Piero

I think you meant "the design of the primary mirror cell" because there isn't anything specific in the design of a primary mirror in terms of shape.

Your UTA is different because the secondary holder is assembled on the ring rather than the aluminium struts which can affect collimation at different altitudes. The same can be true for the installation of the upper truss attachments on the lower ring and in fact evolutions of the obsession telescope (still truss dobsonians though!) have departed from this design.

Besides, in your case one spider is right above the focuser light path. You also chose 18mm thickness plywood for the UTA - there is a lot of discussion in that book about this topic (wood selection, keeping the UTA as light as possible, number of plies, etc).

The mirror cell is a key component of the Kriege development, also because it's an entirely open design and there are reasons for that. There are a lot of thoughts behind that cell..

Talking about granting, I grant that you have been making a truss dobsonian from a skywatcher dob. However the fact that it is a truss, doesn't mean that it is an obsession-like telescope. There is far more than just using trusses in that book, but it looks like to me that you spent very little time reading it because otherwise you would have applied many and many of the engineering ideas reported in it. It is not my business how you built your telescope - in fact to me you can even add wings to it if that makes it more transportable to you. My point is that your pictures do not show an application of his design which includes his engineering choices and principles. You are simply building based on your own design choices.

Therefore the title here is misleading and quite a lot also. If you had a little modesty and most importantly consideration for the readers of this thread who do not have that book at hands, you would change it, because they could actually think that a truss dobsonian is like an obsession dobsonian and that isn't the case at all, in the same way as not every car is a BMW or a Jaguar just because it has 4 wheels.

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For what is worth a visible improvement to your "skywatcher"-based truss dobsonian is to replace the original springs with sturdy ones and remove the locking screws. This is a well known and quick improvement that can be found in almost any astronomy forums.

Nice to see this project moving forward.

However, I feel like the title of this thread is misleading because your project is really about transforming a SW dobson into a truss dobson. The design presented here is substantially different from the Obsession design (as well as any other professional telescope maker) from an engineering standpoint.

1 minute ago, dobbyisbest said:

Thanks Piero.  Just to clarify, are those measurements of the sides the same as the original scope that you got before the redesign, or only after?

I saw your build project and your redesign looks fantastic, btw!

Thank you.

That's correct. I didn't redo the mirror box and rocker box, but only modified their depth and improved air flow. The original mirror cell was ditched though.

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FYI, I expanded my answer above as you were posting.

FWIW, the measurements you ask for my classic 12" F6 dob made by Lukehurst and redesigned by me are as follow:

- rocker box front/back: 487mm, side: 523mm.

- mirror box side: 433mm

Note that the trunnions stick out a bit laterally on each side due to his choices in rocker box side panel thickness (quite likely 18mm in these small telescopes - although it looks like the same thickness in much larger apertures), trunnion design, and exaggerated clearance between mirror and rocker box.

Regarding your sentence "The Lukehurst dobs are also typically well priced according to the ABS archives.", well, that's very subjective. They are more affordable than obsession telescopes because they are made in the UK (including the mirror) AND they are built more cheaply. Some examples about my latter point are: 

- choice of plywood thickness, grade, provenience

- little stainless steel and often use of undersized metal (e.g. lower truss attachments)

- mirror cell made out of wood with "ad hoc" triangles and mirror edge support...

- undersized truss poles with plastic inserts to make lower truss connectors

- plastic in the secondary mirror holder

- paint over varnish in general. I didn't want mine to be painted so he applied 1 coat of sprayed lacquer where he felt necessary leaving parts uncoated (e.g. UTA, wheelbarrow handles, trunnions)

- ...

Both Obsession and Lukehurst dobsons are designed for speed production, just not in the sense of commercial telescopes. There are a few engineering aspects which can be improved in Obsession telescopes and these improvements have been applied by the telescope makers who started off from Kriege's design. Said this, a lot of thinking and sensible choices are in Kriege's design...

If you have time I would encourage you to get a copy of the dobsonian telescope book by Kriege in order to understand concepts and reasons behind the obsession telescope design. Following that you might realise that many aspects in Lukehurst design make little to no sense. How about the views? Well I saw how these were affected by direct experience sadly. On a positive note, this pushed me to understand dobsonian telescope making as well as build my 16" F4 dob (aka Nunki) and then redesign this 12" F6 to make good use of it.

Yeah, what I meant was to build the entire telescope, not just the mirror cell.

