Degrees/Right Ascension

[Kokatha man]

If I don't ask I'll never know - but when looking up DIY jobs for affixing setting circles on a Dob mount the azimuth scale allways seems to be gradated in degrees like the altitude/declination - not in hours and minutes as on an EQ mount.

I realize that the Dob hasn't the sophistication of an EQ and needs to be level and have the OTA's alignment projected onto the rotating table (so that this can be aligned geo south) to make any pretence at this particular circle being an aid to object locating - but why degrees?

Charts give RA - so what's the point? I would have thought that RA positions are constant regardless of the polar axis's angle of alignment: at the Poles (brrr!) a level Dob should be an EQ!?!

Hoping someone can enlighten this old blackfella who's both enraptured and overwhelmed by his re-entry into the world of AA.

ps - if you wish to enlighten me please be kind if this old fool has lost something in translation or mental capacity!!! (have ordered one of Brian Reed's EQ platforms for my 10" Bint.)

[gary]

Hi,

A level Dob is not equivalent to an equatorial mount.

As you appreciate, sky co-ordinates are designated in terms of RA
and Dec. Due to the rotation of the earth around its own axis
and also the rotation of the earth around the sun, the sky appears
to move overhead as a function of time, appearing to revolve around the
celestial poles like clock-work. Therefore, historically, RA is generally reported in
terms of time - hours, minutes and seconds whereas Dec is generally reported
in terms of degrees, arc minutes and arc seconds.

As you also appreciate, if one points a scope at some place in the sky and
if the scope does not track, the heavens appear to move across the field
of view. In other words, the celestial co-ordinates of where the scope
is looking are continually changing with time. If the mount is not polar
aligned, then with the exception of pointing the scope at one of the two
two celestial poles, the RA *and* Dec co-ordinates of where the scope is
pointing will change continually with time.

If one polar aligns the mount, that is, aligns its azimuthal axis with the
celestial pole viewable from your hemisphere, then when the mount is
pointing at any arbitrary place in the sky, only the RA co-ordinate will continually
change if the scope is not tracking. Therefore, one can then conveniently
mark the azimuthal setting circle in terms of hours, minutes and seconds.
When the scope is not polar aligned, making the azimuthal axis in such
a way makes no sense.

When one mounts a Dob on a platform as you propose and then polar aligns the
lot, then marking the azimuthal axis in terms of time then makes sense.
You can convert from degrees to hours, minutes and seconds simply by dividing
by 15.

If one does not have an equatorial platform, one needs to perform a co-ordinate
transformation calculation, going from the mount's co-ordinate system to the
sky's cordinate system. One needs to also know your location on earth and the
current time. This transformation is laborious to do by hand but is readily done by
a programmable calculator or computer.

We happen to manufacture a product that does a similar calculation in real-time multiple
times per second so that one then knows where the scope is pointing in terms of RA/Dec

Best Regards

Gary Kopff
Wildcard Innovations Pty. Ltd.

[Kokatha man]

hanks for the quick reply Gary - and yes, I am aware of your great product and have it on my wish list.

I think you encapsulated everything in your reply, but the nub of the question (for my obsessive pedantic nature!) was that if a Dob (not on an EQ platform such as the one I've ordered) is level and has been aligned such that the longitudinal axis of the scope tube is pointing geo south are not any RA settings callibrated from this alignment indicative of the same positions had an EQ platform been utilized and the scope tube aligned with the south celestial pole?

I relize it would not track because with RA there is shift in Declination as well as east-west movement - but am I correct to think that the RA markings on an assembly that are callibrated with regard to due geo south accord with those same positions (at any fixed point in time) of an assembly celestial polar aligned??? re my comments about a level Dob mount at the north or south poles.

The articulationof my question is most probably the most problematic aspect of this whole issue(?!?) regards, Darryl.

[gary]

Originally Posted by Kokatha man

I think you encapsulated everything in your reply, but the nub of the question (for my obsessive pedantic nature!) was that if a Dob (not on an EQ platform such as the one I've ordered) is level and has been aligned such that the longitudinal axis of the scope tube is pointing geo south are not any RA settings callibrated from this alignment indicative of the same positions had an EQ platform been utilized and the scope tube aligned with the south celestial pole?

