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orientation of DSLR in Newtonian

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#1 Nicole Sharp

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Posted 12 September 2019 - 11:48 AM

I noticed that my Lunar images taken with a Barlowed Newtonian telescope and a Canon Rebel DSLR camera at prime focus appear to be correct-image (not up-down reversed).  Any explanation for this?  I don't think the Barlow lens would flip the image, so maybe the DSLR camera is doing it automatically?  What would happen if the camera sensor was rotated by 180 degrees (so that the hotshoe was on the bottom and not on the top of the camera when doing Lunar observing)?

 

I had the largest dimension of the APS-C sensor oriented to be parallel with the optical axis of the Newtonian telescope.  The image appears to be correct but does look to be slightly off-axis compared to Virtual Moon Atlas, so I am guessing that (somehow) the image is being up-down reversed (twice), but there is still a slight rotation from a correct-image (naked-eye) view since the alignment between the camera sensor and the optical axis was not likely perfectly parallel.

 

I figure maybe the DSLR camera is programmed to automatically flip the image from the sensor, if it is primarily designed to be used with refracting DSLR lenses which may be producing a left-right up-down reversed image?  But it doesn't appear to be left-right reversed.


Edited by Nicole Sharp, 12 September 2019 - 11:58 AM.


#2 vtornado

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Posted 12 September 2019 - 11:52 AM

The camera's software  flips it.  If you take your lens off your camera point it outside, and hold a white piece of paper

behind the back of the lens at its focal plane, you will notice that the image is upside down/left right reversed. 


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#3 Nicole Sharp

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Posted 12 September 2019 - 12:05 PM

Forgive the chickenscratch from a desktop PC without Windows Ink:

 

https://www.nicolesh...n/Capture13.PNG

 

https://commons.wiki...landing_map.jpg

 

It does look to be rotated by maybe 20 degrees or so, but other than that looks like correct-view?



#4 Nicole Sharp

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Posted 12 September 2019 - 12:09 PM

The camera's software  flips it.  If you take your lens off your camera point it outside, and hold a white piece of paper

behind the back of the lens at its focal plane, you will notice that the image is upside down/left right reversed. 

That makes sense, since the lens is a refractor.  But why doesn't the camera left-right flip the image from a Newtonian?  It doesn't know whether it is attached to a refractor or a reflector?



#5 Nicole Sharp

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Posted 12 September 2019 - 12:18 PM

And does this mean you can use Newtonian telescopes for correct-image Terrestrial photography/videography with a DSLR camera?  :-O



#6 Nicole Sharp

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Posted 12 September 2019 - 12:26 PM

Pretty sure that's actually Tycho and not Theophilus down there.  The rotation and non-full phase confused me but otherwise I think everything is correct.  I'm not complaining that the DSLR is giving correct-image views, since that would be pretty cool to try for Terrestrial photography (which I never really thought would be practical with a Newtonian).  But still curious as to why it's not left-right reversed if the camera is expecting an up-down left-right reversed image from a refracting or catadioptric DSLR lens.



#7 vtornado

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Posted 12 September 2019 - 12:32 PM

A refractor and a Newtonian both present what in physics is termed a "real image" which is left/right reversed and upside down.  The diagonal usually used in a refractor flips in the vertical dimension.  The cameral lens does not have the diagonal.

 

Take a few snaps with your telescope outside at a road sign, and see what you get.


Edited by vtornado, 12 September 2019 - 12:36 PM.

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#8 Nicole Sharp

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Posted 12 September 2019 - 12:41 PM

A refractor and a Newtonian both present what in physics is termed a "real image" which is left/right reversed and upside down.  The diagonal usually used in a refractor flips in the vertical dimension.  The cameral lens does not have the diagonal.

 

Take a few snaps with your telescope outside at a road sign, and see what you get.

