GOTO Dobs Astrophotography

[markophoto]

Hi folks

This might be in the wrong section of the forum, so apologies if it should be elsewhere.

I have so far resisted buying a scope (and its been at least a month!!!)

It'll likely be later in the year when I've got enough cash together for what I REALLY want.

Anyway, having realised that a Dobs was probably the best idea in terms of aperature per £, but they wouldn't be suited to much astrophotography I've looked a bit deeper.

I understand that its tracking and field rotation that are the likely issues with Dobs photography. If I bought a GOTO dobs though, which does track, would it do the trick?

I'm looking at this for instance

Dobsonians - Skywatcher Skyliner 300P FlexTube GOTO

Am I missing something obvious that would make it unsuitable for astrophotography?

Marko

[Gerontius]

To deal with field rotation you need an equatorial mount.

[leenewtoastro]

hi mark, i'm a newbie too. i have read and been told while its a very good scope its not best for imaging.

it would appear the scope and mount do not lend them selves well to long exposure/general imaging due to movement type and how images are viewed via the ep / focuser.

the mak newtonian looks the daddy and the more you spend on the mount makes for all the better sturdiness/tracking

heq5 goto possibly

from what i can gather in a few days of research

[reddoss]

The Skywatcher Skyliner 300P FlexTube GOTO is a great scope for visual astronomy. However it can not be used for most types of astro-imaging as it does not track a celestial object in an arc as it moves across the sky with time. Astrophotography requires long exposure times. For example if you use a digital SLR camaera like a Canon or Nikon, exposure times are typicaly 30 secs to 10 mins, depending on the object you are imaging. To do that you need an QE mount. The minimum I would recommend is the HEQ5 PRO, the EQ6 PRO is better as it can carry a bigger load.

You may find this website useful Mounts for Astrophotography

Good luck.

[Space Cowboy]

The auto/goto Dob works fine for planetary imaging but as has been said for deep sky long exposure a EQ mount is needed.

[RoveyDoveyGrovey]

There is this one for a similar price (about £1400) which comes with an NEQ6Pro mount - Sky-Watcher Explorer-300PDS newtonian telescope. - although as I was advised, the size of this thing is quite considerable and i don't know how good this thing would be for astrophotography. i've seen quite a few really nice photos using a 200p scope which might be a bit more manageable. Im a newbie by the way so don't take this as gospel.

[Astro_Baby]

I dont do imaging but I'd forget the 300P - its huge and must tax the NEQ6 (which is only an EQ6 after all with a few trills).

I have a 10" here on an EQ6 and for anything excpet a fixed obs I'd say its too big, too heavy and if you added a camera and imaging gubbins it would put a strain on the mount.

Imaging is a lot more than aperture from reading the imagers and seeing what they use.

[leenewtoastro]

how can a goto dob not visit every point a equi can?

[Space Cowboy]

The mount has to be tilted on a axis the same as the earth for it to produce an elliptical movement. Alt/Az GOTO Dob mounts move in a step sequence so don't follow precisely the orbital plane.

[Astro_Baby]

Basically the stars appear to move in a line thats at an angle - in the UK that line is anled at 51' so it looks a bit like this / an equatorial mount moves in two directions like this X where one line of the X is at the same angle - thats set on the mount.

So the mount, when powered, tracks along the same line as the stars.

A dobsonian is Alt/Az which means it goes up and down and left and right like this + so it doesnt track in the same way. it creates a sort of series of steps. It makes no doifference for visual but its critical for astronomy imaging.

Hope that helps.

[leenewtoastro]

thx. it must be right but getting that to compute in my brain is difficult

for me,if a item arcs the controller would operate both axis to achieve the direction needed! ????

time for diagram.

so if i needed my goto dob to align to the dot or even track. simply both axis would need adjust! ?

so why can they not? or they can just not smoothly?

while the equi would only need 1 axis to move?

.

[Trull]

I like imaging, and I'd recommend that you try using a DSLR on a simple tripod to start, then try it with a zoom lens, then a long lens... you'll note that you get star trails at about 20 second exposures with a 60mm lens, with a long lens its a matter of single seconds for a 300mm. Imagine the trails with a 1600mm scope... pretty nasty! (which is also why a smaller aperture say 100mm f7 refractor is widely favoured for imaging)

Anyway - before taking the plunge I'd seriously recommend contacting your local Astro Soc (and asking for someone local here). There will be a someone that can help you see what we are talking about. I found it impossible to truly understand the concepts until I saw the effects of a slight breeze on an inadequately stabilised OTA...