Hi Darryl.

Unfortunately not.

By the Dob being 'level', I take it you mean that the ground board of the scope
is at right angles to a plumb bob under gravity at the vertical.

If the mount is positioned in this way and if the optical axis (what I believe you
refer to as the 'longitudinal axis of the scope tube') is then pointed at
the South Celestial Pole (SCP), then the scope is still not polar aligned
and this position does not correspond to an equatorial aligned mount.

Keep in mind that that when it comes to polar aligning, one is aligning the
mount and not the optical tube.

In your first post, you mention that you realise a "Dob hasn't the sophistication
of an EQ ....". In fact, that perspective is somewhat flawed and therein one
can find the key to understanding what is going on. A Dob really refers
to a particular popular embodiment of what is known as an Alt/Az mount.
Virtually all popular mounts have two primary axis of motion that are
fabricated at right angles to each other. An EQ mount is in fact just a Alt/Az
mounted tilted over so that its azimuthal axis is parallel to the celestial
pole. In other words, rather than level the base and then point the tube,
one needs to tilt the base up so that the azimuthal axis points to the pole.
That azimuthal axis then has undergone a metamorphoses and can now
be referred to as the "equatorial axis" and one can then validly measure
the angle through which it is turned in terms of what is known as
an "Hour Angle" - which really is just a function of RA and local apparent
sidereal time.

When I mention that to turn an Alt/Az mount into an equatorial mount,
that one need to simply tilt the mount over, an interesting question to
ask oneself is "by what angle does one need to tilt the scope over?"
The answer to that depends upon the latitude of your observing location.

For example, here in Sydney, we are at roughly 33 and a half degrees
south latitude. If one measures the angle from the southern horizon
to the south celestial pole, lo and behold it is 33 and a half degrees
above the horizon. Therefore if one tilts the whole Dob base from a starting
position whereby the Azimuthal axis was initially pointing vertically at the zenith,
through an angle of ninety minus 33 and a half degrees (or 56.5 degrees) so
that now the azimuthal axis is parallel to the pole, then one has polar
aligned the mount.

Hope this helps.

Keep in mind that there is nothing sophisticated about an EQ mount.
It simply is an Alt/Az mount tilted on its side. From a practical viewpoint,
a Dob generally makes a more forgiving partner to manually dance around the sky with
than most EQ mounts.

Hope the above clarification helps.

Best Regards

Gary

[Kokatha man]

Hi again Gary - forgive me for being a "dog at a bone" here, but as previously indicated I think my clarity of articulation is the main problem!?!

Just to clarify - when I say that "I realize a dob hasn't the sophistication of an EQ..." I am merely acknowledging that to track/maintain an object in the FOV an EQM requires only RA inertia (once the object is centred) to do so... Going back to basics this is a function of the telescope's longitudinal axis (the optical axis you refer to) being parallel to the RA axis of the mount, which is of course aligned to point at the south celestial pole - directly related to the latitude as you point out. (Pardon the pun!)

A pedant like me would have to add that although these 2 axis are actually parallel to each other all objects they are targetting are at such great distances that parallax deems them to be one and the same point.

Perhaps the confusion in my diatribe centres on this: when I speak of the levelled mount having the optical axis pointing geo south (not SCP) I am referring to a scribed line on the revolving baseboard of the dob mount that is directly below and aligned with the optical axis of the scope - and the Hour Angles (RA) marked on this baseboard referenced from this scribed point.

This is also to state that regardless of whether the polar axis is set at whatever angle the observer's latitudinal position is; 35, 60 or 90 degrees, all of the RA gradations will correspond exactly to each other: which is why I included the bit about positioning a dob mounted scope at exactly the south pole where the latitude is 90 degrees - if the turntable is level the dob mount assumes EQ status insofar as once an object is centred in the FOV only azimuth/RA inertia would be required to keep the object in the FOV - no adjustment of altitude/declination would be necessary.

Another way of stating this is to visualize RA motion as a circle around its' latitude corrected axis pointing at the SCP. Changing the tilt of this axis to 90 degrees the RA motion then describes a circle parallel or "level" with the ground - ie it takes on properties of a dob mount albeit one where tracking is not facilitated because the declination axis is not fixed at right angles to the RA axis.

