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Kokatha man
02-01-2008, 12:04 PM
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
02-01-2008, 01:13 PM
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
02-01-2008, 02:43 PM
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
02-01-2008, 05:59 PM
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
02-01-2008, 08:07 PM
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
03-01-2008, 06:03 AM
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
03-01-2008, 10:44 AM
Hi Darryl,

You've got it! :thumbsup:

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



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
03-01-2008, 02:42 PM
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
03-01-2008, 05:31 PM
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
03-01-2008, 06:05 PM
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
03-01-2008, 07:48 PM
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
03-01-2008, 10:10 PM
I can't say that i see what you mean, but i had a stab at it! hehe

netwolf
03-01-2008, 10:20 PM
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
04-01-2008, 11:30 AM
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
04-01-2008, 05:34 PM
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
04-01-2008, 08:02 PM
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
04-01-2008, 08:52 PM
:abduct:

Kokatha man
04-01-2008, 09:22 PM
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
04-01-2008, 11:04 PM
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
05-01-2008, 02:09 PM
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.

Kokatha man
05-01-2008, 04:48 PM
Hi Terry - I wasn't going to respond to this subject again untill they'd undone the straps on the special jacket they've got me in and let me out of solitary confinement: but I now understand the blank post Outbackmanyep sent me with the little "teleporting man " icon - it really was a cryptic message!

Ignoring the (more obvious) possibility that it was a rude response based on a poor opinion of my DX3-bred faculties: I must now conclude Outbackmanyep was secretly telling me that some other (weird?) person would soon be teleported along to enable us to expand our various perspectives.

Terry, one of the understandings I evolved as I debunked (with Gary's erudite assistance) my "nonsensical" RA circles (you got that didn't you Outbackmanyep - I called them "nonsensical") was that even if I levelled the baseboard, drew a line on the revolving (azimuth) board of the dob that was aligned with the scope's optical axis and aligned that geo North-South and marked off my RA circle from this - it would be meaningless and afford no benefit because the altitude (declination) axis of a dob is directly above and moving in a manner such that it (the declination/altitude movement) can NOT slew along any particular Hour Angle but merely bisect it at a very arbitrary (for the purpose of targetting) point in the sky. An EQ mount has a polar aligned azimuth/RA axis and an altitude/dec axis set at 90 degrees to it, allowing movement in a plane parallel to the polar axis (ie describing Hour Angles.)

I'm not in agreement re the stars presenting elliptical paths across the sky: setting the Declination on an EQ allows the Polar aligned azimuth axis to define/track any night object's circular passage from East to West via that axis' single revolving movement - but what I think is important for me to recognize in your own (ie Terry's) mental imagery is that, like mine, some points are rather abstruse to express (without modelling) and we may mangle parts of the logic but we do arrive at a somewhat intuitive appreciation of the interacting mechanics of the sky and any mounts we use to observe. Far more interesting from my perspective than just accepting rote-like what this or that represents or facilitates. (Getting into the mental gymnastics as Gary said!)

To this end I can only say to Netwolf that I suggest you obtain a model of my world (they are plentiful this time of year in Oz - there is a ritual about to start called "The Australian Open" where - from an inhabitant of DX3 point of view - disrespectfull things are done to representations of our homeland.)

Netwolf, mark a North and a South pole onto your acquired model of DX3, run, say 4 longitudinal lines from pole to pole with texta to represent the Hour Angles (mark the 0,6,12,18) and then anywhere along the Equator pin another little disk you have scissored out and marked with 4 Hour angles similarly gradated: and give it a little twirl - one of 2 things will most likely happen: (a) you will find yourself sharing a padded cell with yours truly or (b) you may have to revise your prior assertion that 6 will not "align" with 6 and etc etc.

I leave you all for the time being (they want to put this strange jacket back on me) confident that I leave all our readers that much more unenlightened/antagonistic/confused or simply downright stunned by my latest contribution to this delightfull topic.

