lightbidge or orion

[Brian W]

Hi all, once I complete the H-400 I will be moving onto a new observing project which is to replicate what John Herschel was seeing while in South Africa. His primary telescope was a 20 foot f/l, 18" metal mirror with a reflectivity of 67% on a good day.

I have been narrowing down the options and two mass produced scopes are leading the pack; a 16" Meade Lightbridge and the 14" intel from Orion.

I live in the Philippines so bringing in spare parts or needed upgrades is expensive.

Anyone care to voice a preference:shrug:?
Brian

[wavelandscott]

I think mechanical quality will be similar (setting aside the intelliscope option).

I'd probably go for the extra 16 inches over 14 inches bigger mirrors deeper views.

[Brian W]

If deep was the primary aim 16 would make more sense but replicating what Sir J. Herschel is the aim and the 14 might do it as well without the balance and other issues the Lightbridge may have.
Brian

[dannat]

are you going to get the intelliscope? or standard - thr aust company sirius optics has big specials on the intelliscopes at the moment

[Quark]

Hi Brian,

I have a 12" LB but I think the more conventional design of the Orion utilizing 4 sets of struts is far superior. I still cannot believe that Meade would go with only 3 sets of struts. It is possible for the LB to hold collimation but I have to be very gentle with it.

I note that many seem happy with the LB design, perhaps I am less tolerant than most and I expect my scopes to perform. Too many amateurs make excuses for poor performance and blame the seeing instead of the short comings of there scope or mount.

I think using only 3 sets of struts would become an even greater issue when going to a larger aperture.

I have a 16" Meade Starfinder on my own GEM in my observatory, it is highly modified and works well, the difference in mass between the 12" and 16" primary mirrors is substantial. The difference in size and mass of the secondary mirrors is even more significant, definitely requiring a superior truss design to that of the LB, at least in my opinion.

Regards
Trevor

[Brian W]

To answer the questions and comments, intel is not high on my list but Orion's only 14" truss is an Intel.

Buying is still a way off yet as first I need to build the observatory and finish the H-400 (239 caught as of last night)

I find many people agree that the larger LB's are not so good. I have had no complaints with either construction or optics of my 8' LB but it seems to be the one they borrow parts from for the bigger ones.

Again though let me emphasize what I am looking for is a modern scope that will get me close to what Sir John Herschel was able to see with his 18" metal mirror with a 20 foot f/l.
Brian

[dannat]

richard at snake valley wasn't all together happy with his 10" LB

[Quark]

Brian, there is no modern mass produced scope that will go within a bulls roar of comparing with Sir John Herschels 18" creation. No doubt you will be able to purchase a scope of comparable apertures but it will have nothing like the focal ratio. Herschels scope you talk about was approx F13. the performance of it would be nothing like an F 4.5. The FOV would have been nothing like a modern F 4.5 and the images would not have been as bright as a modern scope of similar dia but faster focal ratio.

Cheers
Trevor.

[Brian W]

Hi Trevor, you summed up my dilemma quite nicely. Unless I go the replica route it is not going to be right in whichever way I choose to ignore. I can get the reflectivity close by downsizing the mirror but the fast versus slow is a serious problem. It has been suggested that if I went with an ep with a very narrow fov it would bring the balance in a bit.

Perhaps if I tried this from a different direction? J.H. describes the Triffid Nebula as having a line of 3 faint stars in its core. Stellarium software does show the 3 stars but my 8" reflector doesn't. (on a good night it does show 6 in the trapezium)

In your opinion what would be the minimum size reflector that would allow me to see the 3 :question:
Brian

[Quark]

Brian, not sure that this problem can be approached from this sort of comparison with a smaller faster mirror.

Just ran the spec's of Hershel's scope through a scope performance spreadsheet that I put together a long time ago.

His 18" would have had a limiting mag of about 15.8 with a theoretical resolution of 0.3 arc sec's. Now, I am very confident that from Slough in England this theoretical limit of resolution would never have been achieved.

I think that your best bet maybe to look for a scope with similar limiting mag. 14" aperture will give a theoretical limiting mag of 15.3 while 16" will be 15.5

Next problem is that, due to his long FL and his skill at making eyepieces he was able to use his scope at very high magnification and according to his observing notes, recorded by Williams sister Caroline, was able to use magnifications of over 1000 x.

Your problem is therefore coming up with a scope of similar limiting magnitude and capable of very high magnification. Obviously fast focal ratio and high magnification are not commodities that go together.

Sorry about this, I feel like I haven't helped one bit.

Regards
Trevor

[Brian W]

His 18" would have had a limiting mag of about 15.8 with a theoretical resolution of 0.3 arc sec's. Now, I am very confident that from Slough in England this theoretical limit of resolution would never have been achieved.

I think that your best bet maybe to look for a scope with similar limiting mag. 14" aperture will give a theoretical limiting mag of 15.3 while 16" will be 15.5

Next problem is that, due to his long FL and his skill at making eyepieces he was able to use his scope at very high magnification and according to his observing notes, recorded by Williams sister Caroline, was able to use magnifications of over 1000 x.

Your problem is therefore coming up with a scope of similar limiting magnitude and capable of very high magnification. Obviously fast focal ratio and high magnification are not commodities that go together.

Sorry about this, I feel like I haven't helped one bit.

Regards
Trevor[/QUOTE]
Hi Trevor, actually you and the others are being quite helpful.

However two small points; (1) I am attempting to replicate John Herschel not William Herschel. (2) In particular John's work in South Africa. Not that these points change any of the math just that I like to nit pick. :)

I do not have the bank roll to build a true replica so store bought is my reasonable option. Everyone seems to agree that a reflector with a reflectivity in the mid 90's and an aperture of 12 / 16" with a narrow fov ep is about as close as I can get.

One person with a nasty sense of humor even suggested buying a good 18" mirror and attacking it with steal wool. A slightly nicer suggestion was a good 18" mirror and a moon filter.
Brian