Good luck with this project!

However, why don't you keep the skywatcher mount as it is so that you have a telescope while you build an obsession clone telescope? You would gain in everything. BTW, I made two Kriege-based mirror cells with no prior experience. I'd make one if I were you. 

Well, if I were you I would either wait or get a temporary solution from the second hand market until you have moved to your new house. Doing so, you will be sure that you invest money in the right instrument for the place where you will live for a while.

Last weekend I fitted the original wheelbarrow handles to my redesigned 12" F6 dob. Now both telescopes are ready to be wheeled out from my garage with easy.

“What have you seen in a 5" Refractor, that you've not seen in an 8" newt?”

I would do solar observing with a refractor.

Apart from that, imagination..

It's possible that the myriad is a bit heavier. Ernest measured 854g (see link above).

My first 100s was a Lunt Engineering HDC. I liked it but was not hooked. At that time I used it with my 12" F6 dob and usually jumped from 30mm to 12.5mm.

I have eventually been hooked by 100 deg eps after trying the 9mm and then 7mm APM XWAs. The scope this time is a 16" F4 + PC2 and a focal length of 9mm really feels optimised with this telescope. I decided to swap my N22T4 with Steve's APM XWA 20mm to have a "matching set". Glad this exchange happened!

So for me the "dyslexic duos" are: 30, 12.5mm with the 12" F6 dob and 20, 9mm with this other dob.

5 hours ago, Saganite said:

Hi Neil,

 

I have just parted with an APM 20mm 100 deg in favour of the 2" Nagler 22mm and the weight difference was 30 ounces to 24 ounces for the Nagler 22mm .  I weighed them both.

Mm.. to me they seem very similar in weight and I'm sensitive to this in fact I don't like heavy eyepieces.

According to Ernest, the APM XWA 20mm weighs 689g (page to scroll: https://astro--talks-ru.translate.goog/forum/viewtopic.php?f=32&t=1445&sid=62f5b3f4a78b02a2dc07dc7c8da24868&_x_tr_sl=auto&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp&_x_tr_sch=http ). 

I just measured it too and got 695g. Is it possible that you didn't subtract the tare?

@Littleguy80 FYI.

Okay, my main sets are:

1. rigid case: 4 X 2" eps + barlow

2. lowepro case: 3 X 2" eps

3. Pouch: 6 X 1.25" eps + Barlow

I also have 3 Delos that I need to sell. In the pouch I have 2 other eyepieces which are kept for convenience but could also be sold.

Rigid case and Lowepro case are used together in the field, so that I don't need to put eyepieces in my pockets. The selection of these depends on the telescope used.

In Italy I have 3 eyepieces (stock plossls and 1 ortho) + Barlow.

Well done! It looks better than the original 👍

14 hours ago, ollypenrice said:

Have you tried blowing air across the surface of the mirror? There's an argument which says that this is the best way to break the boundary layer.

I've never tried it and no longer have a Newt.

Olly

No, I have not. The mirror box of my 16" is only 11" deep. My 12" has a slightly deeper MB, but this has holes to the front and back at the level of the mirror surface (not the mirror edge!) to provide passive ventilation.

Both mirror cells are based on Kriege's design, so the back is fully open.

In either cases, the views become noticeable better (easily an extra 100x and more) as the light shroud is pulled about 3" up.

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Update:

Here is a photo for the passive ventilation with the Kriege MC in my 12" F6. The mirror. Has it's lid (see dark circle). The counterweight balancing system has changed since this photo.

My 16" is shown in my profile photo.

In my truss dobsons I pull up the light shroud about 3" from the mirror box. I find this to eliminate the formation of air boundary layer above the primary mirror as this cools down to ambient temperature. I also leave the fan behind the back of the primary mirror on, but at reduced speed.

It's quite interesting to notice how much "poor seeing" is actually very local.

I observed a few galaxies in Leo last Saturday. I quite like the area around the "neck". If I remember correctly, stellarium calls them Leo quadruplet. 

Thanks @John for sharing that article. Like the author, my sky is bortle 4 and we use similar apertures.

The sky was very steady. M3 was superb at 494x with my 16" dob. There wasn't a significant image break down at 714x either. Shame that the sky became foggy around midnight.

I agree with all what Stu said above.

@Flame Nebula I certainly don't want to put you off, but have you already had some experience in imaging with your 80mm refractor?

