Guider camera reflection images

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Guider camera reflection images

Postby David Cejudo » Fri Dec 25, 2020 10:27 pm


I am almost starting with LISA, so i have many doubts. Here is one.
This one is about some strange reflections that appear whenever i get a bright star in the slit as imaged by the guider. To reflections appear, one above, and one below the star.
I wonder if this is normal.
I am attaching two images, a twenty seconds long exposure with no star in the slit and another, one second long with the star Deneb in the slit. Both are jpeg images as i found the forum does not allow me to attach FITS files.

I tried with other stars but i got the same problem. I rotated the camera to try, just in case, but i found that there is no problem with the camera.

I am using the 35um wide slit and an Atik Titan camera as a guider.
Also, the telescope is a 12"Meade GPS with a focal reducer by Optec, a Next Gen 0.5x.

One more question. Is cleaning the mirror slit a good idea? What should i use for that?

Thank you for your comments.
Guider_1sec_ Deneb.jpg
Guider_1sec_ Deneb.jpg (20.71 KiB) Viewed 851 times
Guider_20sec_Deneb.JPG (251.16 KiB) Viewed 851 times
David Cejudo.
Observatorio El gallinero.
El berrueco, Madrid.
David Cejudo
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Re: Guider camera reflection images

Postby Ken Harrison » Sat Dec 26, 2020 3:53 am

This is usual when guiding on bright stars.
It’s caused by the reflections from and within the glass slit plate. Nothing to worry about.
The glass slit plate can be cleaned using a air puffer, if dust persists then a careful wipe with IPA or Baader fluid should work.
Remember the actual slit gap is on the rear surface of the plate as viewed from the telescope side.
"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before....
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Re: Guider camera reflection images

Postby Robin Leadbeater » Sun Dec 27, 2020 5:50 pm

Hi David,

The reflective coating is deliberately on the back side of the slit. If it was coated on the front (like normal telescope optics) the internal reflections you are seeing would enter the spectrograph instead and give ghost spectrum images. You will not notice the reflections once your star is correctly exposed.

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