Kokatha man

omnivorr
20-01-2008, 01:31 AM
:P I hope I don't screw-up adding my lil' diagram..but here goes

lets use terminology that reflects what is goin on... are we not actually aligning with the earth's polar axis? ("pole-stars" whatever, are merely a convenient indicator).... we create the angle of 'tilt' to get our base parallel with a base as it would be if we were on the equator.

this then allows us to pivot the base, around that axis, in 'equal and opposite' rotation to the earth's rotation, thus staying fixed on the same point over time.... as opposed to "pointing to the pole" we are pointing parallel...

think of it like a conrod always pointing at the piston...the gudgeon-pin is parallel to the crank... :D

as for "declination"..is that to do with the tilt of the earth's axis relative to our orbital-plain?? :shrug: jargon annoys me :mad2::screwy:
cheers
Russ

Kokatha man
20-01-2008, 07:27 AM
Greetings omnivorr - good to hear from you: I wasn't going to encourage this thread anymore (it was a promise I made to get me out of "high security") but since I've been let out and have "gone back to my own planet" they won't be able to restrain me anymore - so here goes.....

I actually started all this to ascertain the merit of putting setting circles on a dob mounted scope: ie was there any merit in it?

I wasn't getting into debating any value in declination indicators (I've triggered enough confusion!) - just Right Ascension scales; concluding that if you used them, they only give one (possibly two) correct positions/points in the sky that are correct RA points (both of these are meaningless from the point of useability because you can't slew/move your scope along these longitudinal positions to get the necessary Declination required to fix/pin-point/locate objects because moving the dob mount's altitude axis will instantly change the RA setting you have supposedly set with your RA indicator - even though you have set this scale from a zero or reference point of geographical north south.)

You're right, jargon can be really annoying, but in specific subjects it helps for people who are interested in the same pursuits. (eg your use of mechanical analogies is fine with me, but half of today's mob wouldn't know what the crank is, let alone the gudgeon pin!) It has also become evident to me during the life of this "thread" that many people really have no real innate/intimate understanding of RA (what Gary called "the mental gymnastics in how lines of RA and Dec are mapped out....." but are happily functional accepting the "it is so" viewpoint - and good on them!

However, if we align with the Earth's polar axis (which is an EQ set-up) we aren't aligning our mount base parallel with what it would be at the equator - rather we're aligning it with how it would be at the north or south poles, off-setting it to these positions with respect to our latitude, to arrive at an analogous positioning - which is why a dob mount sitting on the poles instantly becomes an EQ mount.

Having said all this, I will be putting setting circles on my dob-mounted GSO 10" Newt - but only because I'm waiting for my Round Table equatorial platform to arrive; and in that set-up they will have some relevance - especially as this purchase postpones the likelihood or me getting an Argo Navis system for some time!

Regards, Darryl.

omnivorr
20-01-2008, 06:45 PM
Hi Darryl, ...I'd better explain myself before I go causin' any worse confusion or bother to anyone by it. As I'm only just startin to peck at this roast-chicken called stargazing, I'm nowhere near the bones of it yet!

so this is my approach ( no Dob yet, still counting pennies for it)...

base on ground.. compass beside it... line on base parallel to axis thru the tube (i.e. "lightpath" of tube, not pivot-axis) inline with compass n-s. Now, chart to tell me correction from magnetic N to true N,
.. put compass-ring on Dob base , align it with compass, then shift base x degrees according to correction chart for my location......

my base is now aligned true north-south, but still not parallel with the earth-axis.... for this the eq-platform , with correct angle of tilt for my location, goes under base...north aligned with base, thin end north (for southern hemisphere) now the base is aligned with n-s axis and parallel with it... and can rotate on the eq-platform to follow the east to west apparent motion of the sky..... all the above is taken as if there is no turntable, just the base of the dob, or a fixed dob.... there is in fact no need for a turntable that I can see..... tube swings n-s , platform swings e-w... entire hemisphere of sky covered... somebody correct me if I've missed something please!!

so.. to the next step of the path to Go-To, I'd assume some scale applied to each axis....( but whether degrees ,minutes , or mm&inches ..I don't care.. whatever is the convention...) ..some scale or other, in whatever language the direction-sensing apparatus and motors can use to work with the controlling software... a scale on the arc of the tube, and a scale on the arc of the eq-platform.... { keeping the turntable on the base might be a matter for manual-pointing convenience but I could only see it unnecessarily confusing the tube-arc calculations.... therefore, no turntable..and no ring of any scale on the base, but for the simple compass-correction at setup.}

well, that's my plan of approach.. I hope.. (if)when I've had my Dob a while and ready to start "chasing" ;):D

I had in fact thought of a motorised design.. and later strayed across a photo of same/similar someone has made, but now can't find it :(, that demonstrates my approach, completely doing-away with the turntable....
(a member of BAS, I think it was)... I'll enquire soon, when I join :):thumbsup:

Cheers Russ

vinnie
20-01-2008, 08:18 PM
This thread is ridiculous

Dobs are Alt Az/ Gems track

But now you are all up about EQ on a dob????? Yep I could see how I guess, but why?