Reading your posts I suspect that you don't. Of course there is nothing wrong with this, but if this is the case, I would advise to approach this gradually. Imaging is a hobby in the hobby. Everyone in this forum appreciates astro images, but that's only the final result or the tip of the iceberg. Behind that there is a lot of effort, often long hours taking shots, processing, etc. Some people love all of this, others don't.

Moving around an AZ6 is doable but I would not say that it is effortless.. and that's only one bit of the required equipment.

What I am trying to say is to give it a go with a simple and cheap equipment to understand whether this is really your thing.

Like you I was also interested in imaging when I came back to this hobby again, about 10 years ago. I researched half the Internet at that time regarding the equipment required for DSOs or planetary imaging. I decided not to press the button (and have no regrets about this) after mentally putting myself in the context of doing imaging. For instance, I work with computers all day and there is no way that I want to spend my free time processing data in front of a screen. I don't even like the idea of spending "at least" 30 min setting all up and then sitting on a chair watching and controlling my equipment in the dark whilst a lovely sky is above me. Let's not even mention the clouds coming and obscuring the target. I admire people pursuing this "other" hobby with pleasure. For me something like that is enjoyable as a job (e.g. as a way to get data for research), not as a hobby.

As I said above, I really don't want to put you off. My advice is just to take it gradually, incrementing your knowledge and experience with time, and understanding whether you like it or not.

To be honest with you, the same advice is also well applicable to visual observing and that's at least one order of magnitude simpler than imaging.

Hope this helps.

Piero 

If wheelbarrow handles are fitted, moving the telescope around is not tiring at all. I leave my 16" f4 assembled in the garage and wheel it out in a couple of minutes. Seriously, it takes me less time to observe with this telescope than my 4" refractor..

This photo was taken from my previous house. The telescope lived in the living room and was wheeled through that door and steps with ramps. From that door I wheeled it for about 20m (each way).

Your TV eyepieces with a coma corrector will work fine.

Looks very good! Enjoy your new focuser!

15 hours ago, John said:

I wish I could get on with binoviewers. I've tried them a few times, borrowed and even bought a set once, but I just haven't felt relaxed and comfortable when using them 🤔

I reached the same conclusion with binoculars..

My eight eye is not as good as my left one and thankfully I'm "lefty" (..like Al Pacino in Donnie Brasco) . 

I had the 5mm and 9mm Vixen SLV and thought they were very good, particularly the 5mm.

In my opinion though, it is a bit unfortunate that although good they don't exceed in any particular feature apart from eye relief. The Vixen HR are better optically, and a lot of other eyepieces are much better in terms of AFOV. Eye relief helps, but one needs to be seriously astigmatic to need 20mm eye relief below 10mm focal length, particularly with 50 deg AFOV...

The SLV would have been a boom if they had come out a decade earlier at least, I feel. In that case they would have won against TV plossls.

10 hours ago, Flame Nebula said:

I wonder if an Orion Optics VX10 might be a good compromise. Even lighter, at 11kg, and cheaper. Plus at F4.8, it may be better for planetary AP, not require an expensive paracorr. Same length as the VX12. Will 2" make a major difference, in uk skies? 

I am not an imager.

As far as I know optical aberrations and distortions appear very clear in imaging. Therefore, I am not convinced that you can get away without a coma corrector.

Aside from this, it is a question of image scale and magnification. Following that, one needs to get a mount which tracks in a very robust manner. Therefore, due to its long focal length, I think an SCT seems more suitable for planetary imaging, whereas a Newtonian could be more suitable for imaging larger objects requiring less magnification.

It would be good to have feedback from imagers.

Why don't you want to use the 12" with a Dobson mount?

To me that's the way to go, unless you want to do imaging on specific targets. Even so, you are likely better off with a smaller aperture.

At f4, you will need an adjustable chair and a coma corrector. At f5.3 you might want to add the former, but not necessarily the latter.

12" is a good all-around aperture for visual astronomy. Personally I wouldn't go with OO due to their customer service.

Any chance you can build your own dobson? Building my 16" has been one of the greatest pleasures of my life and I started from knowing nothing (not even used a drill..). If you decide to build one, I strongly recommend to get a copy of the dobsonian telescope by kriege. It gives some great insights but most importantly it teaches that everyone can build a telescope with some dedication.

Just a thought of course.

Some long lists above!

I must admit I only sold one telescope: a Lunt Ha 50mm. It was sold as I was not happy with the aperture.

That's all!