Vixen Polarie U periodic error plot? Contemplating tracker ugprade

Hello,

I have an Omegon Minitrack LX4 star tracker which is good for 20mm FF 60MP (A7RV). But at higher focal lengths there is quite a bit of periodic error impacting results, at least on my unit. See my review in progress that includes a plot of periodic error here: https://jtra.cz/stuff/review/omegon-minitrack-lx4/

I am much more interested in landscape-astrophotography than pure astrophotography. That means the Sigma 135/1.8 on A7RV is about the maximum I will want to use anytime soon (I have seen landscape composites with even slightly longer focal lengths). But primary use will be 20 to 50mm. I do not want to throw out many sky sub-exposures because it will make stacking near horizon worse (I do not want to do composites with sky from other day or different sky region).

I am at odds if I should just keep LX4 and limit myself to 20mm and accept significant number trailed images (based on tests with 135mm) at 50mm. Or if I should upgrade or add another tracker.

The trackers for upgrade I consider are:
- Vixen Polarie U as a replacement for LX4
- or Fornax Lightrack II as an addition to LX4

For Polarie U I like its size and weight despite premium price (compared to SWSA 2i, iOptron Skyguider). The Polarie U's weight and needed accessories in my spreadsheet is about 380g heavier than my LX4 set. So it is ok as a replacement. I like that it can be used without WiFi connection as I prefer to avoid losing night adapted vision and mess with touch screen in cold. I like it supports autoguiding if I ever would want to go deeper, but with Lightrack I could probably skip that). With 1200g 135/1.8 I would be close to the limit of 2.5kg without counterweight.

For Lightrack II I like its perfect tracking. The price with scope is similar to Polarie U with scope in my country. I don't mind the tracking time limitation. The setup with accesories will weight 780g more (I don't plan to use heavy Fornax wedge) than my LX4 setup, but even without Fornax wedge it will take significantly more space in my bag. With that size and weight I would use it only for dedicated astro trips rather than mixed daylight landscape and astro landscape trips where I take 16-35/4 and 50-400. So for the mixed ones I would keep LX4 and would be limited with focal length with it. Problem with Lightrack II is that perfect tracking comes only with perfect polar alignment. If I would want to do more deep space photography, then I would probably want a proper mount with guiding in both RA and Declination that reduces need for polar alignment precision and can account for atmospheric changes.

I have seen several reviews that comment on the Polarie U periodic error (I list them with my summary at the bottom for somebody trying to search for that), but what I have learned from testing Omegon is that periodic error might have multiple periods with different amplitudes. So I would like to see actual plot of it to help me with the decision.

Do you have Polarie U? If yes, it is very easy to do the plot. Use longer focal length, like 100 to 250mm (shorter are enough if using APS-C or high megapixel FF). Just align tracker to celestial pole and then rotate the wedge by 5-10 degrees east or west to misalign it. Misalignment will cause drift in declination that will record inaccuracies of the tracking as a wiggly line. Perfect tracking would be just a straight line. Method is also described here: https://clarkvision.com/articles/astrophotography-tracking.mounts/ Adjust aperture and ISO so that 10 or 20 minute exposure will not be overexposed due to light pollution. That's it. Pick a target close to intersection of meridian and celestial equator. Take 10 minute or 20 minute exposure or compose sequential shorter exposures as layers using "lighten" mode. For interpretation a focal length, a sensor size, a number of megapixels and an exposure time is needed.