I think that all (serious) beginners should start with a pair of binoculars and an atlas. Then progress to a GEM and learn how to use it. My opinion.

If this thread is typical of this forum then I wonder why I am here? The slanging is outrageous

I am waiting for replies from a couple of blokes who have asked for reasonable advice re mounts.

After that then if this is the standard of your forums I'm out of here.

you could have called on 45 years experience from me but if this bunfight is typical......... then forget it

Kokatha man
20-01-2008, 08:19 PM
Hi again omnivorr - removing or dispensing the ability to rotate the dob base of your scope (ie the azimuth motion) and taking into account all your other steps (which is how I first postulated this subject in my first post that started this thread) and sitting it on an EQ platform will prevent you from looking at any objects that are not on the meridian line you have set by where you've plonked the mount on the EQ platform. That is, you will only be able to move the scope along the north south line parallel with the scope's light-path axis (as you refer to it) directly above this scope-tube position.

This is because an EQ platform is fixed to the ground and cannot rotate, it only describes the apparent path of an object's motion for a limited arc-movement (approx 1 hour usually.)

Keeping the mounts ability to revolve in the horizontal plane as well as the vertical allows you to point at almost anything in the sky before you then activate the EQ platform's tracking drive. Otherwise you'd have to physically drag the proposed fixed/static baseboard of your "crippled" dob mount around on the fixed platform/base of the EQ platform to sight up objects not on the meridian you've plonked it on! Analogue setting circles will function as a rough setting fix/go to with the unfettered dob mount on an EQ platform.

Hope this makes sense to you, regards, Darryl.

Kokatha man
20-01-2008, 08:51 PM
Dear Vinnie - I'm sorry if you find my thread offensive or off-putting (though you could of course ignore it) but omnivorr replied to me and brought up the issue of EQ platforms.

Many dob mount people use them and find them of great merit: and for the life of me I can't see how this thread could possibly interfere with you (or anyone else's) ability to view responses to "replies from a couple of blokes who have asked for reasonable advice re mounts" (sic) - your quote.

Perhaps you have taken unintended offence (or do not/cannot appreciate) attempts by me to inject a little humour into subjects which we are all passionately interested in. Perhaps you are unable to understand that the content actually is quite serious, and that like yourself, many of us are really quite experienced/knowledgable about areas pertaining to these interests. I myself can call on over 50 years of experience/involvement in skywatching and AA.

One thing I totally disagree with is your confused perception of this thread as containing "slanging" or "bunfighting" - it has not and never will contain such from my part, and I don't believe any other contributor to the thread has done such.

At any rate Vinnie, I wish you the best and (belated) New Season's Greetings and the capacity to surf and view or interact as you wish, regards, Darryl.

ps - back in 1956 I started with my old man's binoculars, the star atlas (mainly northern hemisphere) and The Big Golden Book of Astronomy; with a burning desire and head full of questions and wonder: however, I would never attempt to tell others how they should get started in anything - but perhaps that's because I'm a bit of an anomoly amongst Australian Aboriginal people, a mad astronut and (proud) Kokatha (Western Desert) man!

omnivorr
20-01-2008, 09:19 PM
To Vinnie: sorry u feel that way. ...if u stuck around u might yet learn something, despite your 45yrs of "experience".

I myself have exactly zero experience..or as near to make no nevermind!!

My Mudda has near 70 years of experience ..in cooking. She still can't cook beyond over-boiled veg and dry-cooked meat.......:P;):lol: so forgive me if I flippantly disregard your dramatic coat-fling exit stage left.....

To Darryl, and any other observers...

....if the tube rotates on axis x (parallel to earth-axis), starting from E horizon... while at the same time pointing at any point from the S horizon to the N horizon ( pivot-axis of tube now verticle, at end of fork pivotted from the x axis) ....whatever the tube is pointed at will remain aligned sofar as the scope tracks it by following in rotation on the x axis..... is this not correct????