However, notwithstanding the lack of any tracking benefits that an EQ affords with single motion inertia - when using tables to view the RA of an object at 0 hours UT and converting to local time, these (proposed) Hour Circles would allow the observer to roughly set the dob mount to the given/calculated RA figure and "sweep" using the Altitude motion of the dob mount to (hopefully!?!) target the object.

Which goes back to my real and basic question - why mark setting circles in degrees rather than RA's Hours etc if you wish to gradate the azimuth motion of your dob when you can scribe an optical axis on the azimuth board and reference your setting circle from this, geo align this scribed line with south (also levelling the whole thing) and have a more specific gauge.

Having ploughed through all this Gary; I assure you that your Argo Narvis system, which is next in line on my impoverished income, is likely to make me just wait until I can afford said!!!

Regards, Darryl.

[Kokatha man]

Hi again (again) Gary - thought I'd best post this whilst insomnia was still working: having hoped to check out Venus and Jupiter (ha ha) before daybreak.

Having finally bothered to critique my assertions (hey it's better than paying for therapists and maybe I can spend some of the savings on AA gear!) I see the fatal flaw in my nonsense.

Whilst it still seems to me that with the parameters I've previously described I can rotate the baseboard of the Dob to a point where the scope could be defining a specific Hour Angle/Right Ascension, it would be, because of the Dob's physical construction, only aligning with one tiny point along that RA's meridian .

Because the Dob cannot "sweep" along that meridian (unlike the EQ with its' specifically arranged and aligned dual axes) - the validity for targetting using such aforementioned circles becomes nonsensical.)

Oh well, a thousand apologies and perhaps my one consolation is the bit about a straight Dob being an EQ at the pole: not sure if I'm going to pursue that option of maximizing the plain Dob's potential...

Regards, Darryl.

[gary]

Originally Posted by Kokatha man

Whilst it still seems to me that with the parameters I've previously described I can rotate the baseboard of the Dob to a point where the scope could be defining a specific Hour Angle/Right Ascension, it would be, because of the Dob's physical construction, only aligning with one tiny point along that RA's meridian .

Because the Dob cannot "sweep" along that meridian (unlike the EQ with its' specifically arranged and aligned dual axes) - the validity for targetting using such aforementioned circles becomes nonsensical.)

Hi Darryl,

You've got it!

I won't address your penultimate post as I believe you have now spotted the
various mistakes in it.

Oh well, a thousand apologies and perhaps my one consolation is the bit about a straight Dob being an EQ at the pole: not sure if I'm going to pursue that option of maximizing the plain Dob's potential...

Regards, Darryl.

No need to apologize at all! There is some mental gymnastics in imagining
how lines of RA and Dec are mapped out in the sky that is not always
immediately apparent to many of the enthusiasts I get to deal with.

As you note, take the Dob to the North Pole or South Pole and it is
an equatorial mount when leveled.

At any other latitude, one could tilt an Alt/Az mount over at an appropriate angle
and it becomes and equatorial mount.

Though equatorial platforms do precisely this job, as a rule, most large
aperture Dobs are operated in Alt/Az mode with the base roughly level with the
horizon. The key to large aperture Dobs is large, light weight, thin, affordable
mirrors. However, such mirrors warp under gravity as the scope moves. The
job of a mirror cell is to act as a set of levers to help counteract the effects
of gravity and to try and keep the mirror in its correct shape. Such mirror
cell designs are usually constructed with the assumption that the scope
was operated in an Alt/Az mode rather than the mount being tilted over
so that its Az axis becomes parallel to the celestial pole. This is the
primary reason you see so many large aperture Dobsonian telescopes
whereas most amateur equatorial mounts usually only are fitted with
more modest aperture telescopes. It is also one of the main reasons
why almost all modern professional observatories also prefer the Alt/Az
configuration from a mechanical engineering standpoint.

Best regards

Gary Kopff
Wildcard Innovations Pty. Ltd.

[netwolf]

Daryl,

I spent several hours reviewing your thoughts in my mind and trying to get a mental picture of it. I think perhaps i can offer a somewhat more easier illustration of why it is not so. But not as eloquently as Gary and yourself.

