Doubts about Dobsonian telescope, GO-TO, equatorial mount

[Perene]

Folks, I am thinking of buying a 254mm Dobsonian telescope, but I found the same model with GO-TO.

My primary use would be for astrophotography with the ASI120MC camera. The store said that if I buy the GO-TO model I would not be able to plug an equatorial mount later.

And if I purchase the cheaper one (without the GO-TO feature) there is no way to include the GO-TO (at least not with them).

I have some questions about all of this:

- I heard some users were able to take pictures from planets such as Saturn and Jupiter using the cheaper DOB, with a little more difficulty, but the DOB was not the motorized. When they say "motorized", do they mean the GO-TO model? What is the difference between the GO-TO model and the cheaper one, BUT with the equatorial mount?

- In both cases you can have automatic tracking and finding capabilities using, I don't know, some USB cable/drivers and a software like Stellarium to find specific objects in the sky?

- An alignment with a star, magnetic pole, a compass or something like that needs to be done prior to using the GO-TO feature? How easy can you do that?

- If I wanted to locate objects more easily in the sky and not only use something like the RED DOT finder, for observation besides astrophotography, should I buy the GO-TO model or use an equatorial mount?

The first telescope I used to locate objects in the sky was the Maksutov 90, but with the best eyepiece (Wide Angle 6mm) it was a matter of seconds for them to disappear from my field of view. And after that happens, I had a lot of difficulty to point the Red Dot to that same object again. So if possible I want to avoid this sort of thing in this case, as much as possible. In this case, however, I didn't try to observe using Stellarium and have not aligned the equipment to find objects by itself.

This is an example of picture generated by a "non-motorized" 254mm Dobsonian telescope:

[Avocette]

A lot of issues to cover here! Firstly if your 'primary use would be astrophotography' then you need a good equatorial mount and a much smaller aperture telescope will do fine. There is a relevant thread on SGL you may find interesting http://stargazerslounge.com/topic/198737-skywatcher-80ed-pro-or-skywatcher-150pds/, but really you should get hold of a copy of 'Making Every Photon Count' by Steve Richards [available in the book section of the FLO website, or direct from Steve (Steppenwolf here on the SGL forum)] as a comprehensive  source of practical knowledge and expert advice on AP before you you commit to buying any equipment.

The 250mm Dob is largely a visual tool, and the GoTo version helps you find objects and track them for a while automatically rather than manually as you view them. Flavio has done amazingly well to capture and process many images of Jupiter to prepare the photo attached to your post! I have a similar scope but can only dream of such results!

[Moonshane]

I agree that there are better options than a dob for imaging.

I have no reason to doubt the genuine nature of the images shown but I find it extremely surprising that he can get such good results with manual tracking and would suggest they are atypical of the results you'd normally get with such a set up. it's a completely different kettle of fish again to image anything other than solar system objects

[Stu]

I believe Making Every Photon Count is about Deep Sky Astrophotography rather than planetary imaging so may not be relevant necessarily.

Can the OP confirm if it is purely planetary imaging which is of interest, or long exposure DSO imaging? The answer for each will be very different.

Stu

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[Perene]

I would like to use mainly for planetary imaging, not so much for DSOs, considering that my specific location does not favor too much viewing such objects.

At least in the Wikipedia article it says the equatorial mount is a bad idea for this kind of telescope. These were the reasons stated:

The Dobsonian design has the following characteristics:

Altazimuth mount: An equatorial telescope mount with clock drive was left out of the design. Equatorial mounts tend to be massive (less portable), expensive, complicated, and have the characteristics of putting the eyepiece of Newtonian telescopes in very hard to access positions.

Altazimuth mounts cut the size, weight and cost of the total telescope and keep the eyepiece in a relatively easy to access position on the side of the telescope. The altazimuth mount design used in Dobsonian designs also add to simplicity and portability; there is no added mass or need to transport counter weights, drive components, or tripods/pedestals. Setting up for hard tube dobs simply involves placing the mount on the ground, and setting the tube on top of it. The weight of the Dobsonian style altazimuth mount is distributed over large simple bearing surfaces so the telescope can move smoothly under finger pressure with minimal backlash.

The altazimuth mount does have its own limitations. Un-driven altazimuth mounted telescopes need to be "nudged" every few minutes along both axes to compensate for the rotation of the Earth to keep an object in view (as opposed to one axis for un-driven equatorial mounts), an exercise that becomes more difficult with higher magnifications.

The altazimuth mount does not allow for the use of conventional setting circles to help in aiming the telescope at the coordinates of known objects. They are known for being difficult to point at objects near the zenith, mainly because a large movement of the azimuth axis is needed to move the telescope pointing by even a small amount. Altazimuth mounts are also not well suited for astrophotography.

If this is all true, then wouldn't the GO-TO model be everything you need instead?

Perhaps there are better telescopes, but I am not sure they can offer similar results for similar costs. This model (or the 203mm) was the best thing (for the price asked) I could find. A MEADE telescope, for example, is a lot more expensive than any dobsonian.

