Guide Scope & Camera : Focus

[feilimb]

I recently got my first ever guide scope and camera pair, a QHY 5L II Mono and a simple Orion 50mm guide scope. 

I got to try the camera out briefly this evening and was delighted to find stars very quickly at a number of random points overhead . 

I found that to achieve what looked like good focus , the camera was nearly falling out of the guide scope , and the tightening screws were just barely hanging onto the camera (which already had a 1.25" nose piece adapter screwed on ) . I guess I will need another 1.25" extension ? Has anyone else experienced experienced this with the Orion 50mm guide scope ? 

The other question that came to mind was how good / precise  does the focus need to be for the guide scope / camera combination to achieve good guiding in PHD etc ? To my eyes the focus did look good at a few points , but should I use a focusing tool / FWHM etc to get 'perfect' focus before guiding . . ? 

Thanks for any advice offered ! 

[RayD]

My GP Cam I think is basically the same thing, and came with a 20mm extension which was needed.  I wouldn't be surprised if this is the same for you, so it's not unheard of.

With regards to focus, I think it's literally a case of the better you can get the better the guiding will be.  I only do mine visually but do spend a little bit of time over it, but I do notice a difference if it is out.

[SkyBound]

Focus does not need to be spot on for guiding, in fact slightly off is a little better, to give a very slightly out of focus star, I think PHD works better this way.

[SkyBound]

Just now, RayD said:

My GP Cam I think is basically the same thing, and came with a 20mm extension which was needed.  I wouldn't be surprised if this is the same for you, so it's not unheard of.

With regards to focus, I think it's literally a case of the better you can get the better the guiding will be.  I only do mine visually but do spend a little bit of time over it, but I do notice a difference if it is out.

Hmmmmm, not necessarily true, slightly off focus works better when guiding....

[feilimb]

Cheers that's good to know on the focus, I guess if it visually looks pretty good I'll go with it and see how the guiding goes. 

I did get the 20mm extension, and have it screwed in - but even so the camera is struggling to get focus without being right at the end of the guide scope. Maybe there is some variation in the focal length of these Orion 50mm lenses. 

[RayD]

No problem, I've never heard of that so I guess I've just been lucky 

 

[RayD]

1 minute ago, feilimb said:

Cheers that's good to know on the focus, I guess if it visually looks pretty good I'll go with it and see how the guiding goes. 

I did get the 20mm extension, and have it screwed in - but even so the camera is struggling to get focus without being right at the end of the guide scope. Maybe there is some variation in the focal length of these Orion 50mm lenses. 

Not sure what one you have.  On my Altair one the focus tube is helical and extends, but it's not immediately obvious, so have a close look as your's may extend a bit?

[SkyBound]

PHD seems to like a slightly larger point of light to guide with, rather than a very fine point, it may have something to do with the amount of pixels the light covers being more...not sure but have read many times on this forum that slightly out of focus is better, maybe someone with a lot more experience than me will give some insight..

[Adam J]

Yes the 50mm guide scope was designed to work with the original QHY5 not the v2 you need one of these:

http://www.365astronomy.com/C-Mount-Adapter-with-1.25-Nosepiece-30mm-long.html

I have this setup and it work fine once you have the extension.

This is the second time tonight I have answered this exact question lol.

[RayD]

6 minutes ago, SkyBound said:

PHD seems to like a slightly larger point of light to guide with, rather than a very fine point, it may have something to do with the amount of pixels the light covers being more...not sure but have read many times on this forum that slightly out of focus is better, maybe someone with a lot more experience than me will give some insight..

No I think you're right, it's the SNR that's important, not necessarily the actual focus, which I understand PHD likes to cover several pixels.  I guess my in focus attempts are still far enough out for it to work 

Good job I have a Bahtinov mask on my main OTA as I'm obviously not very good at focusing visually as I've never had guiding problems 

[feilimb]

10 minutes ago, Adam J said:

Yes the 50mm guide scope was designed to work with the original QHY5 not the v2 you need one of these:

http://www.365astronomy.com/C-Mount-Adapter-with-1.25-Nosepiece-30mm-long.html

I have this setup and it work fine once you have the extension.

This is the second time tonight I have answered this exact question lol.

Thanks for the link Adam that looks perfect and it's good to hear it works for you. Yet another adapter to order ..

Thanks for all the tips and info! 

[ollypenrice]

21 hours ago, RayD said:

 

With regards to focus, I think it's literally a case of the better you can get the better the guiding will be.  I only do mine visually but do spend a little bit of time over it, but I do notice a difference if it is out.

Surprizingly this is not so. Craig Stark, creator of PHD, does not recommend tight focus on guide stars. Remember that the software does not guide on a guide star, it guides on the calculated 'centroid' of a guide star and this centroid is far smaller than a pixel. A guide star is far larger than a pixel. So, remarkably enough, we can and do have guide traces measured in fractions of a pixel. Craig tells us that the centroid calculation is more effective if the focus of the guide star is slightly soft.

Now I have never experimented with this, I must say, but I have not touched the focus of my guide scopes in, quite literally, several years but my imaging scopes need focusing several times per night.

Olly