I was thinking exactly that.  Just need to find something far enough away.  That is pretty cool that I don't have to flip images in postprocessing though.  For photos it's not a big deal, but for video would save a lot of work.

 

A helicopter flew over the telescope the other day while Solar imaging, but by the time I reoriented the telescope away from Sol and carefully removed the fragile Solar film filter, it was long gone.


Edited by Nicole Sharp, 12 September 2019 - 12:43 PM.


#9 Nicole Sharp

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Posted 12 September 2019 - 12:54 PM

A refractor and a Newtonian both present what in physics is termed a "real image" which is left/right reversed and upside down.  The diagonal usually used in a refractor flips in the vertical dimension.  The cameral lens does not have the diagonal.

 

Take a few snaps with your telescope outside at a road sign, and see what you get.

 

What would be the orientation from a Newtonian when doing eyepiece projection with a DSLR camera though?  Still correct-image?


Edited by Nicole Sharp, 12 September 2019 - 12:56 PM.


#10 vtornado

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Posted 12 September 2019 - 01:10 PM

Now we are in theoretical ground as I don't do epp. 

However, I ASSUME since the eyepeice does not flip the image of the telescope to your eye,

It will not do anything to the camera either.  Give it a whirl ...

Daylight is your friend to get the kinks worked out.

 

Ohh wait, in epp there is a lens on the camera, so telescope flips, then camera lens flips, then camera sw flips,

ergo I think we are upside down again.

 

Oh btw, it does not have to be a road sign anything that is not symmetrical top to bottom and left to

right will work.  Near my home I have a radio tower that has a light on top and some antenna

masts on the side.  It can be seen for a few miles easily.

 

I have noticed that you are interested in photographing the Mercury transit.

Some larger sunspots are as large as Mercury, some are much larger, but

you could experiment taking sunspot pics to get all the kinks worked out of your

technique.  Perhaps, one of the experts here could tell you how big any particular sunspot is,

so you can get an idea of image scale.


Edited by vtornado, 12 September 2019 - 01:23 PM.


#11 Nicole Sharp

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Posted 12 September 2019 - 01:41 PM

Now we are in theoretical ground as I don't do epp. 

However, I ASSUME since the eyepeice does not flip the image of the telescope to your eye,

It will not do anything to the camera either.  Give it a whirl ...

Daylight is your friend to get the kinks worked out.

 

Ohh wait, in epp there is a lens on the camera, so telescope flips, then camera lens flips, then camera sw flips,

ergo I think we are upside down again.

 

Oh btw, it does not have to be a road sign anything that is not symmetrical top to bottom and left to

right will work.  Near my home I have a radio tower that has a light on top and some antenna

masts on the side.  It can be seen for a few miles easily.

 

I have noticed that you are interested in photographing the Mercury transit.

Some larger sunspots are as large as Mercury, some are much larger, but

you could experiment taking sunspot pics to get all the kinks worked out of your

technique.  Perhaps, one of the experts here could tell you how big any particular sunspot is,

so you can get an idea of image scale.

 

In eyepiece projection, there is only one lens, the eyepiece.  In afocal imaging, there are two lenses, the eyepiece and the camera lens.

 

If eyepieces don't change the orientation, then I am guessing it should be the same as prime focus then.

 

But yes, knowing the correct orientation in the camera will be important for the Mercury transit if I want to keep Mercury centered (instead of Sol).  I need to know where on the Solar disc Mercury will first appear.  Especially if there are no sunspots to reference for a coordinate system, I would have to guess the position of Mercury before the transit begins solely based on the camera orientation.  This would be particularly critical in a Barlowed Newtonian where the full Solar disc is not visible.  Attempting to center on Sol could mean missing Mercury, or if Mercury is visible, it will be off-axis for increased comatic aberration.


Edited by Nicole Sharp, 12 September 2019 - 01:49 PM.


