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DIY 50mm guidescope

michaelmorris

By michaelmorris
in DIY Astronomer

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michaelmorris

michaelmorris

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I’m putting together a new portable observing/imaging rig based around my Orion 80ED on a new-to-me HEQ5 Pro mount. After years of mainly lunar and planetary imaging I recently started trying to image DSOs (with little success so far) and have just started down the somewhat perilous autoguiding route.

I wanted a cheap, small autoguiding system for use with the Orion 80ED. I have been experimenting with off-axis guiding. While this is probably the right way to go with imaging small DSOs with my 8” LX200 in the observatory, this is probably just plain masochism when using a wide field refractor.

The obvious answer was a guidescope piggy-backed on the 80ED. However, the acquisition of the HEQ5 has hit the bank balance somewhat and a new refractor as a guidescope was out of the question. Also I wanted a very small system so that I didn’t have too much stuff to lug around. After reading various articles on autoguiding I stumbled across the concept of the very short focal length, very small guidescope.

As far as I can tell the only purpose-built system seems to be made by Borg and is mighty expensive.

Hutech Astronomical Products -

A bit further research pointed my in the direction of converting an 8 x 50mm finderscope into a guidescope – that’s more like my budget!

http://www.stark-labs.com/movies/NEAIC08_Guiding.html

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3199073/page/0/view/collapsed/sb/5/o/all/fpart/1

http://msfastro.net/articles/finder_guidescope/

http://stargazerslounge.com/diy-astronomer/84726-diy-finder-guider.html

The fundamental problems with converting an 8 x 50 finderscope into a guider are attaching the camera and doing so at a point where it will reach focus. I planned to use my Meade DSI-C as a guider.

A Canadian company called KWTelescope market a pre-modified 50mm finder, but it uses the original Skywatcher finderscope bracket - see comments below for reasons why this is not a good idea.

KW Telescope Kitchener Waterloo Ontario SkyWatcher Products

I eventually found a really nice Skywatcher 9 x 50 finderscope (thanks martin_h). Unscrewing the whole eyepiece plate off the rear of finder, I held the DSI-C up to the rear of the finder and found that to achieve focus I had just 1mm between the rear of the metal finder tube and the front of the T thread on the DSI. I ordered a 2” to 1.25 inch eyepiece adaptor that also had a T thread on it from the Astroboot section of Scope ‘n’ skies ‘website and I was ready.

Luckily a friend (Chris Livingstone) has a good quality lathe and he lopped 14mm off the back of the finderscope tube so that the 2” adaptor would fit. We had a bit of a game getting the thin tube to hold in the lathe chuck. In the end he had to make a plug to fit inside the tube to hold it steady. The inner diameter of finderscope was a gnat’s whisker* (see below) less than 2” so he carefully turned the 2” adaptor in the lathe to get it down to the required diameter.

The barrel of the adaptor now slid smoothly inside the finderscope tube. Finally the modified 2” adaptor was glued in place with a very thin smear of araldite. The new shortened 50mm telescope will now accept 1.25” eyepieces and has a T thread to hold guide camera rock steady

The new 50mm telescope needed to be held in pace very rigidly if flexure was to be minimised. The Skywatcher finder bracket is sprung in one axis and is hence unsuitable for use with a guidescope. Also the extra diameter of the 2” adaptor meant that the finder would no longer feed into the bracket without taking the front lens off first. After a short search I found a very nice Televue finder bracket for sale on UKAstrobuy and sell. This is very solid and slots straight into the finder socket on the Orion 80ED.

The weight break down of this new guiding set up is as follows

50mm telescope = 327g

Finder bracket = 175g

DSI-C = 228g

Total weight = 730g

Overall I’m really please with this neat little set up. Now, to start autoguiding …

If you want to do this conversion without the hassle of cutting the finder tube and whittling down a 2” adaptor, KWTelescopes in Canada sell a special very low profile adaptor to make this conversion hassle free. (Remember when buying items like this from outside the EU to add shipping costs and you will probably have VAT and Post Office VAT collection fees to add to the cost as well). Bern at Modern Astronomy is producing a low profile adaptor similar to the Kwiqfinder adaptor. I understand this will screw straight in to the back of the finder, obviating the need to cut the end off the finder and whittle down a 2” adaptor. Bern told me that he hopes the first examples should be on sale before the end of August 2010.

(*an old imperial measurement of smallness – 14 ‘gnat’s whiskers’ = 3 ‘tinsy bits’ or 1 ‘smidgen’ or ‘smidge’= 1/16 of a ‘tad’)

Below are some before, during and after pictures of the conversion.

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Uranium235

Uranium235

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After reading this, I decided to dig out my 9x50 finderscope and attempt a non-destructive, fully reverseable adaptation for the QHY5.

