Beginners question.. why a DEC motor?

[Samop]

Hi,

As a total beginner I understand what an RA motor is for... on a properly aligned eq mount it keeps it pointing at the same bit of sky....yes?

But I'm not sure what the purpose of a DEC motor is. Us it only used if you've some sort of goto setup? or for tracking the moon? or something else?... I'm sure it'll be obvious when it's explained.

[StevieDvd]

The DEC motor is used to perform the goto or motorised movement dictated by a handset/computer.  For visual users it would keep a target centred, for imaging and remote users it does the tracking and centering of new targets.

Tracking of moon, sun & planets use a special tracking routine as they are not as 'simple' as the astro (sidereal tracking).

[malc-c]

2 hours ago, Samop said:

Hi,

As a total beginner I understand what an RA motor is for... on a properly aligned eq mount it keeps it pointing at the same bit of sky....yes?

But I'm not sure what the purpose of a DEC motor is. Us it only used if you've some sort of goto setup? or for tracking the moon? or something else?... I'm sure it'll be obvious when it's explained.

To look at any object in the sky you need movement on two axis.  Think about it.. with an EQ mount the RA axis is always aligned with the Earths axis, so it rotates around this axis, and when driven in a goto system it will typically use the sidereal rate to counteract the Earths rotation.  But in order to see anything other than the field of view around Polaris the DEC axis is needed to angle the scope.  So rotating the RA axis and the DEC axis a goto mount can be pointed to any target in the sky.  Some mounts only used to be drive in RA, with the user having to manually release the clutches, rotate the DEC axis, and then lock the clutches when the target came into view.  Having both axis driven means the mount should centre the target without any need for user intervention (assuming polar alignment is spot on and the mount has no backlash in the drive train).

Due to the relationship between the Earth, Moon and Sun, the tracking rates are different to sidereal.   

[Pixies]

I have some manual EQ mounts with additional motors - so not used for goto.

True - once you have aligned on your target manually, as long as the mount was accurately polar aligned, the scope will now follow the target. However, for visual observing you don't normally get too hung-up about very accurate polar alignment, so you will find that the target will drift off centre after a while, and if you are at high magnification, it will be moving out of view. Being able to quickly and finely adjust both axes is useful, especially not having to move your eye from the eyepiece. A quick press of the buttons will let you centre things. Slo-mo controls will let you do this manually too - as long as they are within easy reach. Also useful for star-hopping using a wide-angle eyepiece.

So - not essential if you only want a driven mount to follow a target, but pretty convenient. Slo-mo knobs on extensions are a cheaper equivalent,

Edited May 5, 2023 by Pixies

[michael8554]

The Dec motor is nearly always necessary for Guiding.

Otherwise for visual use it could be considered to be a luxury.

To make slewing to a target easier than manually twisting a knob.

And for GoTo (not Push To).

Michael

[Samop]

Thanks all for explaining.

My long-term plan is to do some astro-photography. Planets/Moon and some DSOs.

I have no plans to add a Goto (would like to but can't afford it) so, given that I have slo-mo knob extensions it seems to me that a DEC motor is a luxury that wouldn't really add anything to my viewing and photography abilities. Obvs an RA motor is a must.

Yes?

[malc-c]

16 hours ago, Samop said:

Thanks all for explaining.

My long-term plan is to do some astro-photography. Planets/Moon and some DSOs.

I have no plans to add a Goto (would like to but can't afford it) so, given that I have slo-mo knob extensions it seems to me that a DEC motor is a luxury that wouldn't really add anything to my viewing and photography abilities. Obvs an RA motor is a must.

Yes?

I think you need to re-read the thread... If you want to image DSO's then guiding is a must.  In order to guide, BOTH axis need to be motorised in order to make small corrections in both plains.   You don't need goto functionality, but most mounts that have options for basic drive also tend to have better more precision stepper motors than those which feature in the goto system for the same mount.  An example being the EQ5 from Skywatcher.  For £87 you can buy a standard single axis drive with basic handset  seen here.  For a further £12 you can get the same system, but with dual axis motors like this  Entry level guiding is offered with the enhanced dual axis kit retailing around £150.  All of these drives use DC motors to move the mount.  Stepper motors only come with the go-to option, which also give the further option of interfacing with a PC for full software control.  Naturally give the kit includes a Synscan handset and control board these upgrades are not cheap, being more than double the cost of the enhanced drive at around £360.   

But imaging isn't cheap, and things like the mount plays a bigger part of things compared to visual needs.  Having the option to run precision motors to provide decent tracking with out requiring the software to work hard is something that needs to be considered at the start, it  may also save money in the long run.  If you are unsure of the direction you'll end up going, or the full upgrade (assuming the mount has that ability) is above your budget then choose a mount such as the EQ3 / EQ5 used in this example. that accepts the enhanced drive kit.

[Mr Spock]

Speaking as a visual observer, being able to move an object in all directions at the touch of a button is incredibly convenient and easy.