#12 Kendahl

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Posted 12 September 2019 - 01:42 PM

An odd number of mirrors switches right for left. An even number does not. To get a right-side-up image out of a Newtonian, rotate your camera 180° or rotate the image file afterward.



#13 Nicole Sharp

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Posted 12 September 2019 - 04:13 PM

So I was thinking about this, and the photograph of Luna I took at prime focus appears to be rotated by maybe 20 degrees.  But if the orientation of the camera sensor was misaligned by 20 degrees from being parallel with the optical axis of the telescope, I should have noticed that.

 

What I think would make more sense is that in order to obtain a correct-image view, the camera sensor has to be parallel to the horizon, and not to the telescope.  Since Luna had just risen, it was probably about 20 to 30 degrees in altitude angle, so that would make sense then.  From the perspective of someone standing on flat ground, if the camera sensor is parallel to the optical axis of the telescope, then the view in the camera sensor should be rotated by the altitude angle of whatever the telescope is pointing at (it's like holding the camera at a sideways angle).  I'm guessing then that the only way to maintain a correct-image view when tracking an astronomical object would be to rotate the camera sensor so that the orientation of the sensor is always parallel to the horizon.  Otherwise the view (relative to the ground) would always be rotated by whatever the altitude angle of the telescope is (if the camera is kept parallel to the optical axis of the telescope).  That would only be for Newtonian telescopes on altazimuth mounts though.  If the camera is on the end of the telescope like with a refractor or Cassegrainian, the orientation of the sensor would always be parallel with the horizon, regardless of the altitude angle of the telescope's optical axis (on an altazimuth mount).

 

I'll have to do some experiments, but let me know if I might be wrong about any of that.  For the Mercury transit though, it sounds like I just need to make sure the camera is parallel with the horizon (can use a bubble level on the hot shoe for that), and then maybe I could figure out the position of Mercury before the transit begins (invisible with a Solar filter).  Though the view would begin rotating away from a correct image as the altitude angle increased over the duration of the transit.


Edited by Nicole Sharp, 12 September 2019 - 04:28 PM.


#14 Kendahl

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Posted 13 September 2019 - 02:56 PM

If you look at different photos of the same object on Astrobin, you will see that astrophotographers don't care about orientation. "Correct" orientation is a meaningless concept.

 

A GoTo altitude-azimuth mount, like you said you plan to rent, is like a camera tripod. What starts out horizontal stays horizontal no matter where the camera points. Since the transit will begin around sunrise, I think it will look best if you can orient your DSLR so that the longer side of the image is parallel to the horizon. That will match what a person would see assuming he was wearing properly filtered sunglasses and had sharp enough vision to pick out Mercury on the sun's face. Note that I said "looks best" rather than "correct". "Looks best" has real meaning. "Correct" doesn't.

 

One consequence of this is that the image will slowly rotate clockwise as the transit progresses. Don't worry about it. The image will rotate for a visual observer standing on the ground, too. Suppose there are two sunspots and one is above the other at sunrise. At noon, they will be side by side, with the "upper" one to the right of the "lower". At sunset, the "upper" one will actually be below the "lower" one. For a visual observer to keep them in the same orientation all day, he would have to lie on his side at noon and stand on his head at sunset.

 

The rotation seen by a visual observer, or a camera on an alt-az mount, is called field rotation. It's a problem for astrophotographers whose exposures last for many minutes. Their solution is an equatorial mount that, by its nature, matches this rotation thereby cancelling it in the photograph. My own mount is a German equatorial and my preferred camera orientation has the longer side parallel to the celestial equator. If I try to photograph the transit, I might change the orientation so that it was parallel to the horizon at sunrise. Then, because it's an equatorial, the image of the sun won't rotate over the duration of the transit.

 

You will see Mercury only as small, black spot on the sun's disc. It will be invisible, before the beginning of the transit and after it ends, when it's outside the disc.


