iOptron SkyTracker pushing the limits

[roka]

I wanted to try pushing the limits on my iOptron SkyTracker. I'm certainly no expert with only 3 months of experience but I've used it a lot. Tonight I wanted to see how long of an exposure I could get with my Canon 70D + 400mm f/56 combo. After figuring out that the star trails on all my shots were being caused by the speed switch being on 1/2 speed (oops) I was able to get 60 second exposures. At 90 seconds I was seeing a little star elongation. At 120 seconds I constantly got star trails. So my unscientific test says that about 60 seconds with a 400mm lens is the limit. At least that's the limit I'm working with until/unless I can figure out how to get longer exposures.

I then decided to image M42 and shot 20 subs @ 60 seconds. I only had a 50% keeper rate which was a bit disappointing. Here's the results of 10 subs @ 60 sec, 10 darks, 10 offsets and 10 flats. Stacked in Nebulosity and a just little PP in Capture One since this was just an experiment. Yes, I blew out the core but I wanted to get as much nebulosity as I could. Given that these were shot at my light soaked condo complex with a full moon I'm actually quite happy with it.

Comments, suggestions, opinions, etc are most welcome!

[Woodsey65]

Hi Roka,

If you were on holiday, and under really dark skies then you'd no doubt get even better results!   

I'd be happy with that result.

This is really good for a 60secs. I would have gone with 30secs, a higher ISO and a small lightweight 66mm APO Doublet to let in more light. I'm actually going try this, as the scope I have comes in just 2100 grammes. Add that to my EOS 450D and I'm nearly a kilo within the payload of 3.5kgs. For extra silliness I'll also see what I can do with a 3x Barlow (1200 fl) and a webcam for some planetary!

I've previously benchmarked the iOptron SkyTracker against Vixen Polarie and the SW Star Adventurer. Also the iOptron SkyGuider and AstroTrac TT320X-AG, all with 200mm-400mm lenses. This provides the focal length range that divides these portable trackers into two distinct groups.

The latter two are great at longer focal length for exposures over 60 secs, and of course small OTA's under 100mm aperture, and under 1200mm unguided.

The issue with tracking using 400mm lenses on APS-C DSLR's is that you're getting close to 600mm equivalent focal length, and the moment arm this creates due to mass, means the lens and camera body should be balanced in DEC. Usually it's not, and this can (and will) affect tracking performance especially as most people use the standard mounting points of either camera, or the lens foot if it has one. The Star Adventurer can be balanced in RA, if used with its counterweight.

The SkyTracker could work with, say, a Borg 71 if correctly set up, but again, I couldn't recommend going over 30-40sec for this focal length. Keeping it short means you should have less trashed frames. But the advantages of using a small APO over a 400mm telephoto lenses, is that there is less glass, easier (lockable) focus and more light gathering over a zoom lens to provide greater resolution and detail.

Lovely shot though. Don't care about the core. The Nebulosity wins every time, don't you think?  


 

[griz11]

The 400 is a great lens for AP I've been getting great results with mine. A little heavy for that tracker though. The only two I saw were the astrotrac and losmandy that would handle the weight and still be under the 2/3 rule. My 70-200 works ok but the 400 is really nice. Primes just have less elements no IS or zoom so instead of 10-20 elements it has 6. Gives a nice 2.1 arc sec per pixel ratio on the 70D and 7DmkII. How well did you polar align that might be some of the trailing. I thought I was getting close with the polar scope but best I could do was still almost 2 arc min off. Once you get it right on the money it all settles out. You would have to do a drift align to get it close enough. If it works for 30 sec then its likely some polar misalignment that is causing the abnormal stars. Some of the stars look a little too big which usually means its wiggling around while the exposure is going on. PE is a likely cause. But those are supposed to have pretty good PE from what I've read so I'd go for a really precise polar alignment and try the 400 again. That would be pretty cool if you could get it to work.

Griz

[roka]

Quick update: I'm discovering that some of my problems are "pilot error" as I keep learning about AP. Take last night as an example. I set up the 70D + 400mm on the SkyTracker on my Vanguard tripod. I've started weighting down the tripod by hanging a 10 lb. weight from the center pole. So last night I kept getting star trails no matter what I did or how short the exposure was. When I took the weight off it solved the problem. Then this morning it occured to me that I hadn't tightened down the locking screw for the tripods center pole where the tracker and weight attaches! The tiny swaying of the weight on the unsecured pole caused the star trails. As Homer Simpson would say, DOH!

But last night (after removing the weight) I imaged the Horsehead nebula for the first time. I did 45 second exposures with the 70D + 400mm and had a keeper rate of almost 90%. I'm going to try the weighted tripod again (making sure everything is tight) and see if I can get that kind of keeper rate with 60 second exposures.

[Woodsey65]

Cool Roka,

Coming along nicely! 60secs would meant that you may get nearer 70% (ish) success rate with your frames. Did you increase your ISO?  The PE on these trackers range from 23-40 arcsecs for smaller tracker and from 10 arcsecs for the AstroTrac and as little as 6 arcsecs for the SkyGuider over 10 minutes. These have been confirmed independent tests. I'm getting 6-8mins with my AstroTrac with a 70-300mm Zoom and a EOS450D, and 90% success rate.

Look forward to see some of your images soon!

[roka]

Cool Roka,

Coming along nicely! 60secs would meant that you may get nearer 70% (ish) success rate with your frames. Did you increase your ISO?  The PE on these trackers range from 23-40 arcsecs for smaller tracker and from 10 arcsecs for the AstroTrac and as little as 6 arcsecs for the SkyGuider over 10 minutes. These have been confirmed independent tests. I'm getting 6-8mins with my AstroTrac with a 70-300mm Zoom and a EOS450D, and 90% success rate.

Look forward to see some of your images soon!

2 nights ago I imaged the Horsehead Nebula @ 45 seconds and had a keeper rate of almost 90%. I've been shooting at ISO 1600 but I'm having a lot of luminance noise problems. Maybe I should try ISO 800? I'm an experienced "regular" photographer but I'm still trying to fully understand ISO as related to AP. It doesn't seem to make sense to half the ISO as that would, in terrestrial photography, mean having to greatly increase the exposure time. But I'll probably give that a try and see what I get.

[SiriusDoggy]

Great shot. I wouldn't worry about blowing out the Trapezium area either. If you wanted to bring it out, take another shot at a shorter exposure or lower ISO and composite it back in with PS.