Genesis - Collimated!

[Stu]

Well, Mrs Maksutov has taken the small person to see Granny for a week, so I had some time tonight to attempt to collimate the Genesis finally.

Thanks to Olly, I knew in principle how to do it but was still unsure how it would go. I have read up on various sites of other peoples experiences to give me more chance of success. This article on CN was very useful.

http://www.cloudynights.com/item.php?item_id=2879

I made myself a makeshift artificial star from an LED torch covered with insulating tape with a small pinhole in the centre. This was placed on a chair at the end of our garden, roughly 120ft long.

I had the Genesis on my mini giro which wasn't ideal but good enough.

One of the things I had read was that collimation at f5 is critical to high power performance, something which is obvious to all you dob owners, but not something I have had much experience of, my shortest focal length scope is the f6 dob.

At low power, the diffraction patterns looked pretty good, it was only when I got to mid and high powers, checking the patterns nearly in focus and in focus that I could see where the errors were.

The procedure for the Genesis is very simple. Slightly loosen the three Allen bolts holding the lens cell onto the tube, then gently shift it in the required direction to centre the diffraction pattern. This took a fair amount of fiddling around but eventually I got to something I was very happy with. The problem I found was that when I tightened the bolts up the cell shifted, regardless of how gently I did it.

I found a solution in gently tightening the bolts, correcting the error again and tightening further. I found that the cell shifted when one of the bolts in particular was tightened first, so I did the other two first and finished with the last one. This method worked, and I am now delighted with the test pattern I'm getting.

I suspect this may shift again in future but I am confident I can adjust it again fairly easily.

I did find a jig described on CN which gets around the problem of the cell shifting when the bolts are tightened, this is something I may look into in future.

This clamps around the lens cell, and has three nylon bolts to allow fine adjustment of the collimation. Three holes provide access to the tightening bolts to allow it to be fixed firmly in place without it moving. Quite ingenious.

Anyway, looking forward to giving it a go, but forecast not looking great.

Cheers,

Stu

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[John]

Sounds good Stu and well done ! 

The NP-101 is a Petzval design isn't it ?. Presumably the rear lens set does not need adjustment ?

I had a TAL Apolar 125 refractor for testing quite a while back and thats a 6 element lens system in 3 groups - I'd really not want to meddle with that one ! 

[Stu]

Sounds good Stu and well done !

The NP-101 is a Petzval design isn't it ?. Presumably the rear lens set does not need adjustment ?

I had a TAL Apolar 125 refractor for testing quite a while back and thats a 6 element lens system in 3 groups - I'd really not want to meddle with that one !

Thanks John.

Yes, I believe all these scopes are fundamentally the same ie Genesis, SDF, NP101 etc. they have a longish focal length doublet at the front (achro in the Genesis then I think with ED glass or fluorite in later versions), with the petzval doublet at the rear which reduces the focal length and flattens the field.

The Petzval has no adjustment to it, it is just held in place by an accurately machined cell and fine thread.

Hopefully the collimation will hold with careful handling. I did wonder whether it would be possible to get something machined up which clamps the tube and the cell and provides more stability and fine adjustment. Probably is but I doubt it is worth the effort and complication.

That TAL does sound complex, definitely something best left alone! I guess it's a return to factory job if it gets out of whack?

Stu

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[ollypenrice]

Great. I was wondering how you got on with this. Really pleased that it all worked.

Olly

[Stu]

Thanks very much Olly. With my somewhat hectic and chaotic life it always takes me an age to get around to doing these things, but I get there in the end!

Thanks for all your help on this one, much appreciated.

The forecast actually looks quite good tonight so I'm going to give the Genesis a go to check it out.

Stu

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[Stu]

Well, I had some time out last night with the Genesis (and the 12"). The collimation is not perfect, but the scope is certainly performing very well now. I may have another tweak or may just leave it alone. The CA is much better controlled now too since making all the adjustments.

I didn't buy the scope with high power views in mind, but given the light nights and a desire to test out the optics, I gave Mars and Saturn a go along with a few doubles. I was using the Zeiss 2" prism which may not be ideal at this short focal length (from a CA perspective) but seemed to control scatter very well.