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#15 Nicole Sharp

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Posted 13 September 2019 - 04:29 PM

If you look at different photos of the same object on Astrobin, you will see that astrophotographers don't care about orientation. "Correct" orientation is a meaningless concept.

 

A GoTo altitude-azimuth mount, like you said you plan to rent, is like a camera tripod. What starts out horizontal stays horizontal no matter where the camera points. Since the transit will begin around sunrise, I think it will look best if you can orient your DSLR so that the longer side of the image is parallel to the horizon. That will match what a person would see assuming he was wearing properly filtered sunglasses and had sharp enough vision to pick out Mercury on the sun's face. Note that I said "looks best" rather than "correct". "Looks best" has real meaning. "Correct" doesn't.

 

One consequence of this is that the image will slowly rotate clockwise as the transit progresses. Don't worry about it. The image will rotate for a visual observer standing on the ground, too. Suppose there are two sunspots and one is above the other at sunrise. At noon, they will be side by side, with the "upper" one to the right of the "lower". At sunset, the "upper" one will actually be below the "lower" one. For a visual observer to keep them in the same orientation all day, he would have to lie on his side at noon and stand on his head at sunset.

 

The rotation seen by a visual observer, or a camera on an alt-az mount, is called field rotation. It's a problem for astrophotographers whose exposures last for many minutes. Their solution is an equatorial mount that, by its nature, matches this rotation thereby cancelling it in the photograph. My own mount is a German equatorial and my preferred camera orientation has the longer side parallel to the celestial equator. If I try to photograph the transit, I might change the orientation so that it was parallel to the horizon at sunrise. Then, because it's an equatorial, the image of the sun won't rotate over the duration of the transit.

 

You will see Mercury only as small, black spot on the sun's disc. It will be invisible, before the beginning of the transit and after it ends, when it's outside the disc.

 

I am thinking about canceling the one-day rental for the altazimuth GOTO mount, and getting a manual German equatorial mount instead.  It would be cheaper, I could keep it forever, and it wouldn't need to be powered.  I can still do video clips or timed bursts by manually tracking Sol over the transit in an equatorial alignment.

 

Is the type of rotation I am talking about though unique to Newtonian telescopes on altazimuth mounts, or to all telescopes on altazimuth mounts?  Would there still be rotation in the prime-focus camera for a Newtonian on a nonmotorized German equatorial mount?  I thought the problem is because the camera is on the side of the telescope (perpendicular to the optical axis of the telescope), when the telescope is angled upward.  If the camera was on the back of the telescope (parallel with the optical axis), such as with a refractor or Cassegrainian, the rotation would be less, the same, or different?

 

Knowing the orientation would still be very important for the Mercury transit if there are no sunspots.  Without any sunspots (in white light), there is no frame of reference on a featureless white disc to know where Mercury will first appear, unless you can figure out Solar coordinates from understanding the orientation of the camera relative to the horizon (or the apparent motion of Sol) and the type of mount/OTA used.


Edited by Nicole Sharp, 13 September 2019 - 04:41 PM.


#16 KLWalsh

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Posted 13 September 2019 - 08:00 PM

Any rotation of the telescope not around the Earth’s axis (= polar axis in an EQ mount) will cause field rotation at the eyepiece. Even in an EQ mount not perfectly aligned with the pole there will be field rotation - but the rotation would result in only very small circles if it ran over a full day. In most astrophotos the small amount of field rotation in a mis-aligned EQ mount is too small to worry about.

#17 Kendahl

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Posted 14 September 2019 - 09:35 PM

Field rotation is caused by the mount, not the telescope. An equatorial mount is just an altitude-azimuth mount with the vertical axis tipped over so that it is parallel to the earth's axis. You get field rotation unless the axes are perfectly parallel. The only places you won't get field rotation with an alt-az mount are the north and south poles.

 

If you use an equatorial mount and orient your camera so that the longer side of the frame is parallel to the celestial equator, the frame is more than wide enough to fit the sun but not quite tall enough. It turns out this won't matter because Mercury will enter the sun's disc at its left edge and exit near the top right but still well within the frame.