If you take a Clock and place it level on the ground at the pole. Walk along the 12 hour line to the equator and place another clock such that the 12 hour line aligns with the one at the pole. You will note that though the 12 hour line is the same the 3'o clock (or for that matter any other hour) line is not aligned. You would have to tilt the clock face 90degres to get them to align.

I hope that makes sense.

Regards
Fahim

[Kokatha man]

Fahim - I'm not quite there with your analogy but I'll get back here in my studio when the temp drops a bit more this evening (hopefully with the Dob up on my elevated platform and the mirror cooling fan doing its thing) and go over it.

That is to say that I think I appreciate the analogy but want to absorb it more fully to appreciate what seems to be a rationale as emminently poetic as anything written so far.

Maybe its my artist' bent or my cultural propensity to delve into patterns and intricacies but in many ways I was motivated to start this thread by the desire to evolve different ways of seeing, understanding or explaining things, where many take the dry/barren approach path.

It seems very similar to one of the ways I worked my way through (my own!) conundrum - imagining a Dob's revolving azimuth baseboard sitting at each polar position and being able to join up each gradation from one pole to the other (creating the meridians or Hour Angles) like the paths you walk in your clock analogy.

The one at the equator - hang on now, I've just used a ball and 2 disks to trial this analogy and I'm confused again Fahim!?! - I'll wait till the sun goes down and review it then. From art to abstruse philosophy to (most probably in my case) absurdity, regards, Darryl.

[Outbackmanyep]

Sounds like you are getting confused with your RA axis, on a dob the "RA" axis is the base board centre, or the right / left slewing motion. Your Dec axis is your up / down motion.
To have it polar aligned (which as Gary pointed out is nonsensical) you must tilt the base board so the centre-line of the bolt used to hold the base together is pointing to the SCP, not the tube! But doing this is not in the design of the Dobsonian mount, it is purely an Alt/ Azimuth mount.

[Kokatha man]

Hi Outbackmanyep - appreciate your input but no, I'm not getting confused; especially re RA and DEC, learnt that as a young fella over half a century ago!

But I'm obviously confusing you, for which I'll take full responsibility: it was me you quoted re "nonsensical"!!! (Gary would have been far too polite to use that term himself)

Re polar aligning the Dob I'm quite hopefull my RTP will accomodate that task in its limited fashion when it arrives from the US of A but to get the gist of the thread you'd have to read through the whole thing and I wouldn't advise that to anyone who wants to stay sane!!!

Suffice to say Netwolf has hurled his own curly analogy back at me - and from a process of (mainly) self enlightenment I am now scratching my head to accomodate his analogy: which is the least of my just desserts for starting this discussion.....

Regards, Kokatha man.

[Outbackmanyep]

I can't say that i see what you mean, but i had a stab at it! hehe

[netwolf]

Daryl,

In the attached drawing you can see the first drawing, the circle is the earth the black tangent lines represent the dob bases at the pole and the equator respectively. The red lines are the 3'oclock position on both. As you can see they would project out into space perpendicularly to each other.

The confusion is highlighted in my 2nd drawing left diagram, which i assume is your perception. The red is one base the black is another base the circles seems to indicate that an angle on one is the same as the angle on the other. But this is an illusion because we are confusing the two bases. It is better drawn on the right side or as in my first drawing.

Regards
Fahim

[Kokatha man]