Kokatha man
20-01-2008, 10:05 PM
Hi again omnivorr - our mothers must have used the same cookbook!

All this is very hard just using words: if we could on-line make continuous sketches and point etc it'd be solved in a flash!?!

Reading your last paragraph I understand the x axis as being "polar aligned" (ie parallel to the Earth's axis of rotation) but am not 100% about the "pivot axis of tube now verticle, at end of fork pivotted from the x axis"

From my perspective, if saying the above you are saying "pivot axis of tube which is at the end of fork pivotted from the x axis is perpendicular (at right angles) to this x axis (ie it rotates or spins around a projection or imaginary extension of this x axis) then you have your EQ set-up and the object pointed at will be able to be tracked as it moves from east to west.

Will leave you to ponder my (imagined) re-phrasing of your statement - I'll sign off for the night in case The Thought Police come knocking.....have no idea who would possibly sick 'em onto me though!?!

omnivorr
21-01-2008, 01:45 AM
Oh! Darryl ...yes it would be so much easier if together we could point to the east and raise our arm to follow the stars in the sky... I'd bend my arm at the elbow to point north/south... you'd see it in front of your own eyes... not just my vague words!! you'd see immediately what I mean, and immediately show me if/where I'm wrong, by demonstration in front of my eyes.. ( something like "Netmeeting" setup on our puta's, and drawing on graphics -tablets would help, but sofar at my end I'm not yet "on top" all that
:P )

..but let's say, together side by side we're facing east... I hold my arm out pointed at the horizon, straight, ..my fist at the end is the open end of the telescope tube..... my shoulder is the pivot-point for my arm.. let's say I'm leaned against a stump beside me , on my left, so my shoulders are parallel with the earth-N/S-axis...... now my straight arm moved at the shoulder will follow the movement of the sky.. my shoulder is doing the job of the EQ-platform.

..now I bend my elbow to point my fist N-S...parallel to my shoulder-line, (it will point down to the ground!--my right arm to point north) or my left arm to point south....it will point up to the sky! ) I don't have an elbow that can swing both ways... but a Dob does:D) ... so my fist can point E-W from shoulder movement. and anywhere N-S from elbow movement, along the parallel-to-earth-axis N-S horizon...

doesn't this leave a gap between the "real" N-S horizon and the "polar-aligned" one? ( on the south end)

...well, there's nothing to stop the shoulder axis pointing below its horizon!!, (my shoulder can't swing behind my back, but the EQ-platform mounted arm can!!) and the (left arm) elbow can range in that field too! ...it'll just mean a short duration of track until that object rises above the polar-aligned-horizon of the shoulder (to the East) ....all this is if the object goes behind the earth....

but imagine an object very close to the south polar point (of the sky, not the Earth).. at some convenient distance from earth. In the southern hemisphere. .. that point will not go behind the earth ..... at a certain point from the equator that "object" could be observed without any tracking, it would remain centre-scope, ..only revolving.. like an old EP on a platter :P
....your EQ-mount "crippled" scope could lie flat on either side or be flagpole erect... no difference between'em! ...every point on your latitude focused on the same point purely by the 'alt' angle...

I've concluded from all this : "Azimuth" is a flat-earth concept!!!
:D:P;):lol::eyepop::thumbsup::rofl: :rofl:
:hi:

omnivorr
21-01-2008, 05:38 PM
me again. found photo of where my mind was goin'. I'll shutup now:doh:

Kokatha man
21-01-2008, 07:54 PM
What can I say - that you deigned to imagine some sort of choreographed "pas de deux" as we swung our arms and pivotted our elbows every-which-way, doing a Monty Python version of "The Right Ascension Waltz" - but you told me you were leaning against an old stump: not a monster scope!?! Those instruments you're surrounded by quite rattled my gudgeon pins!

Please tell me that you only own a pair of K-Mart binoculars with one lens broken, if only to alleviate my jealousy!

And to think that I was contemplating one day ammending my profile to boast of my own puny gear: to become what my mate calls a "blowhard." (no offence fellas, I actually enjoy drooling over other people's equipment lists!)

No wonder you wanted to talk about crankshafts and gudgeons!!! Those babies in the photo look like they pillaged a D9 or two!

