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  #81  
Old 23-09-2017, 05:49 PM
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Atmos (Colin)
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Quote:
Originally Posted by troypiggo View Post
I think Mike is showing everyone in the deep sky images section of the forum what an Orion Optics UK focal length around 1100mm can do in the right hands.
As I said, SOME made it out without issues
There are a LOT of threads over several forums about issues with Orion Optics UK telescopes that have had primaries and secondaries with really bad figures, mirror cells that cannot hold collimation over 25į movement, focusers missing parts, the list goes on.

When they get it right they can produce an excellent telescope but you are taking a very expensive gamble as to whether you'll have to send it back at your cost for repairs or it'll work out of the box.
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  #82  
Old 23-09-2017, 06:24 PM
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codemonkey (Lee)
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Originally Posted by Atmos View Post
There is surprisingly very little difference.
Saxon/Skywatcher/Orion are all owned by Synta and therefore exactly the same except for their decals.

GSO is its own, has been around for quite some time although big in the western world, pretty sure they provided the optics and components for the Meade Lightbridge series way back when though. We've only started hearing about GSO quite recently but they've been around for a while.

TS is probably the oddball in all this mix. Their baseline reflectors are all GSO but they do allow for customisation on their ONTC series which can either use GSO mirrors or higher quality ones from another German company (used to be Orion Optics UK but their quality control has really gone down the gurgler).
TS get their carbon fibre tubes custom manufactured, not sure how they compare to the likes of GSO or Synta ones. They do use GSO mirror cells but have some custom secondary supports.

Just to clarify, there is Orion which is Synta and then there is Orion Optics UK which is different... KEEP AWAY! That's about all I can say. Although some good scopes do make it out of there, there are more negative than positive reports/reviews of their products. Customer support appears to be next to non-existent.

Also to clarify, the TS newtonian are largely just rebadged GSO but allow for greater customisation. Their customer support is one of the best in the business.

So to actually answer your question, there isn't much of a difference. When it comes to the non-truss newtonian, the TS ONTC line is possibly a bit better quality as I believe (although not entirely sure) it may have a stiffer CF tube but it has the benefit of having them customise it a bit. You can get them to put on a Moonlite or Feathertouch focuser in factory, you can pay a bit extra for a higher quality primary or get stronger secondary supports or a double vein if you like.
I bought the TS ONTC because I get the impression that GSO and other cheap newts all require a significant amount of tweaking to overcome flex and various other issues. Whether the ONTC proves better remains to be seen, but I'll probably never know because I've never owned a cheaper newt.

Even within the ONTC line there's differences. Some of them come with GSO mirrors, some of them (more expensive ones) come with a OOUK primary.

The ONTC has a 5-6mm thick foam-core carbon fibre tube said to be much stiffer / better than the tubes found on cheaper newts.

Seems that OOUK used to have a good reputation but it turned to **** after a while, particular with the mechanics of their scopes. Supposedly they still produce good mirrors but QA is not so great. TS test the OOUK mirrors themselves to verify that you don't get a bad one. Mine came with a report detailing 0.99 Strehl, but the report came from OOUK and if you do some research you'll find that their reports seem to be largely worthless.
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  #83  
Old 23-09-2017, 07:05 PM
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Originally Posted by codemonkey View Post
.....but QA is not so great. TS test the OOUK mirrors themselves to verify thavt you don't get a bad one. Mine came with a report detailing 0.99 Strehl, but the report came from OOUK and if you do some research you'll find that their reports seem to be largely worthless.

