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>>> When you get to very long focal lengths however OAG becomes technically challenging.?

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In my experience it's not all that challenging. I'm imaging at 3454mm with a monster moag and haven't bothered with star selection (yet?).?

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I had to borrow a quite old SX superstar camera, which adds technical issues, but overall it's pretty much same as in my driver?@ 840mm

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I have an obs partner Martin Pugh who uses AO and definitely has to think about guidestars due to the very short exposure times of the AO unit

On Mon, Aug 9, 2021 at 4:24 AM Michael Ben-Yehuda <mikeby.mikeby@...> wrote:

Tom, We'd need to see the images to see the orientation of the movement and if it's consistent for different parts of the frame and in different parts of the sky. What was the exposure time and what is the pixel size? What is the focal ratio?? You mentioned two different cameras, the ASI1600 and the ASI120. Both of these cameras have similar pixel sizes (3.8 micron vs 3.75) As others have said, when you go from a 400mm to 180mm f/l? with this size sensor it does not provide sufficient resolution. The image scale is around 4.5 arc-seconds per pixel. these cameras are better matched to the 1000mm scope. Even at 400 mm your image scale is over 2 arc-seconds per pixel which is still slightly undersampled in all but poor seeing conditions . You are better off with the asi178 at 2.8 microns or the ASI294mm at 2.3 microns. On the other hand the ASI120 should be ok with an OAG at 1000mm.? Image scale is about .78 and you are slightly oversampled.

It could also be the 180 guider's optical axis wasn't precisely aligned with the main telescope/imaging system. Orthogonal alignment errors can look like polar alignment or cone error. The guide software, by correcting the misaligned guide camera imparts an error on the main imaging system. The differential gets multiplied by the ratio of the effective focal lengths of the two optical systems and by ratio of the pixel sizes (imaging precision).

The impact to your images (oblong or 'stretched' stars) like other tracking errors is still dependent on the effective focal length of the imaging optical system it's precision, where in the sky you are pointing and exposure time. These are the same factors that we normally consider when imaging, so if you see this problem when you're collecting data and don't have an option to reconfigure you should shorten exposure time and take more subs.

1000 mm F/L in combination with today's popular imaging cameras is typical for when these issues start being noticed.

There's a general recommendation in the astrophotography community these days to use an off axis guiding system rather than a guide scope when imaging using long focal lengths over about 1000 mm. This eliminates the focal length ratio multiplier and solves most of the alignment precision challenges with guide scopes that need to have fairly long focal lengths themselves and common mounting rail systems. Ironically there's an argument for using OAG on shorter focal length compact refractors too because of the mechanics of alignment and reduction of equipment load. When you get to very long focal lengths however OAG becomes technically challenging. The longer the focal length the smaller the field of view which can make selecting guide stars difficult. Pixel sizes need to be larger to match the focal length. The new ZWO monochrome planetary cameras using the new Sony sensors have very low noise and are extremely sensitive. Very small pixels can be binned by the low level driver to optimize pixel size.? Precision mounting systems and rails, higher mass, increased stiffness and eliminating dissimilar metals with different thermal coefficients? ?and careful installation followed by testing to confirm orthagonal alignment in is time consuming??

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Brian?

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Brian Valente

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