Jul 10

Refraction Correction

Again I’ve learned something new about the polar alignment 🙂 : Polaris can be at different positions in some apps that takes into account refraction correction!

Wrong position of Polaris?

The iOptron GEM28 mount has a polar finder section on the hand controller. But the position of Polaris seemed to be wrong, compared to the Polar Finder– and the StarAdventurer Mini app which I usually use!

I double checked the time and site settings on the mount’s hand controller. Everything seems to be correct: local time, long/lat, DST and UTC offset.

All 3 screenshots are taken almost at the same time:

  • Star Adventurer Mini App shows 03h23m for Polaris
  • PolarFinder shows 03h24m
  • iOptron hand controller shows 3h15.5mwhy is it different to the other 2 apps?

Polar Alignment Refractive Correction

So I contacted the iOptron support. They answered me, within a few hours (!), the iOptron software takes into account refractive correction (read more at astrocamera.net). Thats the reason for the difference: Atmospheric refraction offset depending on the latitude.

Especially in lower altitudes (below 10°N) atmospheric refraction should be factored in.

But no problem at all, even if I would use the position from the apps. It’s considered to be good enough for long exposure astrophotography. Particularly at my latitude!

AccuAlign Polar Alignment

Polar alignment is very easy with the AccuAlign polar scope. I was lucky yesterday evening to do my first polar alignment with the GEM28 🙂 Alt-Az alignment feels rock solid compared to the Star Adventurer!

This is the iOptron polar scope view: the inner circle group is for the Northern hemisphere. It’s divided into 12 hours (12 full lines) and every hour is divided by 10 minutes (6 short cross lines). Even the radius is displayed from 36 to 44 minutes.

So, polar alignment is mastered on my new mount. Now I need to attach somehow the EF400mm lens 🙂

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