 

You could also make an alt-az mount work if you oriented your camera so that the shorter side was parallel to the horizon (i.e. portrait orientation) and offset the frame so that there was a little bit of space between the right edge of the sun's disc and the edge of the frame. Mercury would enter at the bottom and exit at the top right.

 

I'm not at all comfortable with using a completely manual equatorial mount. It doesn't have to be GoTo but it should have motors. Correcting the tracking errors of a motorized mount, to the accuracy needed for photography, is difficult enough. Trying to twist knobs to keep the sun in position is a huge step more difficult. It would require your undivided attention, with you eye glued to your finder scope, for the duration of the transit. You couldn't spare time to run your camera. For your purposes, a motorized equatorial mount would be best but a motorized alt-az would do.

 

Our first telescope had an Orion SkyView Pro GEM. We paid extra for a polar alignment scope and motors on both axes. With a half way decent polar alignment, the mount would keep an object in view for several minutes without intervention. Then, it was just a matter of tweaking alignment with the hand controller to get the object centered again. We have never regretted the extra expenditure even though it turned a $350 mount into a $590 one.



#18 Nicole Sharp

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Posted 14 September 2019 - 09:50 PM

Field rotation is caused by the mount, not the telescope. An equatorial mount is just an altitude-azimuth mount with the vertical axis tipped over so that it is parallel to the earth's axis. You get field rotation unless the axes are perfectly parallel. The only places you won't get field rotation with an alt-az mount are the north and south poles.

 

If you use an equatorial mount and orient your camera so that the longer side of the frame is parallel to the celestial equator, the frame is more than wide enough to fit the sun but not quite tall enough. It turns out this won't matter because Mercury will enter the sun's disc at its left edge and exit near the top right but still well within the frame.

 

You could also make an alt-az mount work if you oriented your camera so that the shorter side was parallel to the horizon (i.e. portrait orientation) and offset the frame so that there was a little bit of space between the right edge of the sun's disc and the edge of the frame. Mercury would enter at the bottom and exit at the top right.

 

I'm not at all comfortable with using a completely manual equatorial mount. It doesn't have to be GoTo but it should have motors. Correcting the tracking errors of a motorized mount, to the accuracy needed for photography, is difficult enough. Trying to twist knobs to keep the sun in position is a huge step more difficult. It would require your undivided attention, with you eye glued to your finder scope, for the duration of the transit. You couldn't spare time to run your camera. For your purposes, a motorized equatorial mount would be best but a motorized alt-az would do.

 

Our first telescope had an Orion SkyView Pro GEM. We paid extra for a polar alignment scope and motors on both axes. With a half way decent polar alignment, the mount would keep an object in view for several minutes without intervention. Then, it was just a matter of tweaking alignment with the hand controller to get the object centered again. We have never regretted the extra expenditure even though it turned a $350 mount into a $590 one.

 

Yeah, the idea of using a manual GEM was exciting at first, but I quickly realized that having to balance and polar-align the mount in a hurry before the transit might be a bad idea.  So going to try to keep the rental for the Celestron NexStar 6SE.  No wedge though, so will still be altazimuth.  Tracking won't be that great either doing a Solar alignment at Sunrise without a King rate (probably won't be able to do a nighttime alignment).  Will have to realign maybe after the first quarter of the transit so the King error doesn't keep accumulating.

 

My plan would be to orient the largest dimension of the camera sensor to be parallel with the horizon before the transit begins (landscape orientation).  I ordered a bubble level that slides into the hotshoe at the top of the camera, and if that doesn't work, I also have a bubble level that attaches to the tripod hole at the bottom of the camera.  If I can get a full-disc view though, and keep Sol centered, I don't think the orientation would matter except at the very beginning, so I can know where to expect to see Mercury first appear.

