EQ mount question

[5ash (Philip)]

Can anyone enlighten me on " how far from the south celestial pole can a gem mounted telescope see ".I'd imagine it depends on how far the telescope axis is from the mounts polar axis.Is there a formula to work it out?
regards philip

[bojan]

How far..
You mean what is the most "northern" object above your local horizon it can point to?
That depends on your latitude, and the formula is (90 - |La|).
For example, for Melbourne at -37°, we can see the objects at declinations up to (90-37) = +53° or below that. Higher dec are hidden from our view here.
In terms of how far from SCP. that is (180-|La|), and (again for Melbourne) that is 143°
This number is independent of the type of mount.

[Dennis]

I have often seen the formula “90 degrees minus your Latitude” to determine what celestial objects are visible from your location.

So, for my location of Brisbane at 27.5 S, I would expect to be able to view objects up to 62.5 degrees N declination? Such objects would be right on the horizon.

As I write, I’ve just launched The Sky and selected a star (Dubhe) on the N horizon and it has a Dec of +61 degrees.

Cheers

Dennis

[5ash (Philip)]

Im sorry i probably worded my question badly , what i meant to ask was how close to the south celestial pole can a gem mounted scope see .A fork mounted scope can see right to the pole , how close can a gem mounted scope get?
regards philip

[bojan]

The same, if the axes are truly perpendicular.
This is not related to the type of mount.

The only issue with gem is, you can not track the whole day when telescope is pointed to the SCP or near it (actually, if it is pointed anywhere south from zenith), because the tube will hit the stand or tripod sooner or later. This is not a big problem, once you re aware of it.

[5ash (Philip)]

To my mind , if the axes of the scope and polar axis are parallel then as the polar axis rotates the telescope should point and move in a small circle around the south pole. It would therefore never see the south pole as does a fork mounted eq mount.So how many /fractions thereof degrees from the pole would the circumference of the circle be??It would surely have to be related to the distance of the telescope from the polar axis.
regards philip

[Saturnine (Jeff)]

Hi Sash

If your polar axis is aimed at , for auguments sake, Sigma Octans and your scope is centred on the same star, the 20 cm. or so between your axis' will, projected into space, essentially merge. Think of looking at a railroad track dissapearing into the distance, although the rails are parallel, how far can you see before the two lines of the track merge.
Hope that is an easy way to understand the concept.

Regards

Jeff

[mithrandir (Andrew)]

Originally Posted by Saturnine

Hi Sash

If your polar axis is aimed at , for auguments sake, Sigma Octans and your scope is centred on the same star, the 20 cm. or so between your axis' will, projected into space, essentially merge. Think of looking at a railroad track dissapearing into the distance, although the rails are parallel, how far can you see before the two lines of the track merge.
Hope that is an easy way to understand the concept.

Regards

Jeff

Also remember that the field of view is not just the diameter of your scope.

Lets throw some trigonometry at the question. I'll stick to cameras because the numbers are more concrete. The same principle applies to eyepieces.

Take the suggested 20cm offset from the centre of the polar axis to the optical axis of the scope. Now suppose your camera is showing a 1 arc minute field.
Any object more than 676 metres away exactly on the polar axis will be visible.
At 1 arc second that falls to 12 metres.

For an example, a 200mm SCT (f10.0) with a QHY8 at prime focus has an image field 26.6 by 40 arc minutes. The distance for that combination is 26 metres.
Replace the QHY8 with an ATK-16ic (6.2 by 8.2 arc minutes) and it is still only 111 metres.