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image orientation in BYE BYN



OK so Im struggling with an Off Axis Guider.


One of the things I'm trying to wrap my had around is that the OAG is oriented towards the bottom of my T3i (sticks out from the imaging train, directly "down" relative to the camera). However in moving a star or terrestrial object, in live view, from the T3i FOV into the guider camera FOV I need to move things "Up", out of the top of the frame and focus window, before they come into view, from the bottom, in the guide camera view.


Also the vignetting from the OAG prism (if I push it too far in) is seen in the "top" of the live view image. 


Does this imply that BYE is showing me an inverted or vertical flipped image?


My scope is an RC, but terrestrial objects seem right side up in BYEOS live view.


I do note that the guider image is flipped left for right, vs. the BYEOS Live View.



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The camera is right side up, so I don't see how any "auto rotate" setting would effect it. Besides, wasn't that was a pre 4.1.3 bug (showing the thumbnails auto-rotated) that got fixed?


Here's my thinking, and hopefully others can confirm or correct; the symptoms make sense if the light is coming to a point focus in front of the DSLR sensor, and then spreading out again to cover the area of sensor. I know "prime focus" is supposed to be where everything comes to a point, and that it's always drawn that way, but if it was a point then wouldn't you just get a single dot on your image?


However it seems to me that one problem with my theory is that it would seem there would be two points where things in in focus, one on either side of "prime", but I haven't seen that.


I can't seem to find anything on line that describes the image orientation of RCT with an image sensor at prime focus, only eyepieces:


"Image Orientation
The image orientation changes depending on how the eyepiece
is inserted into the telescope. When using the star diagonal,
the image is right side up but reversed from left to right
(i.e., mirror image). If inserting the eyepiece directly into the
focuser without the star diagonal, the image is will be upsidedown
and reversed from left to right (i.e., inverted). This is

normal for the Ritchey-Chrétien design."


Does that imply that the sensor image at prime focus will be "true and correct". That's what I see. Terrestrial objects are right side up and correct left to right, and sky objects agree with my planitarium, but that doesn't help me understand the upside down behavior of the OAG vignette and FOV position relative to the DSLR FOV (unless I am correct in my above theory).



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This is meant to help me visualize where I need to aim the scope to put a guide star in the guider FOV. The larger red box is the FOV of DSLR, and the smaller red box is the size (but not the position) of the OAG guider FOV.


The Orange target, marked "scope", is where I would have to aim the scope to put Alioth in the guider FOV. About 4.5 arcmin BELOW the objects' coordinates, makeing the object ABOVE the DSLR FOV.


I'm also wondering why there is such a gap between the two FOVs. Since I'm seeing vignetting from the OAG prism in the top of the DSLR FOV, I can't move it any closer. Does this mean my prism is not perfectly aligned at right angles to the optical axis?

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In your original note you said that the guide camera was pointing down relative to the DSLR, you did not say that the bottom of the DSLR was facing the ground. Hence my comment about the auto rotate functionality.  People have been burned by auto-rotate in the past.


One thing that is confusing you is that all light does not come to focus at the the same place on the focal plane when in focus.  All light from a point source is displayed at the same place when in focus. Since you are getting light from many sources, you see light across the entire field.


The quote from your previous email is correct.  If you put an eyepiece directly into your RC, the image through the eyepiece will be reversed both left to right and up to down. However, if you remove the eyepiece and put a camera directly into the RC, the image should be correct, both left to right and up to down. This is because the eyepiece reverses the view in both directions.


Light reflecting off of your RC's primary mirror is reversed in both directions.  When the light reflects off of the secondary mirror it is reversed, yet again.  This gives you a normal, unreversed image in both direction when seen through your DLSR's viewfinder which is mounted at prime focus..


However, since the light in the guide camera is reflected off of the pickoff prism, that image will be reversed top-to-bottom but not left-to-right.


Also, it is normal for an OAG that the stars in the guide camera FOV are not seen in imaging camera FOV.


So, digest that and post any remaining questions.

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Thank you and that all makes good sense. 


I'm still stuck with two questions.


1) If my OAG is positioned like this:




so the pick off prism is coming into the light path closest to the bottom of the DSLR sensor,  why do I see the vignetting from the prism on the top of the image, and why to I need to move a guide star "UP" out of the top of the DSLR FOV to get it to come into the bottom of the guidescope FOV?


2) I understand that the guide star should not be in both FOVs at the same time, but assuming the pickoff prism is shoved as far into the lightpath as possible, without obscuring part of the DSLR FOV, why does there seem to be an ~1.5 arcmin gap between when the star leaves the top of the DSLR FOV and enters the bottom of the guider FOV? Is that an indication that the pickoff prism is not properly aligned?

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Question 1) If what you say is so, then stars in the guide camera FOV must be coming from the opposite side of the secondary mirror. The fact that when you move up in the main FOV and it comes in at the bottom of the guide camera FOV is because the pickoff prism reversed up and down.


Question 2) the vignetting that you are seeing could be due to aberrant reflection/refraction off the exteme lower part of the pickoff prism onto the imaging sensor, even before the pickoff prism is physically intruding into the imaging light path.


I would adjust the pickoff prism so you do not see any hint of it in your images and go from there.  The reversals are irrelevant because they will be taken into account during autoguiding calibration. The only time that you will have an issue is if there are no stars in the guide camera FOV.  The typical solution in that case is to rotate the main camera and OAG until you can see a guide star.

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OK thanks I will keep at it.


I have a QHY5L-ii mono (vs the Orion SSAG in the picture) for this rig so about as sensitive a camera you can buy for under $1K. I was trying to test how dim a star I could guide on last night but the clouds kept coming through.


I was successful with a mag 6 star.


It's just so different from the minigude scope where there are always multiple stars to choose from in the guide FOV.


I'll have to modify my cooling rig to let me rotate the camera as well.

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If I understand your question, the explanation is simple. The image in all cameras is upside down on the film or sensor. If you've ever used a camera with ground glass focusing such as a Speed Graphic or view camera this fact is obvious. Therefore, if you insert the OAG prism from the bottom, it appears to enter the viewing area from the top.



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