Newtonian Secondary downsize

[NGC 1502]

Hi.  Seeking advice from those of you with practical experience of building Newtonians.

My 8” F6 Newtonian has a 2” minor axis secondary, so by diameter that’s a 25% obstruction.  The primary has been moved away from the secondary to allow a low profile focuser to be fitted. All works fine.  When collimating it’s apparent that a smaller secondary would suffice to intercept all of the light cone from the primary.  I have a 1.5” secondary I could fit, careful measurements indicate that this would also intercept all of the light cone.  The replacement secondary would be a 19% obstruction.

Well known optical theory says that reducing the obstruction to under 20% should result in more contrasty planetary views and split tighter double stars.  I’d be reducing the obstruction from 25% to 19%.

My question is - would this theoretical improvement result in actual improvements that can be seen at the eyepiece ?  Of course I could try it and see.  But any possible improvement depends on observing with exactly the same observing conditions as previously and relying on memory is a tough call if any improvement is subtle.

Any advice or opinions most welcome 👍

Cheers, Ed.

 

 

[andrew s]

Uses "Newt for the Web" Google will find it. You can put in  your dimensions and it will tell you what results you will get. Field of view, vignetting etc.

Regards Andrew 

[johninderby]

One thing to be aware of when fitting a smaller secondary is the quality of it. Most mass produced secondarys aren’t as flat near the edges so often are oversized to avoid the edge problem.  If fitting a smaller one would get a quality one.

There are pros and cons to doing this. For the minor improvement on planetary with 1.25” eyepieces if using two inch eyepieces edge performance may fall off. Loads of arguments on CN about this. 

So while yes a smaller secondary can slightly improve contrast on purely visual panetary observing there are downsides as well.

 

Edited April 11, 2020 by johninderby

[NGC 1502]

59 minutes ago, johninderby said:

One thing to be aware of when fitting a smaller secondary is the quality of it.

 

Thanks, that’s a good point.  The 1.5” secondary I have is from a 90s mirror set by David Hinds, so it’s a good one.

 It would just intercept the light cone from the primary but use most of the secondaries surface.    I could get a 1.7” (44mm) secondary but I’m guessing that reducing from 50mm to 44mm may not be worth it.

1 hour ago, andrew s said:

Uses "Newt for the Web" Google will find it. You can put in  your dimensions and it will tell you what results you will get. Field of view, vignetting etc

Thanks Andrew.  Another good point, don’t want to lose performance at the lowest powers, nice to have a scope that works well from low to high magnifications.

Further comments welcome please.

Ed.

Edited April 11, 2020 by NGC 1502

[johninderby]

The OO 44mm one would be excellent but pushing it a bit and a fair few quid for an experiment.

https://www.orionoptics.co.uk/OPTICS/ellipticalflats.html

Edited April 11, 2020 by johninderby

[Peter Drew]

I would leave it as is.  I don't think the performance difference would justify the effort, however in these troubled times when something to do is at a premium it could be an interesting experiment, specially as you have the components to hand..  I was under the impression that central obstruction could be an aid to close double star splitting, the obstruction reducing the size of the Airy disc making the centres of the double more obvious, specially on equal bright pairs.     🙂 

[NGC 1502]

Peter, thanks for that.  I doubt you’ll remember me, but we’ve met at Kelling star party a few times and had a chat. I looked through your 6” refractor adapted for Ha solar.

BTW, I once owned one of your AstroSystems 8” F5 on a pillar alt-az.  Stupidly sold it to Fieldview astronomy B&B near Fakenham, always regret that. ( Fieldview now serve the birding fraternity ).

My current 8” is from OO, the primary is engraved 1/10th wave on the back.  It was set up for visual and imaging, so visual focus was achieved with an extension tube, it looked very odd when using one of the larger eyepieces popular these days, with the very tall focuser plus large eyepiece. Set up like that it used every bit of the 2” secondary, it being only just adequate.  

This scope splits sub arcsecond doubles as long as the magnitude is not too dissimilar and the seeing is reasonable. But of course when you get hooked on doubles you always want better !

Ed.

Edited April 12, 2020 by NGC 1502
Extra info

[Peter Drew]

Hello Ed.  I certainly do remember meeting you at Kelling.  It was such a pleasure as I had only known you through SGL up and until then.  Good to know you are still keen and active.       🙂 

[JTEC]

5 hours ago, NGC 1502 said:

I once owned one of your AstroSystems 8” F5 on a pillar alt-az

Just picking up on the above, I own one of the Astro-Systems 6” f6 altaz Newtonians and it is a little gem. Lovely mirror, simple, lightweight, cinch to collimate (rarely needed). It should be nominated a classic. I have a few scopes but it’s a firm favourite. I’d dearly like to find something similar with more aperture! Not sure where if anywhere you’d find that these days. I guess OO is now the only game in town.

