Affiliation
American Association of Variable Star Observers (AAVSO)
Mon, 04/13/2020 - 19:18
I'm in the process of getting my dome slaved to my scope. I suspect that it is not yet setup right, that sometimes the scope is partially obscured by the dome.
What is the impact on the images I take?
Does it just reduce the amount of light?
Will illumination across the image still be uniform? Or one side dimmer?
Thx,
George
Theory suggests that for a near-field obstruction, (1) light flux is of course reduced, (2) out-of-focus star shapes will be warped to roughly match the incoming light field shape, and (3) in-focus star shapes will be a bit broader (smaller effective scope aperture) and will have more diffraction artifacts because of the new edges in the near field.
In practice, it may not make much difference other than shot noise, and if the obstruction is small and you've already had vignetting you may find that it makes less difference still.
Having said that, I would definitely recommend testing this experimentally on one's own system, at least by taking photometric images with vs. without an artificial obstruction (use the dome artificially if you're remote), in order to verify that no relative bias in measured star fluxes arises across the images.
Seems like sky flats would not match unless they were similarly obscured.
Ray
When dome is blocking the telescope, high precision differential photometry will be affected, depending on the location of target(s) and compstar(s) in the FoV. See e.g. graph below that shows CY Aqr lightcurve (Credit: Taavi Tuvikene). Dips at ~0.81 and ~0.92 show the effect when manually rotated dome blocked ca 1/4 to 1/3 of the aperture of an 80 cm telescope. The distances between CY Aqr and two compstars were ca 5' and ca 9' along RA. Simple measurement uncertainty is most probably smaller than the size of points.
Best wishes,
Tõnis
Thank you all for your comments..
I've done an examination of the dome shutter positioning and there is clearly a problem. So now I have a project to improve it! First step I'll attach a ring of lasers to the end of the scope so I can check the pointing precision with the shutter closed. Then its a matter of optimizing the 5 parameters that drive the dome azimuth equation. Getting that equation into a programming environment has been a bit of a bear. I've found versions in VB and in C# but need it it python.
Hopefully I'll end up with a procedure that I can share!
Hmm. With a ring of lasers I could have an active pointing adjustment. If I'm pointed well, there will be no scatter off the slit edges...
Cheers,
George
Maybe if you had that laser battery you could shoot Mr.Musk's satellites out of the sky...
Hi George,
Using lasers for active pointing is kinda similar to what was done with the 200" in the early days, using a "phantom telescope" model:
https://www.astro.caltech.edu/palomar/about/history.html
The best paper for determining dome azimuth was by my graduate advisor (Kent Honeycutt) and one of his students (Jim Kephart):
https://ui.adsabs.harvard.edu/abs/1982PASP...94..605H/abstract
the exact equations are in there in a generalized form. We implemented dome control and full automation as part of an IMSAI 8080 microcomputer with 64KB of memory and 4MHz CPU speed...
Arne
Arne, thank you for good reference!
I have particularly enjoyed the approach of Toshimi Taki, because it really explains how those complex equations form. It's a simple ( :-) ) linear algebra:
http://www.brayebrookobservatory.org/BrayObsWebSite/BOOKS/matrix_method_rev_d.pdf
Best wishes,
Tõnis