I helped someone recently after he was struggling with an Orion UK telescope. To cut a long story short the optics were rubbish, and Orion blamed the customer! Would not touch them with a barge pole. Worst business morals Iíve ever experienced. GSO while not an A list product, at least will replace a dud at no expense to the customer and kudos to them for that.
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  #84  
Old 23-09-2017, 07:07 PM
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I'd go with the skywatcher 10" cf or TS carbon fibre. get a feathertouch or moonlite, good coma corrector and investigate barlows if you wanted the extra focal length.

as for the cheap options the build quality of my 12" skywatcher is way better than my 12" GSO (steel tube however).

didn't know that OOUK went downhill thats interesting.
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  #85  
Old 23-09-2017, 11:47 PM
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Quote:
Originally Posted by Atmos View Post
It's more to do with image scale than focal length. For your camera something around the 1000mm range would be good. So a 8" F/5 or 10" F/4 or 12" F/3

http://www.astrobin.com/full/287438/D/
This was shot at 677mm on a night of awful seeing.
I remembered this article from way back which has a great nomogram showing the relationship between pixel size, focal length and image scale.

Interesting article as well.

http://www.skyandtelescope.com/astro...ocal-reducers/
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  #86  
Old 24-09-2017, 09:25 AM
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I generally operate off of sample images I like and what was the gear used. A very objective empirical approach rather than theoretical.

Which images do you like that were taken by gear you can afford is the million dollar question?

In your example 10 inch carbon fibre RC. I have seen many excellent images from these scopes.

Generally, smaller sensors are more forgiving of the optics. Large sensors start testing the correction of the optics and the strength and lack of flex of the focuser and frame of the scope.

Avoid closed tubes due to thermal currents if you can. Or at least make sure that have fans.

My Honders has a 14 inch tube and the optics are 12 inch plus it has fans as well as removable back cooling plates. The point being great care has been taken to manage thermal tube currents. Keep that in mind when selecting an RC type scope.

Greg.
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  #87  
Old Yesterday, 03:10 PM
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Yes this is a really useful thread, dare I say 'award winning! 😁

I've heard from one technical expert how bad Orion Optics can be, having to recoat a new expensive mirror! And another industry supplier saying pretty disappointing things about their tactics. Just rumour of course, but enough for me to be glad I've never used them.

At these longer focal lengths, once off focusing at start is not possible. Temp changes cause the focus point to vary too much
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  #88  
Old Yesterday, 03:32 PM
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Quote:
Originally Posted by Peter Ward View Post
The houses were not a few hundred metres away....several kilometres in fact.

I thought they pretty clearly showed how aperture influences image resolution.
(sampling between the two systems by dumb luck was almost identical)

Deep sky imaging simply has a few more layers of image abberration by looking through a tad more atmosphere...

Earth's atmosphere is nearly 500 km thick ( even if most of it is within less than 20 km of the surface. )
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  #89  
Old Yesterday, 05:31 PM
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This is a very interesting thread. Thank you.
May I ask for some suggestions please? I have been imaging with a 6" RC reduced to about 1000 mm with a reducer. With my atik 460 the pixel scale is about right for my seeing (0.9 arc sec per px). However I have recently acquired a refractor which has only slightly shorter focal length so I feel that it is now redundant and I would like to take this opportunity to upgrade it to something with longer FL so I can image galaxies that are even smaller. I am thinking about a 8" RC with a flattener will give 1600mm at 0.6 arc sec per px, or shall i jump straight to a 10" for 0.46 arc sec per px? Would that be way too oversampled? I guess with the 10" I have the option to use a reducer to bring it to 1600 mm focal length but at a faster speed, but the jump in cost is quite substantial esp as the 8" carbon tube is on special at the moment.

Ps. I don't like imaging with my edgehd because the few times I tried it it dewed up so quickly and mirror flop was a bit of a pain. It will cost me almost as much to upgrade the focuser to something decent than to buy a new 8" Rc OTA and move the moonlite from my 6" to 8". I will likely keep it for visual or planetary only or to sell it.
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  #90  
Old Yesterday, 05:57 PM
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Quote:
Originally Posted by Slawomir View Post
Earth's atmosphere is nearly 500 km thick ( even if most of it is within less than 20 km of the surface. )
Having spent the last 30 years operating airliners in the tropopause Iím fairly familiar with how density decreases with altitude

(interestingly, if a turbofan could still produce the same thrust at altitude as it does as sea level, there would be enough thrust there to get to orbit)