• Hi Outbachkmanyep, Netwolf and any fool who has attempted to follow this thread.
• Firstly to you Outbackmanyep: my only criticism of your last post is this - if you can't see what I mean then surely you're agreeing with my previous reply to you; ie trying to understand this thread is a sanity issue. For your lack of (its) comprehension you have now been given extended leave from "the farm."
• You Netwolf, who has had me drawing clock faces on coins and trying to stick them onto a tennis ball; that was really sneaky of you: I've now had all my own "farm" leave cancelled indefinitly. I was fooled by that image of you on IIS - it wasn't fair of you to take off your white coat and test me like that with those timepieces! (Bet they were fake Rolex anyway.)
• But seeing I'm now imprisoned here indefinitly behind a mesh of logitudinal and latitudinal (oops, make that RA & DEC) bars, I'm going to make your life hell!?!
• On my planet - Dunlop xpro 3 - which has a similar orbit to Earth's but appears from a distance to be lemon-yellowish and hairy/fluffy, there is a substance extracted from Sirius B (for Blutack) that clings to the surface for dear life. However, Earth money that has 3 o'clock faces drawn on it in texta constantly dissappear into deep space (under the flooring) when one tries to fix it to this marvellous substance.
• But on the few occassions when I have made these coins defy anti-gravity the little (3 pointing) hands can be deemed to project off parallel to each other into space or - if one extends them in their allotted directions around the surface of xpro 3 - they do intersect each other at right angles (your perpendicularly.)
• Which could give credence (for a raving loony like myself) to my old assertion about putting RA markings on the azimuth board of a dob mount, referencing them from a line directly below and in alignment with the optical axis of the scope. If you then align this line geo north - south (Not SCP Outbackmanyep!)and set your pointer for the RA markings on the fixed (lower) baseboard along the same alignment - would any attempt at an "Hour Angle" fix produce 1 (maybe 2?)correct RA positions? It is a given, thanks to Gary's kind encouragements (hey, your not one of Netwolf's colleagues are you bro?) that the dob setup would not allow you to select any Declination except those 1 or 2 arbitrary positions.
• You will notice, Netwolf, that I am merely speculating this possibillity nowadays: I will learn how to behave and one day all of you mob are going to have to let me return to my home on Dunlop xpro 3 to further my analysis of those curious properties of Sirius's "Pup."

[netwolf]

Daryl,

Imagine a sphere around your dob at the pole of DX-3 this is your reference sphere. Now move your dob further up towards the equator of DX-3 and imagine another sphere around you scope. Note how these imaginary spheres are not aligned. So how would a RA reading at these two positions be the same? The only way is to tilt the base such the two spheres are parallel to each other.

Regards
Fahim

[Kokatha man]

Fahim - at last you've teleported to my planet (DX3) and thankfully left your darn watches stuck on 3pm back on Earth! (Did I ask you before whether they were 24 hour ones? - if they weren't that could well explain our previous differences; RA is a 24 hour model.....)

At last our fertile imaginations have synchronized (this could only occur on DX3) and would go so far as to say that we could substitute your "reference sphere around my dob" at the Pole (I'll let you pick which one, as you are a guest of mine on DX3) into an actual calibrated RA circle on my dob.

I think you will find that when we take my other dob (I have many) to the Equator we can give it a spin and you will find that any given Hour angle will intersect (or at least be directly below/corresponding to that specific Hour Angle on the dob at the Pole) twice in a single revolution of the dob mount at the Equator.

Again I will emphasize that this will be an absolutely arbitrary alignment (due to the altitude mount of the dob being centered directly above the azimuth pivot point it will afford us no ability to coordinate our scope onto anything -except to be able to claim that we could locate two points in the heavens at any particular RA.) Something akin to one of your damnable watches stuck on 3pm - they'll tell correct time twice a day, ie as long as they're not 24 hour ones......

Darryl - ps I'm about to start a new thread proposing Left Ascension and positive Declination for us Southern Hemisphere mobs - we've been kept upside down and on the bottom of the AA heap for far too long...

[Outbackmanyep]

[Kokatha man]

Listen up good Outbackmanyep - if you want to speak to me on DX3 it's no good sending a blank post with telepathic mind messages encrypted into it - we're not that flaming bright on this side of the universe: you have to shout really loud too, (obviously) I'm on another planet and a long way from Earth.....

[netwolf]

Daryl can you provide any drawings of your analogy? Perhaps I am visualizing what you are saying incorrectly.

What we need is a white board some marker pens and some chairs. oh and some white coats. Brings back memories of study with my mates back at uni.


Regards
Fahim

[Terry B]

Boy this is confusing but I think I get the problem.
I think the question is: Why cant you use the azimuth circle on a dob as a RA measurement even if it is not aligned at the SCP?
Both turn as the earth turns. The reason that it doesn't work is that if you trace out the track of a star across the sky as it moves along the RA axis it appears to produce an arc that is elliptical in shape across the sky. The shape of the arc will vary depending on you lattitude. On the south pole the star will produce a circle and this would be the only place that the aximuth circle would work. The azimuth circle would measure a circular track only and this would not work anywhere except on the south pole.