Finding perfect focus with V-Curve fitting based on a hyperbolic function

This article is about finding the best possible telescope focus by approximating a V-Curve with a hyperbolic function. A V-Curve is the result of moving the focus of a telescope from an outside focus position into the focus and then further into the same direction out of focus again. As probably not surprising this curve is called V-Curve since it is similar to a V. The focus can be measured using different measures - for example the "Half-Flux Diameter" (HFD) or the "Full width at half maximum" (FWHM). In this case the HFD is used. This way a V-Curve comes into existence where the focus position is shown on the x-axis and the focus measure (HFD) on the y-axis (right figure).

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Trifid nebula M20 & open cluster M21 from La Palma

LocationLa Palma / Spain
ObjectTrifid nebula (M20) & open cluster M21
Guidingyes, QHY5-II Mono via OAG
Telescope8" GSO Newtonian
Barlow lensnone
MountEQ6 Syntrek
Luminance8x 600s, bin: 1x1
Red7x 150s, bin: 2x2
Green7x 150s, bin: 2x2
Blue7x 150s, bin: 2x2
Total exposure~2h12m
Tonight I decided to image the Trifid nebula (M20) which is quite "close" to the Lagoon nebula M8 from my last session. The seeing tonight was around ~2.4" which could have been better - but certainly it was still sufficient.
Back in 2013 I already imaged the Trifid nebula with the same equipment but from Boeblingen. It is interesting to see the difference here.
For post-processing I used the free software DeepSkyStacker and GIMP. The full resolution images is available here.

Clear skies!

Stargazing time on La Palma - Casa Las Flores

It's stargazing time on La Palma again - this time from Casa Las Flores. The little house is located in Aguatavar - close to Tijarafe on a height of about 580m. You have a power supply outside the building, free view to Polaris from both terraces and in July the bright part of the Milky Way is perfectly visible between 22:00 and 4:00 local time. Furthermore you have a coffee maker, a table, chairs, a fast WLAN internet connection and a barbecue. What else do you need? 😀

The image is a single frame recorded with an Canon EOS6D in combination with a Samyang 2.8/14mm lens (15 sec. exposure time, ISO4000) . The post processing has been done with rawtherapee.

Tonight I only test my equipment - for tomorrow I plan to record the bright nebula M8 in the milkyway. So stay tuned for the result!

Astrophotography from La Palma - The Coma Galaxy Cluster - Tacande Observatory

LocationLa Palma / Spain
ObjectComa galaxy cluster
CameraModified Canon EOS6D
TelescopeR120S / 120mm
Barlow lensnone
MountEQ6 Syntrek
Luminance6x RGB frame / 240s
Total exposure~24m
Tonight I again spent the night at the Tacande Observatory on La Palma. There we observed the Coma galaxy cluster. The result is this photo taken with a modified EOS6D camera.

For post-processing I used the free software DeepSkyStacker and GIMP. The full resolution images is available here.

Clear skies!