Autoguiding using a finderscope

I took my first tentative steps in autoguiding last night, taking the budget basement route as usual.

Following a growing trend, I used a finderscope as a light and cheap guidescope. The finderscope was effectively free, being the 8 x 50 that came with the 200 mm GSO Newtonian, which was lying around unused because a red dot finder is so much more convenient to use for aligning the CG5 GoTo mount. The guiding camera was a Philips SPC880 fitted with an IR/UV filter (approx €35 from Morgan computers) attached to the finderscope using an adapter (€34 from Modern Astronomy). The original threads on the finderscope were knackered, and it cost €20 to get the damaged thread ground out. I used MetaGuide (free) to control the guiding. Everything went reasonably smoothly and after a little playing around with the guiding parameters, the results were pretty good. I took a series of 2 minute exposures of M13 using the 200 mm f/5 reflector with a micro four thirds format camera, with a "crop factor" of 2, so the effective focal length was approx 2000 mm. About 90% of the images were useable (by my standards), far far better than the hit rate without autoguiding.

The main problem was the breeze which caused some wobbling in declination, so the stars in the final stacked image (11 x 2 min) are still not perfectly round. The link to the image is:
www.flickr.com/photos/45468979@N04/5791696460/in/photostream

All in all, the experience was less daunting than I feared, and the improvement in the tracking exceeded my expectations for the mount. Also, it was very interesting to see the data and graphs produced by MetaGuide. They show how big an improvement was achieved in tracking, but also show how even small puffs of wind, or the slightest of touches, affected the scope. I had no idea just how sensitive the tracking was. I hope to continue playing around with my new toys over the next few weeks and am optimistic that longer exposure times will be feasible.

Any comments and criticisms are welcome, and if anyone is interested in the details of the setup I would be happy to explain.

Thanks for reading,
Mike

Thats a nice effort Mike. You are well on your way. Do you have any issues finding guide stars?
My first attempts with a DMK camera and Megrez 70 went well. My recent attempts have been plagued with issues like software crashing constantly and polar alignmnet problems. I am determined to nail this one though.
Keep up the good work.

John

Very nice!

I considered using a finder as a guide-scope but I stumbled upon scope and skys' 'car boot' section; 60x700mm refractor for £20 and a set of 5 inch scope rings for £20. Hopefully they'll work out but I'm looking forward to dealing with flexture and all sorts of other fun.

Pretty good effort there Mike, well done! You still got colour and definition despite the trailing, but that was out of your control a bit. I never heard of a finderscope being used as a guidescope. Would you recommend it to a newbie in 'guidescoping' now that you gave it a go?

Seanie.

Mike - that's a really good report.

Is the astrometry.net report of your pixel scale correct? At 3.39 arcsecs / pixel your image is considerably under-sampled. The seeing FWHM of your stellar images is likely to be less that one pixel. Yet a surprisingly good image for that.

I have heard of the use of finder scopes as guide scopes before - I've even heard of half a binocular being used to good success.

Would you have a direct link to the details of the adapter to connect the webcam to the finder scope?

The small aperture of the finder scope and lack of sensitivity of a webcam must make make finding a usable guide star challenging - as John has already suggested.

Mark C.

Thanks for the feedback guys. I'll try to deal with some of the issues you raised.

The adapter is available here: www.modernastronomy.com/accessories.html (first item on page). It worked exactly as advertised, though there was a problem with the threads in my finderscope, as I mentioned in the original post. It accepts standard 1.25" filters as shown in the photo below. A spacer ring must be removed from the front of the scope in order to achieve focus, which is then fine tuned by screwing the objective assembly in/out on the tube (see msfastro.net/articles/finder_guidescope/ ). The whole assembly is so light that flexure is unlikely to be a problem (though I can't prove that). It is also light enough to use the original finderscope mount, which allows you to move the finderscope easily when searching for a guide star.







The webcam ( www.morgancomputers.co.uk/c/556/Astronomy/ ) needs to be "flashed" on a Windows XP machine to make it act as an SPC900, for which drivers are available for Windows Vista and 7 (I use a little netbook with Windows 7). That is a little complicated, but detailed instructions are available online, and it all went smoothly for me. Alternatively you can buy one that has already been flashed. It's sensitivity is not great, but so far (only two nights and about four targets) I've been able to find a guide star without any problems. I leave "automatic exposure setting" on in MetaGuide and it seems to be able to guide successfully on faint stars even though their fwhm values are huge. It helps that the finderscope is approx f/4, so it is reasonably "fast".

The image scale of the actual subs from the Panasonic GH1 camera is 0.89 arcsec/pixel - the image on the flickr page was resized to reduce the filesize - so the original image has adequate resolution. I suspect that the seeing and air pollution, as well as the accuracy of the guiding, will be the limiting factors in the resolution of the images.

So does it work, and is it worth it? Bearing in mind that I've only been out for two sessions, the simple answer is that it massively improves the quality of the images obtained using the CG5 mount. ALL of the 2 minute subs I got last night, in calm conditions, were good enough for me, whereas less than 20% would have been useable without the autoguiding. The stars are much rounder than those I was was getting previously and seem significantly smaller, though I don't know how to attempt to quantify that. Bearing in mind the low cost, it is definitely worthwhile, despite the more complicated setup. It should also be possible to use the webcam for planetary imaging, and for refining collimation and facilitating drift alignment.

Is it as good as a more conventional autoguiding setup? I have no experience of alternatives, so I don't know the answer. It is very unlikely to meet the requirements of expert astroimagers, but for backyard astronomers like me, with modest equipment, it seems to be a very good way to take the step up to autoguiding. It would be nice to compare the results against those obtained with a setup like the you have Martinus - perhaps we can try to organise that when you have got it operational. It is interesting that Orion (USA) now offer a complete package of this type, albeit with a better camera ( www.telescope.com/Astrophotography/Autog.../4/sc/60/p/99631.uts ).

I'm very happy so far, but it won't be right for everyone.

Cheers,
Mike