 

And no, the GEM I was considering was the Celestron AstroMaster.  It's the cheapest I could find.  No polarscope, shaky lightweight tripod, and the clockdrive is bad enough that trying to motorize the mount would actually make it more difficult to use.


Edited by Nicole Sharp, 14 September 2019 - 09:59 PM.


#19 Alex McConahay

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Posted 15 September 2019 - 08:53 AM

>>>>>  but the rotation would result in only very small circles if it ran over a full day. In most astrophotos the small amount of field rotation in a mis-aligned EQ mount is too small to worry about

 

In the case of any one image, I agree there would be little, if any, evidence of field rotation. However, over the duration of a Mercury Transit, there will be enough rotation to make the path of Mercury over the face of the sun curve substantially. 

 

Of course, one could rotate the frame, aligning the image of the sun one after the other in the series of images until the evidence of field rotation disappears. To do so, though, would require features (a sunspot in white light, or a prom in Ha) on the sun. And features on the sun may be a problem with the weather it has had lately.

 

Alex



#20 Kendahl

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Posted 15 September 2019 - 10:30 AM

Yeah, the idea of using a manual GEM was exciting at first, but I quickly realized that having to balance and polar-align the mount in a hurry before the transit might be a bad idea.  So going to try to keep the rental for the Celestron NexStar 6SE.  No wedge though, so will still be altazimuth.  Tracking won't be that great either doing a Solar alignment at Sunrise without a King rate (probably won't be able to do a nighttime alignment).  Will have to realign maybe after the first quarter of the transit so the King error doesn't keep accumulating.

 

My plan would be to orient the largest dimension of the camera sensor to be parallel with the horizon before the transit begins (landscape orientation).  I ordered a bubble level that slides into the hotshoe at the top of the camera, and if that doesn't work, I also have a bubble level that attaches to the tripod hole at the bottom of the camera.  If I can get a full-disc view though, and keep Sol centered, I don't think the orientation would matter except at the very beginning, so I can know where to expect to see Mercury first appear.

 

And no, the GEM I was considering was the Celestron AstroMaster.  It's the cheapest I could find.  No polarscope, shaky lightweight tripod, and the clockdrive is bad enough that trying to motorize the mount would actually make it more difficult to use.

My experiences photographing eclipses and transits has been that I needed to tweak tracking at least every half hour to keep the target from drifting too far. Selecting the optimum tracking rate will, at best, extend the interval between tweaks.

 

Some suggestions:

  • With solar filter in place, use your telescope, Barlow and camera to take a few photos of the morning sun. This will enable you to select ISO and shutter speed. It will also tell you how much of the sun's disc you can get into the frame. (My calculations indicate that the frame is more than wide enough but not quite tall enough.)
  • Transits and eclipses occur now and then. Sunrise happens every day. Take advantage of that to rehearse the procedure you intend to use for the transit. Most likely, you will encounter problems because one or more aspects of the project won't work the way you expected. Repeated rehearsals enable you to find problems and fix them before the day of the transit. By then, you want everything to run smoothly. The fix may be to settle for a more modest goal.
  • Establish some minimum goal that you can be sure to achieve so that you will get something out of the transit even if it's not as much as you want. A handful of still photos of Mercury on the sun's disc that you make into a Powerpoint presentation would qualify. As you get more confident, you can increase the complexity and sophistication of your minimum goal.
  • Leave yourself plenty of time to set up before the transit begins. Rushing leads to mistakes you wouldn't make if you weren't under time pressure. You don't have to wait for the sun to get a preliminary alignment. Use a star near the eastern horizon.
  • There is an astronomy club in Cumberland (www.cumberlandastronomyclub.org). Some of its members are likely to have plans for observing the transit. My experience with such clubs is that they welcome new members and are willing to assist them. The club or a member might have a decent equatorial mount to lend you for the transit. Just having a place to set up before sunrise would help.



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