Guide stars

[Kevin Gurney]

Following up my question on target location ..

I was using guide stars other than the target, but I think I have seen a discussion (not here) where people are sometimes able to use the target star itself if it bleeds over the edges of the slit. It requires that the guiding software can cope with bimodal images and I was wondering if PhD2 can do this?
 Or is it normal to always use a non-target star?

cheers

Kevin

[Andy Wilson]

Hi Kevin,

I find this depends on the field and the brightness of the star. I typically guide on another star in the field and there are usually several candidates, with my guide images of 5 to 10 seconds. However, this may depend on how good your mount is at tracking.

I use PHD2 and have guided on the target star on occasion. I’ve not found any problem with this, but there does need to be a strong signal outside of the slit. I’ve not looked in any detail as to how this works, whether it needs bimodal guiding or not. If the star is bright enough then I tend to notice it dimming rather than disappearing or splitting in two. This is usually the case when I am taking a spectra of a bright star and so the fainter stars are not easily visible in the guide image.

Cheers,

Andy

[Mr Nicholas John Atkinson]

Hi kevin,

I use Maxim dl 6 where there is an option after calibration to guide on all the stars in the field rather than just the guide star. At one time I used PHD with but have found it much easier and more reliable. The other advantage is the point telescope here command. I have ASCOM pointing to The SkyX and Maxim links into this. I have T point add on which will allow very accurate polar alignment but also will correct any pointing errors giving a high degree of confidence. The other advantage of Maxim is the ability to use 2 cameras. I expect with the £/$ exchange rate this will be more expensive but is worth avoiding the frustration of spending hours centering the star.

Cheers

Nick

[Robin Leadbeater]

I use PHD 2 to guide both on the slit or offset using a field star. The later version had some features added compared with the original PHD to help with spectroscopy guiding, particularly the ability to guide on a specific XY position which can be saved and recalled and to nudge the guide position a fraction of a pixel at a time.  It is not perfect when guiding on the hamburger shaped split star image though as it has a tendency to hunt. Others swear by AstroArt which has a special algorithm for guiding on the slit and is  being improved in other areas specifically for this application.

http://www.spectro-aras.com/forum/viewtopic.php?f=8&t=771

To find my targets (some of which are extremely faint when I am using the ALPY 200 – down to mag 17) I use a webcam mounted on the guidescope plus guider image to check and update if neccesary the alignment on  nearby bright stars (using an EQ6 with eqmod and Cartes du Ciel) then zone in on the required field using the guider image.  Astrometry. net usually solves the ALPY guider field for me if necessary but usually comparing the guider image with the projected DSS image in CdC gets me there.  For me though spectroscopy was the driver that pushed me to a simple permanent setup. The setup and teardown time was just too time consuming.

http://www.threehillsobservatory.co.uk/astro/observatory/observatory.htm

Cheers

Robin

[Kevin Gurney]

Thanks everyone…

I’ll certainly be more confident trying guiding on the target star, and the fixed XY technique in PHD2 looks interesting…

I did look at the Maxim website, but I would rather avoid learning a whole new set of tools just now..

Unfortunately my ‘land-rights’ circumstances mitigate against a permanent set up, but I might manage something ‘semi-permanent’ in future. If so, I’ll show some pics – it might be innovative 🙂

cheers

Kevin

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