Field of view orientation with dobsonian

[Stephen_M]

Hi,

I'm sure this is an easy question for someone, so I'm going to post it rather than research myself! It's something that sketching has made me think more about.

I'm wondering, is there an easy way to discern what the orientation of your field of view is on a dobsonian? I get that it's reversed, and if I wait a minute, I can tell the direction of travel around polaris.

Is it the case that 12 o'clock on the eyepiece if your eyes are parallel to the telescope body will be 6 o'clock if you were looking up from the primary mirror to the sky? Or is it something else? I would consider myself to have good spacial awareness, but just can't figure this out!

Thanks, Stephen

[cajen2]

Everything is upside down, so south = north. The other thing that always confused me is that Betelgeuse, for example, is on the east side of Orion as you look, not the west, as directions are reversed when you look at the sky.

Edited March 8, 2022 by cajen2

[Stephen_M]

Thanks for the reply, perhaps I have not phrased my question correctly.

I get that it's inverted, but for sketching I am hoping to learn how the layout of the field of view is affect by where the telescope is pointed and at what body/head/eye position I am looking through the eyepiece at. With the dob, I can obviously either look through with my eyes perpendicular to the telescope, or parallel, or potentially any other angle between 9 o'clock, 12noon and 3 o'clock. I'm not particularly flexible, so the angle will depend on what's comfortable for me!

Using my clock reference, is there an easy way to orient yourself? Thanks.

[Louis D]

Since the Newtonian's field of view is flipped and reversed, it's a 180 degree rotation if your head's major axis is perpendicular to the tube.  As I always like to point out, if you walk around to the other side of the tube and look over it into the eyepiece with the crown of your head pointing to the ground, the view is correctly oriented, assuming the tube is parallel to the ground.  Thus, if you rotate your head 180 degrees, you counteract the 180 degree image rotation.

If you were observing at zenith with your head rotated 90 degrees to the eyepiece, you'd have counteracted 90 degrees of the rotation.  So, simply subtract the number of degrees your head is from being completely perpendicular to the tube from 180 to figure out how much the image is still rotated.  For instance, at 45 degrees altitude, you'd be 45 degrees from perpendicular, so 180-45=135 degrees of image rotation.  At 60 degrees altitude you're still 30 degrees from parallel to the tube as at zenith, so 180-60=120 degrees rotation.  Only observing at the horizon will you truly be 180 degrees rotated.  I'll let you work out whether the rotation is clockwise or counter-clockwise.

[Nyctimene]

Just watch the movement of the stars in the field of view of your (undriven) scope, easier done at higher magnifications. In a Dobsonian (=Newtonian) scope, the stars will leave the field in the West direction. 90 ° counterclockwise to this direction is North. Mnemonic: stars enter from the East and vanish in the West.

Hth.

Stephan

Edited March 8, 2022 by Nyctimene

[Stephen_M]

Thanks everyone. Those descriptions are really helpful, the other thing that springs to mind is to look at a few object in the garden before it gets dark.

I guess I'm hoping this skill will be useful for multiple things such as sketching, and also interpreting star maps to star hop. Thanks again.

 

[Pixies]

14 hours ago, Nyctimene said:

Just watch the movement of the stars in the field of view of your (undriven) scope, easier done at higher magnifications. In a Dobsonian (=Newtonian) scope, the stars will leave the field in the West direction. 90 ° counterclockwise to this direction is North. Mnemonic: stars enter from the East and vanish in the West.

Hth.

Stephan

I just sing, "Go West..."

[Zermelo]

9 minutes ago, Pixies said:

I just sing, "Go West..."

Ah yes, but do you do the actions at the same time?

 

 

 

[Mandy D]

Because a Newtonian reflector has two mirrors, there can be no lateral inversion of the image at the focusser. However, because of the location of the focusser, there is an apparent rotation of the image. With the focusser in the normal location, this rotation will be 45 deg to the axis of the tube. An eyepiece will normally rotate the image by a further 180 deg, whereas a camera at prime focus will not. For imaging it is normal to rotate the camera through 45 deg anti-clockwise wrt the main axis of the tube.

[Louis D]

2 hours ago, Mandy D said:

Because a Newtonian reflector has two mirrors, there can be no lateral inversion of the image at the focusser. However, because of the location of the focusser, there is an apparent rotation of the image. With the focusser in the normal location, this rotation will be 45 deg to the axis of the tube. An eyepiece will normally rotate the image by a further 180 deg, whereas a camera at prime focus will not. For imaging it is normal to rotate the camera through 45 deg anti-clockwise wrt the main axis of the tube.

Good point.  If you take out the eyepiece and look at the image of the moon directly down the focuser tube by pulling your head back a bit, does it look upright and non-inverted?  I'll have to check next time I'm out with mine.

[Mandy D]

13 minutes ago, Louis D said:

Good point.  If you take out the eyepiece and look at the image of the moon directly down the focuser tube by pulling your head back a bit, does it look upright and non-inverted?  I'll have to check next time I'm out with mine.

Well, if you think about it, your camera does nothing to the orientation of the view, so what you see through it's viewfinder will be oriented exactly as per the scope without an eyepiece. So, yes, the Moon will be upright and non-inverted, subject to the 45 deg rotation I mentioned previously. I did take some photos when I was checking all this earlier and may post them tomorrow.

[Mandy D]

For reference, here are the photos I took when playing with my Newt in daylight. The first pair are with the camera aligned with the tube. The second, with the camera rotated 45° which corrects the orientation and the third shows what a normal eyepiece does to the image. If we rotate the eyepiece view so that the image is correctly oriented, everything will be the correct way round. i.e. no lateral inversion.

[Louis D]

I was mistaking the eyepiece's 180 rotation for the scope.

[Mandy D]

3 hours ago, Louis D said:

I was mistaking the eyepiece's 180 rotation for the scope.

I only realised it was the eyepiece and not the scope the first time I put a DSLR on one and was confused to see an erect image in the viewfinder!

[Stephen_M]

Thank you for this. Definitely highlights how difficult it is to understand the orientation of everything!

So with an EP, 180 degrees for the EP, and 45 degrees where the focusser is?

[Louis D]

1 hour ago, Stephen_M said:

So with an EP, 180 degrees for the EP, and 45 degrees where the focusser is?

Sounds about right.  You can see that the horizon has swapped corners with the eyepiece inserted vs. the camera view, but is still slanted 45 degrees.

[allworlds]

Old thread, but I think you're mistaken in attributing an inversion to the eyepiece. It's cameras that invert the view in software compared to what is physically on the sensor, to undo the inversion that's caused by a normal camera lens. (With an SLR's optical viewfinder, the reflex mirror and pentaprism sort the image orientation out.) Eyepieces magnify the image without inversion, as can be seen by a ray diagram such as https://i.stack.imgur.com/PHzEC.gif,

My claim could be tested by putting a sheet of tracing paper (or a ground glass pane if you have one) at the focal plane to directly observe the real image.