Gondwana Telescopes - a new choice in dobsonian telescopes

[AG Hybrid (Adrian)]

311,000 steps!! Wow.

[mental4astro (Alexander)]

The alt axis resolution of my 17.5" dob is 650500 steps!

And it can be four times that.

[AG Hybrid (Adrian)]

And here I am rocking a pathetic 10K. Well its been a few months since I used my 12" but when I was 10K seemed pretty good. Getting a perfect alignment was more important then the resolution. To be fair, 10K is accurate enough for a 12" in my experience to find anything on my phone or tablet. Probably because anything smaller would be too dim for me to see anyway.

[Wavytone]

Adrian I have the 311k encoders on my AZ8, thats 4 arc sec per step. They are quite simply a game-changer.

With SkySafari, there are a few interesting tricks:

- I can leave a high power eyepiece in my mak (9mm, 300X) and it will locate objects spot on every time. Forget finders - and forget swapping eyepieces. In the first night under dark skies it pulled in more DSO's than I've seen in a very long while.
- in suburban sydney skies it makes it possible to reliably find DSO's where there are simply no bright stars to use for star hopping.
- it is possible to distinguish between individual stars in a cluster. Simply impossible with 6k encoders !
- trying to find a nova in a busy star field you can be quite sure you have the right star if you reference a known star nearby.
- you can be quite sure of what is (or isn't) a moon of Saturn or Jupiter,
- you can measure the relative positions of things quite accurately, such as reasonably wide doubles - directly from the encoder positions - allowing for constant offset each time.

Lastly, and this one is a special trick - at 4 arc seconds resolution the position will update so frequently and precisely I was able to let a friend look through the mak at Saturn at 300X, while I had Sky Safari zoomed in to the maximum and used that to manually track Saturn using the image on the iPhone (this mount has manual worm drives on both axes). With low-res encoders that would simply have been impossible to pull off.

The accuracy is inherently only as good as your calibration on reference stars so frankly you really do need a large finderscope with 20X magnification and decent cross hairs to nail the reference stars to a minute of arc or better - eyeballing out-of-focus circles is simply not good enough to show what this can really do. Using a zero-power finder, RDF or Telrad etc is simply letting yourself in for disappointment later.

Admittedly there a few unknowns - the mechanical errors in the mount (the axes wont be precisely perpendicular), plus the effects of flexure in the tripod, mount and scope. The latter are significant in dobs as well as cheap metal mounts like the EQ5/EQ6. The other unknown at this level of accuracy is atmospheric refraction and so far I do not know whether SkySafari or the Nexus DSC are compensating for this. Maybe, maybe not. So far I've only had it out a few nights and not long enough to really test these aspects but what I do know is that it does reliably locate objects to better than 10 arc minutes with the AZ8. And it will do this all night from the initial calibration. No resetting, no recalibration required.

[ausastronomer (John Bambury)]

Originally Posted by Wavytone

Adrian I have the 311k encoders on my AZ8, thats 4 arc sec per step. They are quite simply a game-changer.

Admittedly there a few unknowns - the mechanical errors in the mount (the axes wont be precisely perpendicular), plus the effects of flexure in the tripod, mount and scope. The latter are significant in dobs as well as cheap metal mounts like the EQ5/EQ6. The other unknown at this level of accuracy is atmospheric refraction and so far I do not know whether SkySafari or the Nexus DSC are compensating for this. Maybe, maybe not. So far I've only had it out a few nights and not long enough to really test these aspects but what I do know is that it does reliably locate objects to better than 10 arc minutes with the AZ8. And it will do this all night from the initial calibration. No resetting, no recalibration required.