I digress, adding extra image abberation in by pointing an optical system vertically doesnít change the systemís intrinsic resolution. It all still goes south from the starting datum.....
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  #91  
Old Yesterday, 06:01 PM
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I'm sorry.... I seem to be losing the plot here...
The resolution obtained in long exposure images is limited by the seeing conditions, not the theoretical optical resolution of the optical system.
Over sampling I think is generally better than undersampling..
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  #92  
Old Yesterday, 06:34 PM
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Quote:
Originally Posted by Merlin66 View Post
I'm sorry.... I seem to be losing the plot here...
The resolution obtained in long exposure images is limited by the seeing conditions, not the theoretical optical resolution of the optical system.
Over sampling I think is generally better than undersampling..
It’s not that simple.

Seeing affects smaller apertures differently to larger ones.

While the larger aperture typically presents an image that fuzzes in and out, it remains positionally stable on the sensor.

The smaller aperture images presents less perturbed airy disk, but it is positionally unstable.

Seeing clearly varies from night to night....but to assume 2 arc sec is as good as it gets is nonsense. Some nights fleetingly present sub arc second seeing.

Having the aperture to gather signal quickly in those conditions is simply another reason as to why aperture rules

Last edited by Peter Ward; Yesterday at 06:35 PM. Reason: dammed autocorrect!
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  #93  
Old Yesterday, 06:57 PM
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Peter,
I'd be surprised that those " fleeting moments" can actually impact on a long exposure image....lucky fast frame imaging is a different issue.
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  #94  
Old Yesterday, 06:58 PM
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Quote:
Originally Posted by Peter Ward View Post
Having spent the last 30 years operating airliners in the tropopause Iím fairly familiar with how density decreases with altitude

(interestingly, if a turbofan could still produce the same thrust at altitude as it does as sea level, there would be enough thrust there to get to orbit)

I digress, adding extra image abberation in by pointing an optical system vertically doesnít change the systemís intrinsic resolution. It all still goes south from the starting datum.....
For the record, I'm converted and saving for the biggest telescope my current mount can handle. One beautiful day, it will be either a fast 10" Newton or a 10" RC.
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  #95  
Old Yesterday, 07:13 PM
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Quote:
Originally Posted by Merlin66 View Post
Peter,
I'd be surprised that those " fleeting moments" can actually impact on a long exposure image....lucky fast frame imaging is a different issue.
A big decending high pressure cell (the kind that usually causes fog in the mornings) gives hours of marvellous seeing. I am surprised you do not seem to have experienced these conditions during a run of imaging sessions.
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  #96  
Old Yesterday, 07:29 PM
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My practical experience is in holding a target star image for subs of >10 min with a C11 at f10 on a slit gap of 20 micron - this is less than the average FWHM I experience.....
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  #97  
Old Yesterday, 07:58 PM
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Quote:
Originally Posted by Merlin66 View Post
My practical experience is in holding a target star image for subs of >10 min with a C11 at f10 on a slit gap of 20 micron - this is less than the average FWHM I experience.....
What can I cay? You definitely need bigger+ faster scope
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  #98  
Old Yesterday, 08:04 PM
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I agree with the larger aperture, but slit spectroscopes need f10. They don't work effectively at faster f ratios...
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  #99  
Old Yesterday, 08:55 PM
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Quote:
Originally Posted by Merlin66 View Post
I agree with the larger aperture, but slit spectroscopes need f10. They don't work effectively at faster f ratios...
Really? I used a SBIG self guiding spectrograph (18 micron slit) for about 12 months on a 14Ē F8 system. The spectrograph was somewhat immune to F ratio, (worked well at F6) but I found dispersion from the grating made aperture my friend.

TOTALLY off topic BTW.....
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  #100  
Old Yesterday, 09:08 PM
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Just to finish the OT discussion...
SBiG no longer manufacture spectrographs...
The current commercial Littrow instruments (Shelyak) are optimized for f10.
The point was regards the FWHM size of the target star image in long focal length instrument.
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