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EEVA and dobsonian?


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After many years of using a Vixen 4” refractor to observe mainly the planets and moon, I’ve recently bought a 12” dobsonian (Meade Lightbridge), and have added encoders to allow it to be used with a Nexus II. The idea being to begin to visually observe some of the DSOs that I’ve only seen as images in books or on the web up till now.

I’ve been following the threads in the EEVA – Reports section with real interest, particularly the Arp, VV and Hickson galaxy groups, and have been spending days reading as much as I can find about these groups, some of which I’d not heard of before. If only the deep cloud cover would clear! I’d like to try to see some of these distant galaxies, but am concerned that I’ve bought the wrong telescope, and should have bought an SCT or RASA, on a guided mount. Is it possible to do any form of EEA with a dobsonian? Also, are people here generally using a form of video astronomy, or some form of night-vision monocular? The Televue or OVNI NV equipment is way beyond my price range, but I seem to be reading that nothing else is suitable for astronomy – is that broadly correct? Apologies if these are obvious questions - I'm a bit worried that I could have spent my budget on more suitable equipment.

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Using a camera will need a tracking mount so that the longer exposures can be built up. There are tracking platforms that you can pop a dobsonian on and it will track for a while, would allow you to make use of the aperture. You’d need to look out for reviews and what accuracy they could give you.

 

peter

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Hi David

While a few people have tried (and succeeded to a certain point) in using undriven mounts for EEVA, nobody that I know has actually continued past the point of initial experimentation. The lack of a driven mount is compounded with the long focal length of most large-ish Dobs. It just isn't a lot of fun trying to keep the object centred for long enough. So yes, you really need a driven mount. I suppose you could try your existing Dob with an equatorial platform but I don't see many people doing that either for EEVA.

On the other hand, you don't need a large aperture in the same way you would with visual work. A 6" f5 reflector is quite easy to mount and will show a lot of stuff; an 8", preferably at f4 will show more than enough interesting objects for a lifetime while producing bright images quickly. Others use smaller refractors and still others use SCTs, which are a very adaptable scopes when paired with focal reducers. RASAs are fine but few seem to use them for EEVA (and there are downsides, like inability to use a filterwheel due to excessive occlusion of the light path since the camera is positioned at prime focus, but then again, use of filterwheels in EEVA is also a minority pursuit).

The key is to ensure that any scope you choose is fast (which might require the use of a focal reducer), and to choose a sensor that is well matched, so you don't end up with blocky pixels due to excessive undersampling. And also to ensure that you can reach focus with the sensor, given the format of the camera body, and likewise that you clear the mount for those mountings/scopes where the camera is attached at the base.

My own sweet spot is a very simple system consisting of a sensitive (big pixels) mono guide camera that weighs 50g and slips into the drawtube of my 8" f4 imaging Newtonian, which is mounted in alt-azimuth. I do use a filterwheel so with a mono camera I can also do colour or narrowband without swapping anything in or out -- its a very flexible approach. It also enables me to take darks quickly.

Some people here do use NV devices, but most of the reports you mention that are accompanied by images come from people using CCD or CMOS cameras. I see PeterW has replied as I write this, so I imagine you will get some NV opinions too!

All the best

Martin

 

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The request was for non-NV. NV being real-time doesn’t care what mount you use. For faint stuff I find it an advantage to have a manual mount as you can pan around and let peripheral vision help pick things up. Of course then you have to do the finding, which can be hard as NV shows stars different apparent brightnesses, so goto can be a help.

As noted for whatever EEVA you want to do, decide what sort of field of view you want for the sorts of things you want to observe and then find the fastest optic to give you the focal length you need for the sensor you use. It might be that you need more than one system for large nebulae and small globulars and galaxies.

 

peter

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Hi David,

A 12 inch Dob is a great size for DSOs. My suggestion would be enjoy using it visually to get to know the sky. Go for galaxies brighter than mag 12 and then ponder using the EEVA technique to observe.

I had a 12 ", then 15" and finally a 20" Dob and enjoyed 20 years of visual observing but in the end I ran out of targets that satisfied me. Even in the 20" the sort of observations you see here in the EEVA section are way beyond even a 20" scope

I use a Dob for EEVA but it is fully driven as well as a SCT.