Mars showed some nice detail, especially with the Mars A filter. I do like the colour and contrast this filter gives. The North polar cap, Syrtis Major, Hellas frosting and a few other dark areas were visible at x166 (3mm on the nag zoom), very nice. I could also detect the 92% phase which was good to see.

Saturn showed well too, seeing was mixed so Cassini wasn't clear in this scope, although it was jet black in the 12"!

I split a few doubles too. Izar was nice at x125, and just about split at x100. Porrima split at the higher powers, and the double double also split very nicely. I reckon the scope is probably at 90% at the moment in these types of targets but will be fabulous on widefield under a dark sky.

As said, possibly a bit more tinkering to do, but may leave that until I can make up a proper jig for it. Pretty happy for now. Not wishing the summer away, but some dark August skies on the Veil could be lovely :-)

Stu

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[Joves]

Sounds great, Stu. Very glad to hear you're starting to get some views that are near to what you hoped they would be when you first purchased this scope.

I've been spending a lot of time with my Genesis SDF and find it to be a real gem. Has fast become my most used scope, it is just a really nice all-rounder.

[Stu]

Thanks Aaron, great to be making some progress at last! Just needed a little time to myself to concentrate on it, wasn't too tricky. With a little bit of refinement it will be cracking!

The SDF sounds lovely :-)

Stu

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[Stu]

Btw, what is the difference between the SDF and the TV101? They seem quite similar?

Stu

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[Stu]

Finally had a decent clear night with no other commitments last night, and had a nice long session from 11 until 1am with the Genesis and TV85 side by side on my Giro-WR mount (more of which elsewhere).

I had thought the Gen needed a tweak on the collimation but last night, after acclimatising for a while it seemed pretty much bang on, very nice diffraction patterns. I had a look using a continuum filter to just show the green and everything looked centralised even at high mag.

I spent most of the time playing around with different configurations - diagonals/prisms and balancing systems and just getting used to the scope with different eyepieces.

I know that the 17 ethos produces lovely results in this scope, a 3.4mm exit pupil for a nice dark sky, with a 3.4 degree fov.

I was quite surprised however that, despite the Genesis' reputation for a flat field, I was still getting field curvature with the ethos. I popped my TS flattener into the front barrel of the 2" prism and bingo, nice sharp stars all across the field :-)

In terms of observing, I saw lots, and very little if that makes sense. I spent most of the time just panning around Cygnus and Lyra, taking in all the wonderful clusters and asterisms in that area without worrying about identifying them.

M27 looked very nice indeed with UHC filter, surprisingly bright. The double double split at x83 so I'm happy that the optics seem to be in good shape, and my old favourite Izar also looked nice.

The CA seemed much reduced from pre re-alignment sessions which is great, and it is barely noticeable except on very bright stars. The star colour in clusters is very pronounced, and lovely to see.

So, finally happy! :-) I feel like this is now a scope which is working well, and I know how to get the best out of. The Baader Prism worked very well in it, despite the short focal length, and to me, improved the sharpness noticeably. I intend to get away to some dark sites this autumn and winter to properly trawl the Milky Way :-)

Stu

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[Joves]

Great news, Stu. Glad to hear it all seems to be sorted with the Genesis.

Not long ago, I finally managed to get my SDF out under quite dark skies and was amazed. The view was stunning and kept me having to remind myself that I was only looking through a 4" scope.

Look forward to hopefully hearing more reports of your time with it.

Aaron

[Stu]

Thanks Aaron. I really intend to use this as a grab and go scope and get out to some dark sites. It's the sort of thing that I can 'chuck' in the car easily, and it is light enough to go on my photo tripod and a Giro-WR mount which is super portable.

I do keep thinking that it is probably the short focal length which allows you to get lovely wide fields without over large exit pupils that really makes a difference. I keep seeing thing which are surprisingly bright and clear even under my poor skies, so really looking forward to going somewhere decent.

Stu

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