Another critical underlying factor is the construction quality of the encoder itself. Not only do errors in the construction quality of the mount and telescope come in to play, as the encoder resolution increases, the construction quality and mechanical alignment accuracy of the encoder disc and shifts within the encoder housing becomes critical. At the price point of these mega resolution magnetic encoders used on amateur telescopes (very cheap ); it's highly likely that radial and axial alignment errors and concentricity errors of the encoder disc itself will offset any gains that may have been achieved by any additional rotational resolution gains of the encoder itself. Mechanical alignment errors of a few microns even will swamp any gains by the extra rotational resolution of the encoder itself. To the extent in fact that its quite possible that in the field a high quality 10k or 32k encoder might outperform some cheaper > 100k encoders. Of course you might get lucky. There will certainly be some really well made >100k cheap encoders go out and you're well in front. There will also be some that aren't so great at these price points.

To get the mechanical construction precision required on a consistent basis to take full advantage of these mega resolution step encoders you would need to spend several thousand $$$$. They are out there by the way. You can get them from a company like Heidenhain http://www.heidenhain.com/ Unfortunately a high resolution pair will cost as much as most telescopes.

Cheers

[mental4astro (Alexander)]

John, I welcome your considered post.

From your post I see that you are not familiar with the Nexus systems nor these extreme resolution encoders. Knowing your support for good quality products, Serge from Astro Devices and myself would love to host you to try out the various Nexus systems and these extreme resolution encoders on a variety of different mounts. You will be able to familiarise yourself with the unique features of the Nexus units, and when coupled with these new encoders just how many more possibilities are opened up.

Your knowledge on astronomy is well known and respected. Serge and I would love to share in your experience too.

Alex.

[ausastronomer (John Bambury)]

Originally Posted by mental4astro

John, I welcome your considered post.

From your post I see that you are not familiar with the Nexus systems nor these extreme resolution encoders. Knowing your support for good quality products, Serge from Astro Devices and myself would love to host you to try out the various Nexus systems and these extreme resolution encoders on a variety of different mounts. You will be able to familiarise yourself with the unique features of the Nexus units, and when coupled with these new encoders just how many more possibilities are opened up.

Your knowledge on astronomy is well known and respected. Serge and I would love to share in your experience too.

Alex.

Hi Alex,

I appreciate your's and Serge's offer !

I did spend a brief amount of time with a Nexus device at the Ozsky Spring Star Safari held last October just north west of Crookwell. It belonged to one of the US visitors (I think Robert Werkman); but I am not sure which model it was. I have also spent a bit of time looking over the Astro Devices Website and browsing the user manual (can't remember which device), but certainly haven't studied any of the devices, in depth.

As you could appreciate I live over 2 hours out of Sydney. I try to avoid coming to Sydney (unless I have to) as I don't enjoy the amount of traffic and the number of people who can't drive worth squat. I enjoy the dark skies, ocean and river views and the green rolling hills offered by a rural lifestyle, as opposed to the "concrete jungle" of Sydney. I was born and lived in Sydney for 40 years; and no amount of money would get me back there
In addition, as I have got older, combined with having been around the sky a lot of times, I also have a lot less motivation to go out and observe in the middle of winter.

That all having been said I would be happy to meet up with you and Serge at a mutually convenient time and location. I am always happy to learn about new products and to see what else is out there.

If you care to send me a PM with your mobile phone number I would be happy to give you a call over the next few weeks and make some arrangements to meet up.

Cheers,
John B

[Astro Devices]

Originally Posted by ausastronomer

At the price point of these mega resolution magnetic encoders used on amateur telescopes (very cheap ); it's highly likely that radial and axial alignment errors and concentricity errors of the encoder disc itself will offset any gains that may have been achieved by any additional rotational resolution gains of the encoder itself. Mechanical alignment errors of a few microns even will swamp any gains by the extra rotational resolution of the encoder itself. To the extent in fact that its quite possible that in the field a high quality 10k or 32k encoder might outperform some cheaper > 100k encoders.

John, those encoders are priced approximately at the same level as 32K or 40K encoders from US Digital.

By the way, the encoder I use on altitude of my telescope (18" SkyWatcher) gives me the accuracy of ~12 seconds of arc. These encoders do not require any tangent arms for the altitude.

Also, you will find that US Digital do not provide the accuracy of their encoders.

Having higher resolution encoders allows for much higher accuracy if using a local sync as well - that is even if there are mechanical issues with the telescope.