Mike

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Thanks very much for all the responses.

I'm really struck by Mike's comments that EEVA can exceed a 20" scope, and I think that is what I have been realising as I've been reading through the various EEVA reports. Nevertheless, I will probably stick with what I have, and see what how visual observing goes over the next few months - there is already a lot for me to discover, and I have already blown my budget. Hopefully we will have some dark, clear nights over the winter months. 

That said, is there an NV monocular at a reasonable price (say up to £1K?) that would enhance what I can already see with my 12" dobsonian? £5.5K is way beyond my price range!

 

 

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37 minutes ago, David J H said:

Thanks very much for all the responses.

I'm really struck by Mike's comments that EEVA can exceed a 20" scope, and I think that is what I have been realising as I've been reading through the various EEVA reports. Nevertheless, I will probably stick with what I have, and see what how visual observing goes over the next few months - there is already a lot for me to discover, and I have already blown my budget. Hopefully we will have some dark, clear nights over the winter months. 

That said, is there an NV monocular at a reasonable price (say up to £1K?) that would enhance what I can already see with my 12" dobsonian? £5.5K is way beyond my price range!

 

 

Regarding nv monoculars, unfortunately not. Even second hand you would need to pay £2-3k to get one good enough for the strenuous demands of astronomy.

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5 hours ago, callump said:

Hi Mike,

what sort of Dob and drive system do you have?

Thanks,
Callum

 

15" Truss Dob with encoders and servo cat motors, using Nexus. Much prefer Nexus to Argo Navis.

Mike

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  • 3 weeks later...

Callum you might look at OnStep as a low cost alternative to commercial offerings, it's Arduino based Open Source, emulates an LX200, and can use several different 3D printer motor control boards (I use MKS GEN-L boards on my scopes) so no soldering is involved. Implementations can be as simple as a motor controller, two steppers, and a laptop and you're off to the races.

https://onstep.groups.io/g/main/wiki/3860

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Thanks @GordTulloch - interesting stuff. Nice to see it has INDI support too. I don't really have a requirement at the moment, but just interested to see what people are actually using. I have been following the Alt-Az initiative stuff from its early days too... I have been thinking about a dedicated EEVA scope with a large mirror - but it is a bit of a day-dream project at the moment...

Callum

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On 10/11/2020 at 17:13, Mike JW said:

15" Truss Dob with encoders and servo cat motors, using Nexus. Much prefer Nexus to Argo Navis.

Mike

i hated argo navis compared to nexus. good choice

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6 minutes ago, faulksy said:

what mirror is that ?

That is a 24" Pyrex military surplus mirror 2" thick currently F2, this winter I'll put it on a machine and grind it back to F3.3 to for use as a dedicated EVAA platform for my local RASC Centre. End result will look something like this (bit of a mockup atm based on Hubble Optics UL24, currently designing the structure in Fusion 360):

 

 

NTT24EAA.png

Edited by GordTulloch
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8 minutes ago, AKB said:

No secondary mirror... How will collimation and focussing work?

Yeah as Mike says no secondary (true prime focus) so collimation is simplified. There'll be a motorized 3D printed Crayford focuser where the secondary usually is with a coma corrector and field rotator, with a three point tilt adjustment for collimation. Collimation will be as normal at the mirror end (accessible from the front.)

Phase 2 is grind a secondary mirror but this scheme gets the scope up and running as quickly as possible, the degree to which it gets used will dictate how fast Phase 2 gets completed!

Mike I love your mirror cell design on the 20", exactly what I was contemplating but you filled in some gaps thanks!

Sorry to hijack this thread!!! Move along nothing to see here :) I was just too thrilled with my recent acquisition not to share!

Edited by GordTulloch
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40 minutes ago, GordTulloch said:

There'll be a motorized 3D printed Crayford focuser where the secondary usually is with a coma corrector and field rotator, with a three point tilt adjustment for collimation. Collimation will be as normal at the mirror end (accessible from the front.)

Thanks. That’s what I needed to know!  Sorry for aiding and abetting any hijacking.

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