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Optical weirdness - Off Axis Guider abberations


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Hi all

I am having some issues in setting up an OAG on my Officina Stellare Veloce RH200 Astrograph, I had hoped that it would be a straight swap from my APO as the camera configuration was staying the same. however i found that i did not have sufficient back focus and had to remove around 13mm of spacing from between both cameras and the OAG, as a result the focus position of the guide camera was lost, I assumed that setting it would be as easy as when i set it all up on my  80mm APO... well we all know what they say about assumptions. 🤣 Instead of seeing the out of focus image of a star as the usual ring shape, i found there were two arc shapes which appeared to converge as the focus point was reached leaving a kind of elongated "X" Shape.

As a sanity check to see if  i was near focus i decided to aim the guide camera at the moon, I found the image was a little soft but still pretty close to focus.but the strangest thing was a dark pillar that extended about 1/2 way up from the bottom to the centre of the image, It looked like there was some physical obstruction away from the focal plane. However inspection has found no evidence of any such obstruction.

I then got to thinking about the optical path and the behaviour of the light along it.

First off a description of my optics / Imaging train, The Astrograph has an aperture of 200mm and a focal length of 600mm, the focal plane is 60mm beyond the focuser being fully wound in and the field is supposed to be "sub micron" flat up to 51mm diameter. The Main imaging camera is an EOS 50d (back focus 44mm) which is fitted with the Teleskop Service 9mm OAG using the direct EOS mounting ring option. The OAG is configured so that the pickup prism is as close to the centre as possible without casting a vignette on to the main sensor. I've measured this to be about 12-14mm from the centre of the optical axis. Connecting the OAG to the focuser is a 2" shoulder-less nose piece which allows the OAG to butt right up against the drawtube,

We start at the focal plane where a sharp in focus star converges to a point that is around 8 micron diameter (around 2 pixels on my EOS 50d) If i then move my focuser by 1mm the ring produced by the out of focus star is approx 100 pixels in diameter (this equates to about 0.5mm diameter on the sensor) when you project the spread of the light cone out to 50mm forward of the focal plane (which is approximately where the prism lies) the ring of light works out to be a diameter of around 24mm.

Now the prism of the OAG is 6mm x 10mm which means that only a section of the 24mm ring will fall on its surface therefore projecting an arc back towards the focal plane, does this seem like a reasonable explanation as to what is happening here? although i am not sure how this could cause the 2 arcs to converge in an X shape or even how it could cause the dark "pillar" on the image of the moon.

Does anyone have any advice as to setting up an OAG on a fast compound astrograph like this? I had imagined that OAG would be easy as there is tonnes more light coming in compared to the 80mm APO which incidentally never failed to find a guide star.

I would like to have had the foresight to save some of the images from the guide camera but alas i didnt think about it until afterwards. I'll try again tonight if its clear.

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  • 2 weeks later...

No idea really but have you tried moving the stalk /  prism further in towards edge of chip ? Blow everything out with blower brush ( hair or eyelash in path ) rotate guide cam to see if pattern moves to see if camera or oag ?

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A picture would help :) - does it change if you move the star to different positions? e.g. does it get worse as it gets futher away from the center of the telescope FOV ?

Edited by SamAndrew
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I have the pickup prism as close to the main sensor as possible without causing too much vignetting and everything has been inspected and is meticulously clean. Although this afternoon i noticed that tightening the lock screw for the OAG focus position causes the guider to tilt, it looks like the lock screw was grabbing the stalk so I have rounded the tip of the screw, everything seems to snug up nice and square now and daylight images seem sharper so I will try it out when its next clear.

I have noticed that the image from the QHY5 is very noisy which makes me wonder if that is adding to the issue preventing Maxim DL from finding a guide star.  I looked in to trying my old Atik 16IC-S Camera which is more sensitive but has slower image download, but unfortunately due to its rather large diameter it fouls on the backplate of the telescope. 

I must admit that i keep forgetting to save images, Next time I'm set up I will make sure to grab some.

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