Best Regards,
Serge.

[Wavytone]

John that might be true if you were trying to achieve raw blind pointing accuracy using absolute encoders - using a telescope in the same manner as a theodolite. However it is doomed to fail for the obvious reasons most amateur mounts have huge mechanical errors and flexure in the mount and whatever it sits on which is certainly into minutes if not a degree or more for dobs on wooden bases.

However there is an easier way which all of the DSC's use - and that is to use incremental encoders (orders of magnitude cheaper) and to calibrate on several reference stars. This deals with most of the unknown mechanical errors and flexure in much the way an auto guider does - by simply measuring the reference stars with the mechanical errors and flexure present.

The only thing these can't solve - and which an auto guider does solve - is atmospheric refraction.

Forget about pontificating on hypotheticals. Get out in the field and see what a scope equipped with these and and Nexus or Sky Safari actually does.

[Wavytone]

John:

What you suggest is true if you were trying to achieve raw blind pointing accuracy using absolute encoders - in just the same manner as a theodolite, and it is partly why a decent theodolite with circles reading to 1 arc second was (and is) a very expensive beastie.

But you are also implicitly assuming this is the way the DSC software works, which is incorrect.

None of the DSC's rely on absolute encoders. That they all require calibration on reference stars gives away the fact that they all use incremental encoders (orders of magnitude cheaper) or are counting stepping motor "steps" (how all those GOTO mounts work which require the axes to remain locked after calibration - Celestron CPC, Meade LX, Lightswitch). Either way its the same approach. Even the mounts with encoders invariably have incremental ones - not absolute encoders.

The point is to calibrate on several reference stars, the more the better if they are spread across the sky and encircle the horizon. This deals with most of the unknown mechanical errors and mount flexure in much the way an auto guider does.

There's also an opportunity for some smart cookie to figure out a way to calibrate these encoders as-fitted to a mount, right down to the step. I'm thinking of some sort of dingbat that could be set up in a basement. Solve that and these would be as good as absolute encoders.



The only thing these can't solve - and which an auto guider does solve - is atmospheric refraction.

Suggest that instead of pontificating on hypotheticals based on false assumptions, get out and try a scope fitted with these, and see what it can do. Even better try the trick of using SkySafari at maximum zoom to track Saturn blind, while someone else observes through a scope at 300X or more. That is simply not possible with low-res encoders as the angular resolution and feedback frequency is too low. You're welcome to visit and have a decker through my Santel at the same time.

[ausastronomer (John Bambury)]

Originally Posted by Astro Devices

By the way, the encoder I use on altitude of my telescope (18" SkyWatcher) gives me the accuracy of ~12 seconds of arc. These encoders do not require any tangent arms for the altitude.

Best Regards,
Serge.

Good evening Serge,

Thank you for your response. With reference to these high resolution encoders which do not require a tangent arm on the altitude axis, am I correct in assuming that at this time these types of encoders are only compatible with the Nexus DSC and cannot at present be used with the Nexus, or Nexus II ?

Cheers,
John B

[Astro Devices]

Originally Posted by ausastronomer

Good evening Serge,

Thank you for your response. With reference to these high resolution encoders which do not require a tangent arm on the altitude axis, am I correct in assuming that at this time these types of encoders are only compatible with the Nexus DSC and cannot at present be used with the Nexus, or Nexus II ?

Cheers,
John B

Hello John,

Thank you for your question.

The high resolution encoders can be used with any of our Nexus units - Nexus DSC, Nexus and Nexus II.

Best Regards,
Serge.

[Wavytone]

John they're also available as magnetic ring encoders which make it possible to fit these on shafts where a tradition disk-on-shaft type won't fit without major surgery to the mount. The read head is non contact - a slight gap between the ring and the head, so no force and no backlash. And they're able to be installed without a housing.

[Steffen]

Originally Posted by Wavytone

The only thing these can't solve - and which an auto guider does solve - is atmospheric refraction

Sky Safari can correct for atmospheric refraction (among other things). The setting is somewhat hidden under Settings > Precession.