted and await a clear night.  However I still guess I will get "half moon" guide stars even if they are in focus with my adapters I have now added?

Any thoughts please?

Steve

[Zakalwe]

They all do that Sir. Luckily, PHD doesn't seem to care one jot and it can still calculate the star centroid.

[sloz1664]

26 minutes ago, Zakalwe said:

They all do that Sir. Luckily, PHD doesn't seem to care one jot and it can still calculate the star centroid.

Same here. I also set mine slightly out of focus. A little bit more meat for PHD2 to get hold of.

Steve

[kirkster501]

Phew, thanks guys, feel a bit better now ??

[Davey-T]

Yes, same here as well, always had banana or seagull shaped guide stars in my OAG, doesn't bother PHD.

Dave

[Merlin66]

If an OAG was designed and built properly, the guide star image would be no worse than the star image at the edge of your imaging camera FOV.

Most of the problem comes from the inability to tilt the prism relative to the optical axis. If this feature (prism tilt) is available, the guide images are much, much better.

 

[skybadger]

Agreed with the latter. It is another degree of freedom to control however. I have the TS OAG and the Brightstar. The latter had a prism tilt facility. But if you position the pickup right on the edge of the camera for you will get the best stars. And I find it's worth spending the time to focus as sharp as possible since you want as many stars to guide on from all over the sky as possible and sometimes they are hard to come by.

 

[michael8554]

Reply from the experts to a similar question of OAG star shape on the PHD2 forum:

Over the years, people have not generally had problems with using comatic stars for guiding.  I did it myself for years with an LX200GPS.  The algorithm we use is a classic centroid calculation, so no assumptions are made about the shape of the star or where the brightest pixel happens to be.  The algorithm generally works fine so long as the star is reasonably well-sampled and has a continuous brightness profile – e.g. doesn’t have a big dark doughnut in the center or some other aberration that creates a bi-modal brightness distribution.  

Michael

[Shibby]

When you say you did't have enough inward travel, do you just mean the focus? (i.e. not the prism?)

Because, you should be able to move the prism inwards, getting it as far in as possible without overlapping the sensor area. Unless you've already done that?

[knobby]

I'd move it in until vignetting shows, then pull back out til it goes.

But ! if its working as is then don't worry.