Fun with a largish newtonian

[John]

I was having a few peeks and a play with one of these last night. It's a JMI NGT 18" newtonian. Interesting design that was on sale during the early 1990's I believe. Compact and innovative equatorial mount design that tracks, when the motors are working (not last night !). Quite a handful to actually use and the eyepiece height is not friendly unless a small set of steps is at hand although the upper tube assembly does rotate to move the focuser and finders to the best position. I only managed to see the moon and Jupiter last night because of cloud cover but the views were quite impressive. The scope belongs to my society so I will hopefully get some more opportunities to observe with it under darker skies

JMI did a 25" scope as well

[Alan White]

Interesting scope John.
For a minute I thought you had managed a US based star party trip on the quiet.

[John]

1 minute ago, Alan White said:

Interesting scope John.
For a minute I thought you had managed a US based star party trip on the quiet.

No but it's a nice idea. I fancy this one in the Florida Keys

http://www.scas.org/Home/WinterStarParty

[Alan White]

A Winter star party in tee shirts and shorts, now that's most unfair.

[cloudsweeper]

Very interesting design that, John.  I like the idea of the rotating secondary assembly, but just wondered whether it might impact unfavourably on collimation.  Any thoughts or experience on that?

Doug.

[John]

42 minutes ago, cloudsweeper said:

Very interesting design that, John.  I like the idea of the rotating secondary assembly, but just wondered whether it might impact unfavourably on collimation.  Any thoughts or experience on that?

Doug.

I haven't had enough time using the scope as yet but I have read reports that the rotating upper tube assembly can affect collimation. Apparently while the rotation mechanism works, it's none too precise and adjustments to collimation can be required after rotation.

 

 

 

[cloudsweeper]

Just now, John said:

I haven't had enough time using the scope as yet but I have read reports that the rotating upper tube assembly can affect collimation. Apparently while the rotation mechanism works, it's none to precise.

 

Glad you said that John.  It shows I'm getting to grips with this fabulous pastime!  ?

Doug.

[John]

Sir Patrick Moore's 15" Fullerscopes newtonian has a rotatable upper tube section as well but I suspect that was a little better engineered:

 

[25585]

BAS must be quite large and well funded. Does it have its own private dark site, or special access to one away from public access?

[JeremyS]

2 minutes ago, 25585 said:

BAS must be quite large and well funded.

I was a member in the early 80s and it was quite big then. We met in the HH Wills Physics Laboratory at the University. Happy days!

[niallk]

Cool!

I remember seeing that Skywatcher were developing prototypes with that kind of mount for 10"-14" newts a few years back, around the same time that the stargate structure was also being prototyped for larger apertures, but then shelved it.

While an interesting design (allowing astro photography?), the SW mounts did look possibly heavy/cumbersome imho.  The JMI looks more streamlined!

[John]

21 hours ago, 25585 said:

BAS must be quite large and well funded. Does it have its own private dark site, or special access to one away from public access?

It's about average now in terms of membership numbers, compared with other societies. Not particularly well funded. The observatory site we have is a small piece of land rented from a farmer at a "peppercorn" rent. Darker than if you live in Bristol but not as dark as we would ideally like. We have a 4m "homebrew" dome with an interesting old 12" newtonian in it and the 18" NGT in a massive roll off shed type structure. We are in the process of updating all this but it will take time. 

We do get some excellent speakers (Prof Chandra Wickramasinghe last week) and do quite a bit of outreach which I really enjoy

 

 

[25585]

23 hours ago, John said:

Sir Patrick Moore's 15" Fullerscopes newtonian has a rotatable upper tube section as well but I suspect that was a little better engineered:

 

Shows well how big an aperture 15" is, for anyone considering over 12. My dodgy shoulder twinges at the thought. 

[John]

3 hours ago, 25585 said:

Shows well how big an aperture 15" is, for anyone considering over 12. My dodgy shoulder twinges at the thought. 

I'm pleased to be able to use the BAS 18" from time to time. 12" is the largest I can manage at home and that has to be an Orion Optics based dob to keep the weight manageable. It's my knees with me

[JeremyS]

12 hours ago, John said:

It's my knees with me

Too much praying for clear skies, John?

[John]

6 hours ago, JeremyS said:

Too much praying for clear skies, John?

Begging to be allowed to keep 6 telescopes when I'm supposed to have a 3 scope limit more likely .....

[Peter Drew]

On 24/05/2018 at 18:28, cloudsweeper said:

Very interesting design that, John.  I like the idea of the rotating secondary assembly, but just wondered whether it might impact unfavourably on collimation.  Any thoughts or experience on that?

Doug.

I've built large reflectors with rotating upper assemblies and it is indeed tricky to engineer them with minimal collimation effect in use. I subsequently made them with just the secondary mirror rotating. Four focusers, three visual and one photographic were used, the diagonal shaft had click stops for accurate rotation and each focus mount was collimated to suit its position. Seemed to work ok.   ?