About the author from that Jupiter photo, the author took similar ones as you can see from here. He claims (and even posted a few pictures from his telescope) they were all generated this way. See more here:
https://www.flickr.com/photos/93257060@N04/sets/72157635199644635/

If he is telling the truth how hard can it be?

[ronin]

To understand the problems it needs a little imagination of the situation in your mind.

Picture an imaginary constellation of an Arrow.

That rises in the evening in the night sky, and it rises point first.

It will come up vertically, pointy bit first.

By midnight it will lie due South and be horizontal, again pointy bit pointing to the direction of movement, but basically no bit is any longer Up.

When it sets then it will be heading vertically down, the pointy bit now being at the bottom.

An Alt/Az mount does not account for that rotation of the object as it travels across the sky.

What started out as the top finishes as the bottom.

On an Alt/Az mount the top "edge" remain at the top whereever the scope move to.

The Equitorial mount rotates the same as the object in the sky does.

If he is telling the truth how hard can it be?

Withhold my views on the truth, but images like that are not easy, and certainly not easy on a manual mount, Jupiter also has an added compliction for getting detailed images. It's rotational speed is high.

[nicks90]

how hard can it be?

hahahahahahahaha

sorry, that wasnt very constructive.

But you often see people post up to the t'internet "I did this, then plugged in this, clicked this and then out popped THIS!!!"

dead easy eh?

absolutely not. Nothing in imaging is easy. I suppose once you have done it a million times, it then becomes second nature. You have worked through all the trials and tribulations, equipment that works and equipment that doesnt work etc and in the end you may be able to produce some good results without too many hassles.

But that certainly isnt easy to get to.

as an example: I own a 12" dob with tracking. basically the same as the goto dob, but i have to find the object first and then it tracks it. This should be perfect for taking some videos of solar system objects and then stacking them in registax and getting some amazing images of jupiter etc. With that much aperture it will be crystal.

nope.

Getting the dob accurately set up, levelled and pointed north etc to get it to know where it is and where to track is not completely straight forward. Power issues / batteries then adds another factor into the mix, especially when the power starts to die after a couple of hours...

Every camera / webcam will need different settings in sharpcap programme. So you spend ages fiddling with gamma, gain and a million other settings so you can a) see the thing on your screen, when you do stack them its doesnt turn out like an over exposed onion. One can be exclusive of the other too! so it makes getting it on the screen and focussing quite entertaining!!!!

Then I had the issue that I couldnt achieve very good focus. hmmmm turned out i didnt have enough "in focus" for the webcam of my choice. So the only way of doing it is now with a barlow. Ok can now get focus, but the barlow made the image degrade due to reflections.

choice was to now buy a new higher quality barlow or a different camera. I chose a better barlow.

Issue is i;m now trying to image and track at 3000mm focal length! that is not easy.

after 2 hours of trying, I got one 30 second avi to stack and that wasnt in great focus.

I have now given it up as a bad job.

If i want to image, I'll get a goto EQ mounted SCT and buy a decent planetary imaging camera and find ALOT of patience from somewhere Zen-like.

[Stu]

I would agree that something like a C925 on a decent EQ mount would make a lot more sense than a dob for planetary imaging.

Stu

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[Perene]

About the dobsonian telescopes...

This is a translated version from the 254mm GO-TO manual:

https://dl.dropboxusercontent.com/u/53143167/DOB254GOTO.pdf

In pages 7 and 8 you can see instructions to install the primary mirror.

The 203mm manual does not mention any of this. Only explains how to install the optical tube.

LINK: https://dl.dropboxusercontent.com/u/53143167/DOB203.pdf

Before someone says they are different, both PDFs can be applied to all dobsonian models, such as 200, 254, 305, 355, 400...

So (without having looked the original manuals) I need to ask you: do we really need to install the primary mirror as explained in the first manual or not?

This is not the first time I am seeing different manuals. When I received the MAK90 telescope, the original manual only mentioned the mount, but didn't cover the red-dot finder, which was explained briefly by the store's translated manual (which in turn didn't mentioned the mount, it was only talking about the optical tube with equatorial mounts).

[Cornelius Varley]

I believe that the larger flextube dobsonian telescopes are sent out with the mirror packed separately, possibly the larger solid tubed version as well. This is a sensible precaution to avoid damaging the large heavy mirror during transit. 

[Perene]

I was just told that if :

1) I purchase the Dobsonian with the GO-TO feature and wants to use an equatorial mount such as the AZ-EQ6 this is impossible because the DOB GO-TO is designed in a way that prevents the optical tube from being adapted to an equatorial mount.

2) The equatorial mount already have the GO-TO feature in it (I don't know anything about EQ mounts, that was one of my doubts). 

And if I decide to purchase the DOB with GO-TO and somehow tries to use the telescope without feeding any energy (not even with batteries) this is impossible.

