Refractor Fanboi drives a 16” dob…

[Stu]

I’m a bit of a refractor fanboi really, but have always appreciated the benefits of larger aperture scopes for deep sky observing as they allow for much larger image scale whilst maintaining surface brightness levels. In addition, the extra resolution really counts for objects like globular clusters.

I acquired this scope about seven months ago and due to the general craziness of life, moving house etc, last night was the first time I used it. I thought about trying tonight instead as the forecast is better, but I have a 4am start on Wednesday….

The scope is home built, but to a decently high standard, and seems loosely based on an Obsession type design. The Alt bearing folds down too so it is quite compact when disassembled. It also has very handy latches to lock it in a flat position for lifting and transport. I bought it with an EQ platform which allowed me to sell another one on, so the net price was very reasonable for a 16” truss dob. 

Assembly was easy enough, about fifteen to twenty minutes but will be faster next time as I get used to it. The whole thing feels very rigid and with less flex than previous Sumerian scopes I’ve had (16” Canopus and Alkaid), but then it is a lot more substantial so it’s to be expected. The design is a little unusual in that the rocker box is actually quite light, but then you add a series of counterweights underneath the mirror to balance it up. This generally works well, but I found with heavy 2” eyepieces and a Paracorr, it does actually need more weight at the back which I will sort.

The motion was ok, but more jerky than ideal so I will have to work on that, plus it was tending to pull down at lower altitudes although still holding position but it made accurate pointing harder than it should have been. When I bought it, it had a small RDF but I replaced that with a Telrad mounting which works well, mounted up high, above the UTA it was easy to sight through.

Collimation was a bit of a challenge, and I’m not sure it is correct yet, it’s mainly the secondary position which needs work. The secondary is HUGE, it seems much bigger than is needed so that’s certainly something I will look into changing in future but it works for now so I’ll just get some use out of the scope before making any changes. The focuser is an old dual speed Moonlite which seems perfectly serviceable. It stuck a bit initially but is now running freely and will be fine I think.

I wasn’t expecting massively high optical standards from this scope as the mirror is of unknown origin and quality, but overall it seems quite acceptable for some deep sky work from dark skies, and I will  easily be able to get this down to the coast for some sessions possible within a few weeks, if not then in the autumn. I’ll examine the star test in more detail once I’ve nailed the collimation. First impressions are good though, the Trapezium was well defined and the E star showed up ok, but I see this as more of a low and medium power scope than high power for planetary or doubles.

I tried first without the Paracorr to see what it was like. At f4.2 I was expecting coma and yes, there was plenty there in a 21mm Ethos. Adding the Paracorr (Type 2, recently acquired) tightened things up very well indeed, sharp stars to the edge once I sussed out the correct setting, which handily enough seems to be ‘A’ for the 31 Nagler, Ethos’, XWs and Leica zoom which makes life very easy. The focuser handled the weight without any sag or slop which is great.

The skies last night were pretty bad in terms of transparency. I could barely see the sword stars in Orion so I really wasn’t expecting much visually. So it proved really, galaxies such as M81 and 82 were there but very muted. 82 showed some mottling in the centre and was much larger than I usually view it but will be so much better with darker and more transparent skies. I viewed M51 as two cores with faint halo, and even M42 was not what it should be.

Best objects of the night were M97, The Owl nebula which was large and showed hints of eyes, and M3 which was beautifully resolved right into the core, but again would have been stunning had the skies been better. The Auriga clusters looked lovely as ever, large in the eyepiece and filled with plenty of stars, their distinct shapes and different characters were clear. I love the bright orange star in the middle of M37.

The full list of what I observed is:

M81/82, M42, M35, 36, 37 and 38, M51, M97, NGC2169, M65 and 66, Double Cluster, M3 and the Eskimo nebula.

I also tracked the ISS for a few minutes using the 21mm Ethos at x80, and picked up some good detail.

So, this was generally just a test out session to see that I have a working scope, and I do, so next step is to get is under a decent, transparent sky and see what it can do. Watch this space 👍

 

[tomato]

Great post. I bought my first visual scope last year, (a 16” SW flextube Dob, on a if some is good, more is better, approach to visual) and yes, I have fallen foul of the big scope, a pain to set up, syndrome.

As an imager I find I want to use it more when the lighter evenings are here as I have more time to set it up rather than rush to get the imaging rig going, and then I don’t want to start moving it and setting it up in the dark. I know that  doesn’t stand up to logical scrutiny, but that’s the mindset I have got into.

[Stu]

2 hours ago, tomato said:

Great post. I bought my first visual scope last year, (a 16” SW flextube Dob, on a if some is good, more is better, approach to visual) and yes, I have fallen foul of the big scope, a pain to set up, syndrome.

As an imager I find I want to use it more when the lighter evenings are here as I have more time to set it up rather than rush to get the imaging rig going, and then I don’t want to start moving it and setting it up in the dark. I know that  doesn’t stand up to logical scrutiny, but that’s the mindset I have got into.

Thanks. Yes, it can be a challenge getting the motivation to put a large scope out. I need to get myself more organised, but last night wasn’t too bad; I used a garden trolley to take everything down to the bottom of the garden so there wasn’t too much lugging to be done. It’s probably no worse than setting the AZ100 and 130mm LZOS up, but psychologically it’s harder for some reason.

I also need to make a screen to shield my eyes from the lights down one the local leisure centre and supermarket which stopped me getting dark adapted, oh, and persuade Mrs Stu not the put all the lights on!! 🤪🤪

[Alan White]

Who would have thought it, @Stu a fully paid up member of the Dob Mob! 😉

Looks a really nice scope, the secondary holder is an interesting design.
I hope your newer skies are a mjor bonus with that size aperture.

 

[fwm891]

@Stu Great write up. I finished building a 16 inch with John Nichol optics late last year and really enjoy getting it under a clear sky. I'm hoping this haze clears today so I can have another session.

Clear skies

[Louis D]

Looks like the designer chose a path similar to Hubble Optics for the upper cage.

Edited March 22, 2022 by Louis D

[niallk]

Congrats Stu, looks like a very nice scope, very Obsession UC-esque!

Looking fwd to hearing more

 

[Stu]

19 hours ago, fwm891 said:

@Stu Great write up. I finished building a 16 inch with John Nichol optics late last year and really enjoy getting it under a clear sky. I'm hoping this haze clears today so I can have another session.

Clear skies

Thank you 😊. Yes, I’ve seen your lovely dob. The Nichol mirror will be excellent, if funds ever allow I would consider upgrading this one but I’ll see how it goes first. Plenty of other mods required first!

[callisto]

Lovely scope Stu 

[ollypenrice]

There's always good work for a large Dob. When I read your statements about more weight being needed behind the mirror, I thought of my own years with 20 inch. On mine, I fitted a counterweight arm outboard of the mirror box and extending below the mirror, but the low profile of your scope wouldn't let such an arm clear the ground. It's admirably compact. Enjoy the view!

Olly