But even if you do only have the K-Mart nocs, I'll still look forward to seeing your posts on IIS.....
Regards, Darryl.

duncan
21-01-2008, 09:11 PM
Man thats one hell of a mount in the photo Omnivorr. How much does that baby cost? Cause it's what i need for my 12" Dob!
Cheers:eyepop:

omnivorr
21-01-2008, 10:24 PM
Hi fellas, I know nothin' about it except that he stole my idea!!:eyepop:

here's the link.. http://www.qldastrofest.org.au/html/tony_hs1_05.htm

Now I really want to go to Astrofest!!!
:D

(PS as yet Darryl I don't even have those Kmart nocs :p, no man hath greater envy than I in this regard :P )

Cheers
Russ

alistairsam
18-10-2010, 02:22 PM
Hi
was looking for threads explaining Dec and RA gradations for EQ mount setting circles and found this thread.
I'm kind of getting my head around the Equatorial co-ordinate system and can understand how RA is divided into 24hrs, but can someone explain the How the Dec co-ordinates work.
I'm using an equatorial fork mount, and the RA axis is set to 38deg for Melbourne. That said, if I align my RA axis to point to the SCP, and the tube is parallel to the RA axis, my understanding is that the Dec is 90Deg. But I read that the Dec is divided into 4 quadrants of 90 deg.
So if I move my OTA on the Dec axis by 90 deg toward azimuth (alt/az) or overhead, would that be 0 deg Dec? what happens if I move further up?
to clarify, I'm starting Dec at 90 deg pointing to the SCP when RA is aligned to SCP, I then move only along Dec by 90 towards North, then Dec becomes 0, and how does the Dec axis increment or decrement? to -10deg, -20 deg and so on?

found this pic http://en.wikipedia.org/wiki/File:Setting_circles_equatorial_mou nt.jpg

secondly, in the eq co-ordinate system, if all objects have constand RA and Dec co-ordinates, what actually changes depending on time of day? is it the hour angle? I'm using stellarium to visualize.

Terry B
18-10-2010, 02:47 PM
Whe the scope is aimed at the SCP it is actually pointed at -90deg dec. As you point it away from the SCp towards the celestial equator you approach 0deg. If you go furter north you are then aiming into the +ve Dec area. the N celestial pole is+90 deg.

alistairsam
18-10-2010, 03:29 PM
Thanks. what happens if you go below the SCP? does it go from -90 to zero again? so SCP is -90, below that say -80, -70 and so on?

Terry B
19-10-2010, 10:01 AM
Yes
If you are standing on the south pole and look at the zenith you are looking at the SCP. Going any direction away from that is moving to less negative declination.
For me at latitude 29deg south, the SCP is 29 deg above the horizon. From that point (at -90 dec), if I move my scope say 10deg down it will be pointing at -80deg dec. The stars at that spot will move around the SCP with time and 12 hours later they will be 10deg above the SCP. As this shows it doesn't matter which direction you move away from the SCP it will still be moving to less negative dec.

alistairsam
19-10-2010, 06:20 PM
thanks. as the position of the objects in the equatorial co-ordinate system don't change significantly (we're rotating and not the stars), do we use the Hour angle to find an object?

eg, position of sirius from stellarium at 0011hrs at Melbourne is

RA/DE(j2000): 6h 4m 8.8s / -16d 43' 1.1"
HA/DE : 17h 56m 22s / -16d 43' 44"

and at 0311hrs is
RA/DE(j2000): 6h 4m 8.8s / -16d 43' 1.1"
HA/DE : 21h 56m 22s / -16d 43' 44"

so if I were to have a list of objects with their RA/DE values and I use just the gradations on my equatorial mount, after polar aligning, how do i find out what the RA angle would be or where the object would be?
I'm guessing it would be based on an offset of the RA angle from the observing time and position

Terry B
19-10-2010, 06:32 PM
I have never used the hour angle just the RA and DEC.
Yes you are correct. You set the RA by offsetting against a known RA.
Even for a goto scope it has to be originally aligned to a known point but the computer will then compensate for the rotation of the sky with time.
If you start at a known star and set the RA setting circle on your scope it will only be correct at that time without tracking. If the scope tracks then the RA will remain correct until you turn off the scope.