All that was used were 2 aerosol can tops, some masking tape, a craft knife and a file. I found what the focus point was with the cam before creating an insert from one top (packed out with marking tape so its a snug fit) and a spacer made from the other top (which has a slightly larger diameter). The thickness of the spacer determines the focus, Ive tested it on some houses about 500m away and they come to focus ok - but ive yet to test it on stars (cos there arent any!!)... better take the file with me in case I need to make any fine adjustments :D

If it works it means I dont have to go through the hassle of swapping the ST80 between main scopes (and therefore wrecking the alignment). Thanks for the great idea!! :)

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Uranium235

Uranium235

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Update:

Had a clear patch for about 20min just, so ive fine tuned the spacer... its now spot-on. I managed to get a screen grab too, there arent any other stars as i was shooting this through what turned out to be quite a thick cloud. But its down to a point and the profile looked good, so a few more finishing touches and it should be ready for business :)

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Uranium235

Uranium235

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Thanks man, your guide was the main inspiration though :)

Hoping to give it a proper test tonight (guiding with it), but im amazed at how much weight its saved. The 6" newt feels a lot lighter.... which is welcome at the moment as ive done my shoulder in...lol.

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astrovirus

astrovirus

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After reading this, I decided to dig out my 9x50 finderscope and attempt a non-destructive, fully reverseable adaptation for the QHY5.

I have 2 small remarks, first you can also buy the seperate adapter ring that fits the finderscope threads and has T2 for the QHY5 (KWIQ adapter, KW Telescope Kitchener Waterloo Ontario SkyWatcher Products) for a non destructive pefect fit of QHY5 and scope, and second I would suggest swapping out the standard holder for the finderscope for a more stable one. Many users have found that the standard holder isn't sturdy enough.

I have opted to do both and am very satisfied with the performance so far, 5 to 10 min guided subs are no problem, although I did spend some time with finding the right PHD settings. (see other equipment below)

Good luck

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Uranium235

Uranium235

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What cal step length did you use? 1500?

I'll take a look for a better bracket, but for now I can just tape up the qhy5 cables to keep it as stable as poss. Its cost me £0.00 so far, so keeping it at that price would be great. However, if I take to it as a serious competitor for the ST80 then I might very well end up buying the adapter kit for it.

Am I right in thinking that this would not be suitable for guiding over 1000mm?

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astrovirus

astrovirus

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These are my current settings which work for me. I have seen several reports of guiding C8 and above at F/10 with these finderguiders, and those guys were really satisfied. This was amongst others what convinced me to persue this way.

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Uranium235

Uranium235

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Cheers man, will try those settings when i get the chance (hopefully tomorrow). Last night during my clouded out attempt to set up, I noticed that I can further finetune the focus by loosening the ring just below the first lens, then its just a case of screwing/uncrewing the lens to move it backwards/forwards. After youre done you can lock it in place by re-tightening the ring, that gives me a couple of mm either way to play with :)

I think you must have read my mind when you mentioned the C8...lol it was that scope that I had in mind because the CG5 would be able to take one of those + a finderguider (but not the ST80)

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MickyWay

MickyWay

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Just had this reply from Brian Dernesch (KWIQ) Re: Adapter,

'We ship our products worldwide and have many KWIQ Guider users in the UK, Europe and Asia.

The KWIQ Adapter is priced at $40CAN- shipping(small packet) will run about $12CAN', which is about £32.

Not too bad for something purpose made....if only I had a lathe...

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astrovirus

astrovirus

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The KWIQ Adapter is priced at $40CAN- shipping(small packet) will run about $12CAN', which is about £32.

Not too bad for something purpose made....if only I had a lathe...

That's exactly why I ordered one. I don't have a lathe, and furthermore I wasn't able to find the threadsize on the finderside on he internet. However, you can always measure this off course. But it's nicely priced.

@Uranium235; And yes, it works perfect, graph looks very similar to my recent guiding graphs. Are you using my setting in PHD?

Good luck with imaging

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Uranium235

Uranium235

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Yes, same settings apart from the guide pulse which I upped to 350. But thats probably down to the fact I didnt polar align my mount, I just plonked it down into the markers i drilled into the patio. It went wayward a couple of times to be honest, but that wasnt down to the guider - but my haste to set up :p

Just working on stacking last nights results - will post em a bit later in the deep sky section.

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MickyWay

MickyWay

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If you're trying to source an adapter for this mod, Bernie at Modern Astronomy is now supplying a 50mm finderscope to QHY5 camera adapter that he has tested and works very well : Accessories @ Modern Astronomy

I've asked Bernard to supply me with an adapter for the DMK21, and this is now nearly ready, so I'll post when I have more news.

Thanks, Michael, you really have started a very interesting alternative to the 'heavy' guidescope.

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