Lastly, the hobby aint cheap!

[angusb1]

thx. it must be right but getting that to compute in my brain is difficult

for me,if a item arcs the controller would operate both axis to achieve the direction needed! ????

so if i needed my goto dob to align to the dot or even track. simply both axis would need adjust! ?

so why can they not? or they can just not smoothly?

while the equi would only need 1 axis to move?

The alt/az mount needs both axes to track as the movement relative to the earth's rotations is a curve, so the alt/az mount needs a series of vertical and horizontal steps from both axes to roughly follow the curve. A polar aligned equatorial mount has a right ascension (RA) drive which rotates in a curve around a central axis which points towards the north pole in the northern hemisphere), so the movement is smooth and only needs a single motor to move the OTA in a curve that follows the rotation of the earth. The declination axis of an equatorial mount moves the OTA away from or towards the curve of the RA movement.

Hope this helps.

[leenewtoastro]

yep got it angus.

[Rossco72]

if you took your crosshair, the alt/az dob (like my 350) would keep the target star in the centre, however all of the other stars would form arcs circular traces as the field rotation isn't accounted for. The EQ mount is aligned by setting the angle to the celestial pole so that the mount tracks around the rotation, taking it out of the equation.

[Astro_Baby]

Using your diagram an EQ would track in a diagonal from the centre of the cross hairs and follow the same motion as ALL the other stars. Your dot/star would be moving in an arc which the eq would follow on only its ra motor.

An alt az would be going up a bit, left a bit, up a bit,left a bit etc and would track the dot, unfortunately all of the other dots would also be moving in a clockwise direction.

The effect would be the dot in you diagram would be centred in every pic, the dots around it when using an alt/az would move progressively up and to the left but not at the same rate as the dot thats being tracked.

In photographic terms each frame would have the dot centred but the dots around it would move into different places in each subsequent frame.

If you took a long exposure which would be essential for deep sky imaging the stars around the target would show trails, if you used stacking, which is where you take a lot of shorter exposures and stack the images your target would be centred but everything else would always appear in the wrong position on subsequent frames making stacking impossible.

Its tough to explain in text and youd really need to see an eq in operation to fully understand it I think.

[leenewtoastro]

nah. i got that now greatly explained by both replys.

i sort of mentioned is it only 1 axis that would track in my closing sentence

[yorks]

thanks, this thread has cleared up a few questions I had. Like Lee I am am confused beginner

[leenewtoastro]

thanks, this thread has cleared up a few questions I had. Like Lee I am am confused beginner

OI OI :)

[sixela]

The mount has to be tilted on a axis the same as the earth for it to produce an elliptical movement. Alt/Az GOTO Dob mounts move in a step sequence so don't follow precisely the orbital plane.

The tracking is identical (the fact that it needs two axes is largely irrelevant. If you use an equatorial mount there's also a Dec correction from time to time unless the polar alignment is perfect and the orthogonality of both axes is as well, and the resolution of the steps or microsteps is enough not to be an issue). The problem is that the field rotates around the object.

Suppose that you're at the North pole and look at stars while they move (that's what an EQ mount does: it tilts the Dec/RA axes until one axis is pointing to the celestial pole). The top of your nose will always be pointing to the celestial pole that is overhead.

Suppose you do the same thing here (at 51°N). When the object is due East, the top of your nose isn't pointing towards the celestial pole (which is 49° to the left). When the object is in the South,the top of your nose *is* pointing to the celestial pole (if you follow it beyond the zenith). When the object moves to the West, your nose is "up" but the celestial pole is now 49° to the right. So as you can see, the line "up" from the object actually doesn't always point to the same objects around what you are looking at during the object's travel.

The worst field rotation occurs when an object passes overhead. An Alt/Az mount has to flip Az 180° instantly in order to track and the image goes upside down at once!

Watch this and follow an object in sight to see field rotation at work:

On an astrophotograph, this is what field rotation does (no prizes for guessing where the guiding star was):