I don't get it, I had a Maksutov 90 and although I could power the mount to try tracking objects in the sky, I could perfectly use the optical tube manually and point to objects myself. In case there are no battery packs/energy available to plug the telescope.

The store has just said that if I purchase the Dobsonian with GO-TO this cannot be done.

Does that apply to the equatorial mount, too, if one day I decide to buy that mount and use in a non-motorized Dobsonian?

[nicks90]

what a load of rubbish

i have a SW 12" dob with tracking - not goto, but it has all the other motorised movement, up down, left, right and the ability to track (well,,, thats another story for my other thread!)

and without power you can still move it about perfectly well. It has a lot more resistance than a normal 'manual' dob, as you are moving the motors and encoders - but I nearly always find everything manually and only use the handset for fine tuning the object into the centre of my field of view and then pressing 'track'.

everything else is done manually by holding on to one of the very convenient black knobs on the upper part of the scope. You do NOT need it powered.

although i have to admit i rarely use it completely unpowered. I have a small 12V house alarm/motorbike battery (approx 4"x6"x6") that i use to power it. more than enough for a couple of evenings viewing between charging. Sits hapily on the front of the dob base under the little eyepiece tray as its so slim.

however i would say that if i was using the motors to move it completely - never touching the thing at al - then i dont know how long the battery would last. But waiting for that 'coffee grinder' to chug its way around would do my head in. Hence I use it manually.

[ronin]

I can understand (1) being correct, the OTA will have to connect to the base via a drive mechanism and that would likely prevent moving the OTA to an EQ mount.

You may find that people start with goto, then decide to do a bit of manual, then return to the goto.

The manual bit will mean the scope is pointed at one bit of the sky but will think it is aimed at another. So any movement made will be based on an incorrect position.

For (2) You may well be able to use the goto manually but that is not really what is intended and with motors engaged you are using the scope to drive the motors not the motors to drive the mount. I suspect the supplier is not not overly keen to guarantee a scope being used "not as intended". Ultimately you have to use the equipment in a reasonable manner.

On the Mak you had I suspect it was a case of you were driving the mount with a scope sat on it, the goto dobsonian you may well be driving the base in Az but I think you will be driving the actual OTA in Alt. A different mechanical setup. Related back to (1) the OTA will not be able to be swapped over to an EQ.

[Owmuchonomy]

I recently looked at getting a VX series Orion Optics OTA with a OO Dob mount. This can easily be adapted to then go on my HEQ5 mount. I'm not considering it now for various reasons but it did seem an appropriate compromise.

[AndyWB]

And if I decide to purchase the DOB with GO-TO and somehow tries to use the telescope without feeding any energy (not even with batteries) this is impossible.

Well, what you were told seems to disagree with the info given out by Skywatcher. From a reputable supplier, about the 250px Flextube GoTo:

The patented dual-encoder design allows you to manually move the telescope anytime and to anywhere you wish - with no need for realignment. Slewing to an object on the opposite side of the night sky no longer feels like watching a kettle boil. Simply push the tube close to the object and let the computer do the rest of the work.

I thought that the dual encoder design quite neat - that you could then just manually move the scope, but that the GOTO would know where you'd moved it to and adjust tracking appropriately.

All that said, I try and avoid electricity.

[ronin]

Didn't know where the dual encoders start to come in on the different models.

If the one being considered has them then by rights it should be able to be moved manually and the software will/should know where it has moved to.

Which would also imply that the system can be moved manually as well.

Still concerned that the OP is wanting to do astrophotography on an Alt/Az mount, it is the wrong mount as field rotation will occur and so imaging DSO's is likely not possible.

Seem to be in the situation of get a big visual scope then try to use it for imaging, which is not the best way of doing it.

If you want to image then buy an imaging setup.

[Dude_with_the_tube]

Skywatcher already has the dual encoder system on they new dobs.its exactly the same system as orion dobs are using and YES you can:

1.use the scope without powering it up in manual mode simply by releasing the Alt and Az clutches.

2.use the scope powered up and aligned via controller or by releasing clutches you can push it to wherever you want,then tighten clutches and use again powered up.Dual encoders will see where you have moved the scope so your alignment will not be destructed.

i have tested it on mine and it works perfectly fine.

As for using Alt/Az dob for AP,well,you will be limited to max 30 secs exposures due to being in Alt/Az,however,i have seen some cracking images of people using these scopes for DSO and Planetary AP with great success.

Nothing is impossible  

[Perene]

I asked a store if it's possible to use an equatorial mount for the NON-GO-TO dobsonian (254mm/10") and they said I need at least the Az-Eq6.

And for me to remove the tube and adapt to the EQ mount a dovetail and mounting rings.

However, they advised strongly against this claiming it was too hard to do it, from past experiences. Instead they said we should purchase the OTA alone, and then the EQ mount.

But can we find these products? Even if we can, what is the difference between them and the OTA from the collapsible dobsonian?

Plus we also have this one:

Everything th store said is also valid for it? What is the difference between both dobsonians?