Tarantula Nebula - Need help to remove star-trails

[PETER LOO]

6 Dec 2012 the South Sky looks like at its best for this year, managed to collect 2 hours of ngc 2070 (Effectively approx 1 hour as using long exposure noise reduction feature of the camera), combined the 50 shots (40 X 1 mins @ISO 200 and 10 X 40s @ISO 400) produced better details but can't fully remove the star trial using the "align + rotate" feature in Nebulosity, sifu or anyone please help, is it that the lens spherical aberration also play a part to the trials as shown in the photo attached? The photo has already been crop

[orly_andico]

Peter, these aren't all star trails.

You also have field curvature, because some of the trails are pointing inward toward the center.

I assume this is with the Meade 80ED triplet? even if it is a triplet, at f/6 it will still have pretty strong field curvature. Do you have a flattener for it? I would strongly suggest a flattener, they are quite inexpensive ($150 range). These are pretty much identical (an achromatic doublet) so the Orion one, or Astro-Tech, William-Optics, etc. would all work fine.

Alternatively if you can't get a flattener quick, make sure the object is centered on the camera. And then only use that central portion (crop). Your image looks like the Tarantula was off in one corner because the curvature is asymmetric.

Another issue is focuser sag. If the focuser sags a bit, objects at the top and bottom of the frame will be elongated due to being out of focus. I am fighting this problem right now. Sadly the only solution for that is an upgraded focuser, specifically a Feathertouch. But fix the field curvature problems first and 70% of your oblong stars (should) go away.

[Gary]

So much nebulosity and stars! Thanks for sharing. 6 Dec was indeed a special night.

[PETER LOO]

Thanks Orly & Garry, the camera used was 500-D I-R mod, mounted with 200 mm F2.8 lens with 1.4 X extension. Maximum opening at F4.0 used, hence, may need to fall back to the Meade 80 ED though, which supposed to be a APO F6.0 Triplet ? (actually i don't fully understand what all this meant as i am more an electronic guy than a season astro guy) & the focuser is not so smooth. Tried visual (25mm ep) on that night, seems able to see the core of the Tarantula but not certain, a small little patch (to be verified next round). Also, thank Orly for the suggestion to do a proper balance, (i heck care last time), it improved the tracking a lot supprisingly ! The mount motor i thought suppose to be noisy are now much quieter. Also for light exposures, stacking the many frames together saw them deviate by much less.

[orly_andico]

The 200/2.8L shouldn't have so many aberrations.... is your polar alignment good? It might be field rotation.

A camera lens is a very undemanding OTA. I'd expect perfect stars across the field.

[PETER LOO]

Yeh ! good news then if it is not the lens cos need to rely on it heavily for the faster speed. The mount alignment was not very accurate, as again i did not spend time to do a 1 star drift alignment due to "kiasu mindset" (afraid that the good sky would be quickly vanished once the cloud comes in but it didn't happen until about 2 hrs later.

[beginner]

Hi Peter,

Not bad for a shot out of the window.
May I ask whether the photo is at 100% crop?

Thanks for sharing.

[PETER LOO]

Thanks my friend, not too sure if it's more than 100% crop as just dragged a box around the better portion, the edge stars look extremely bad, it might caused by the low altitude near horizon n aggrevated by seeing n poor optics etc.. don't say my poor skill, lol...

[cataclysm]

Incredible shot from the window peter!
Looks like you are experienceing field rotation from the photo. From my experience, it's a real challenge to get precise polar alignment from the window, hence it's worth praising you for the efforts!

[PETER LOO]

Thanks Kielvin, our sifu for the constant encouragements, will start to consider guiding it although meanwhile always encounter difficulty in choosing a suitable star in SG, (the red stars in LMC NGC2070 region also looks weak to use as guide stars here).