[John]

My first "proper" telescope was an Astro Systems 6 inch F/6. I used it to observe Halleys Comet

 

[JTEC]

Here it is 🙂  It’s wearing an extra light shield and a Telrad in place of the original optical finder

[NGC 1502]

Hi JTEC, thanks for the pics, my 8” version was identical in design. The pillar alt-az is recognisable from a mile away, nothing else quite like it, brilliant, smooth, simple, stable.

Regretting even more selling it 😢

Ed.

[JTEC]

All of those things, 1502,  minimalist and clever.  Rob built me a 10 inch on a GEM in, I think, ‘79.  Again, in a different way, an excellent scope. As you did with yours, I sold it and later wished I hadn’t. I drove up from Kent and sold it to a bloke in Bolton. I set it up for him and he said ‘Ah, you look in at the side do yer?’ I should have reclaimed it there and then.

Leaving aside Obsessions and the like, is OOUK the only place to go now for a Newtonian with better than average optics, do you think?

 

[NGC 1502]

2 hours ago, JTEC said:

All of those things, 1502,  minimalist and clever.  Rob built me a 10 inch on a GEM in, I think, ‘79.  Again, in a different way, an excellent scope. As you did with yours, I sold it and later wished I hadn’t. I drove up from Kent and sold it to a bloke in Bolton. I set it up for him and he said ‘Ah, you look in at the side do yer?’ I should have reclaimed it there and then.

Leaving aside Obsessions and the like, is OOUK the only place to go now for a Newtonian with better than average optics, do you think?

 

 

 

I seem to recall Rob was Rob Miller at Astrosystems ?  Not certain about that....

Hmmm, your question about better than average optics.   One of my local club members has an optical workshop with full testing facilities and he knows how to use it.  He’s of the opinion that lots of massed produced mirrors are good, probably good enough so that it’s only in exceptional seeing conditions that a premium mirror will show an improvement in what’s seen at the eyepiece.   Of course this can be controversial and I understand that.  But given the return policy of most dealers, if the scope is a dog then you could return it.

The problem with that is you have to be able to try the scope in good conditions soon after it arrives, and you need to be able to accurately collimate, cool adequately.  Plus you must know how to evaluate the optics being fussy but not over fussy. Star testing can easily show optical errors that are of no real consequence in practical terms.

Don’t know if that helps 😳

Ed.

 

 

 

 

[JTEC]

Thanks, 1502. Yes, it tends to confirm what I’ve heard from others who’ve tested mass-produced mirrors, for example, those used in the Skywatcher Dobs: that many of them are quite good.  I’ve not seen actual figures but the opinions came from people who are technically expert and know what they’re talking about. I suppose there might be issues around consistency.

Peter Drew told me that the mirrors in the Astrosystems 150 f6 scopes were typically at least 1/8 wave - and certainly the images have a feel of quality.  I think it is noticeable in average conditions and perhaps has to do with smoothness, etc. Whether or not there’s a straightforward relationship between required accuracy of figure and available seeing I don’t know, in the way that there is with aperture. I can see how there might be but I suspect it’s more complicated than that - for example, with planetary observing where overall seeing might be ordinary but with rare moments of excellence that you’d not like to miss out on. I guess you wouldn’t want to be thinking ‘Well my average seeing is 2-3 arc sec, so I only need an average mirror’.  There are probably people on the forum well-placed to comment. 

I’ve owned, I think, 6 Newtonians of varying sizes over the years and, size for size, this one is a standout.  It looks like a simple design to replicate and, as your 8” shows, was scaleable. You’d have thought a 10” might be viable.

Yes, it was Rob Miller. 👍🏻

 

[John]

Rob Miller went over to the USA to work with Roland Christen at Astro Physics on some of their amazing mounts.

I believe he and Peter Drew were involved in the design of Astro Systems (Luton) scopes.

David Hinds made many / most of the mirrors for these so they are very good.

[Astrobits]

On the original question in this thread, my first query is just what size the secondary holder is. If that is the same ( or nearly the same) size as the current secondary mirror then that will need to be replaced with one as small as the new secondary mirror.

Nigel

 

[NGC 1502]

3 hours ago, Astrobits said:

On the original question in this thread, my first query is just what size the secondary holder is. If that is the same ( or nearly the same) size as the current secondary mirror then that will need to be replaced with one as small as the new secondary mirror.

Hello Nigel, thanks, that’s an important point.  

But no worries, the possible replacement secondary I have ( 1.5”/38mm ) is already mounted on a suitable holder.  If the replacement secondary was fitted to a larger mount, it would of course defeat the whole idea of downsizing.

That’s what’s brilliant about Newtonians and Dob mounts, ideal for DIY and optical tinkerers.  Can easily get OCD 😇

Ed.