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Sculptor Galaxy - Saxon 200DS 1st light


hjw
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I finally got around to get myself a 8" f/5 Newtonian with a GSO coma corrector and had my first go at the Sculptor galaxy (NGC253). The picture was taken from 33 frames, 180sec at ISO800. First off, I was impressed with how much light the scope collects compared to the ED80. I could already see the dust lanes on the individual frames. I noticed that I still had coma in the corners of the full image and searching the internet found that the coma corrector requires a 15-20mm spacer to work with DSLRs - Bummer! The stars are wider than what I am used to. According to the Bhatinov mask, focus should have been pretty spot on but compared to the ED80 my star width went from 3.6 to 5.6 pixels. Is this due to the missing spacer ring, still not perfect focus or that my mount (HEQ5) is struggling with the weight, or do I have to live with it? Just trying to understand what is normal for a reflector and what I can improve.

Thanks in advance and clear skies!

HJ

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Great image!

Reflectors have more stuff in the light path than refractors. This not only gives you those typical star spikes, but also diffraction "halos". In your image, you can see the effect of the mirror clips around the bright stars. Stars are usually not as tight in reflectors as they are in refractors.

You can reduce star size during processing.

Star size in pixels also depends on focal length. Compare star size in arcseconds instead:

Arcsecs = pixel-size (microns) x 206 / focal-length (mm)

Btw, did you use flats? There's still some vignetting, especially at the left in your image.

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Thanks Wim, no, I haven't used flats. With the ED80 there was never a need to do so and this was virtually the first day of using the scope. I took some darks though but adding them during processing made things much worse. It probably will take some time to "master" the beast :)

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Darks and non-cooled camera don't always get along well. Instead, use dithering (= move the fov about 12 - 15 pixels in a random direction between exposures), and let the stacking program get rid of hot pixels.

Tony Hallas has a good youtube video where he explains the benefits of dithering. Google "Tony Hallas DSLR astrophotography"

Imo, flats and bias frames are the more important calibration frames when it comes to dslr imaging.

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2 hours ago, wimvb said:

Darks and non-cooled camera don't always get along well. Instead, use dithering (= move the fov about 12 - 15 pixels in a random direction between exposures), and let the stacking program get rid of hot pixels.

Tony Hallas has a good youtube video where he explains the benefits of dithering. Google "Tony Hallas DSLR astrophotography"

Imo, flats and bias frames are the more important calibration frames when it comes to dslr imaging.

I would love doing dithering but the Pentax can not be tethered. I think I have to try harder to find a solution... And I will try using flats and bias frames.

Thanks

HJ

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