Claims about Polarie U tracking performance I have found. They vary quite a lot, but it also depends which sky portion is used for measurement. Close to celestial equator is the hardest for tracking accuracy, close to a celestial pole is easiest:

• Unguided:
  • 20” in 4 minutes to 20” in 10 minutes PE [1, 2] which implies on 60MP FF: 135mm 4min trails up to 4px, 50mm trails up to 2px.
  • 1-2min without trails 200mm 20MP FF (equivalent to 115mm 60MP FF) [3]
  • 120s without trails at 237mm APS-C 1.6 Canon M6 24MP (same pixel density as 60MP FF). Two units performed the same. [4]
  • 7px PE amplitude over 15 minutes with 20MP FF 250mm (equivalent to 143mm 60MP FF) where 2 minute exposure had almost no trailing.
• Guided:
  • RA RMS between 0.7”-1.7”, usually 1.1” [6]

[1] https://www.cloudynights.com/topic/874050-vixen-polarie-u/?p=12716049
[2] https://www.dpreview.com/forums/thread/4581571
[3] https://stargazerslounge.com/topic/387555-vixen-polarie-u/
[4] https://blog.goo.ne.jp/tkfactory-japan2014/e/7b479be083610ee72c22874109795120
[5] https://reflexions.jp/tenref/astro/equipment/mount/10758/
[6] https://www.cloudynights.com/topic/795369-round-stars-crappy-mount-guiding/?p=11518820

I have several tracker units. One is the Light track 2, It's quite a nice unit. But it (like any other tracker) is only as good as the polar alignment.

The setup to use a polar scope is rather poor with it requiring it to be callibrated first.

Greg.

Hi!

Have both and use both, but your Polarie U errors sounds like a fairy tale to me. The main wheel of the Polarie is much smaller than that of the Sky Watcher and anything about 135 mm is challenging. I like the Polarie for MW panos with a 35mm lens but wouldn't recommend it for >135mm.

The Fornax LT II is a different story, but you already know Clark and his summary I assume. I use the Fornax up to 300mm and that is not too complicated. Yes, that tracker is a little bit sensitive to wind and getting up any 100 min at night is also a pain in the butt, but OK ....

So in summary: The Polarie is in my backpack for hiking to nightscapes while the Fornax is in the car for moderate DS from dark spots in the mountains.

BTW: We have 12P Pons-Brooks right now ....

I do have the Vixen Polarie U but hardly have any time these days - at least not enough to help you with an actual periodic error plot. However, I can at least share some subjective user experience with you which hopefully can be useful for your decision.

Let me assume that if you are contemplating about the Polarie U, you also factor in getting the optional polar scope, which is very expensive at another min. 300USD (but strongly recommended!).

I came from using the MSM tracker with simple laser pointer polar alignment and only used ultra wide angle lenses (18mm). For my 50mm lens this would not be good enough though and the switch to the Polarie U was a huge upgrade. My main use cases are milky way panoramas. I have made several of these with the Polarie U and up to 85mm without issues over the past years. While I have also tried a 135mm lens on it successfully, those were only a few test shots and I have no long-term experience. Imo, the sweet spot for panoramas is in the 50-85mm range anyway. Depending on the model, with 135mm you will also be pushing the tracker from a weight perspective.

The following are my maximum exposure times on the Polarie U with no visible trailing, retested a few times over 2-3 years in good conditions and fresh polar alignment with the separate scope:

50mm: 420s

85mm: 240s

I am sure a bit longer can be squeezed out when spending even more time doing a very thorough polar alignment, but this would be purely for academic curiosity as such exposure times are unnecessary. Probably gives you an idea about the limits of a 135mm though.

I was out last night with the Polarie U and the Fornax LT II, chasing 12P Pons-Brooks, but I got a lot of light pollution from polar light. At the end I only used the Polarie (and a fixed camera for the aurora). Some remarks:

1) Setting up the Polarie U (polar alignment) is fast, around 1 min for me. I use the Polarscope and the wedge which is very easy to adjust. Many other wedges for trackers are just a 'pain in the butt' (too flimsy and hard to adjust).

2) After the Polarie wasn't working with the counterbalance set (it was well adjusted, but the Polarie wasn't turning, just making a 'knack', probably an overload safety) I used the Camera directly on the tracker and it was working fine (Canon 6D mk2 plus RF 85/1.2). 30 sec was no problem but due to the strong 'light pollution' I could only use 10 sec, chasing the comet.

3) You gain more signal with a longer exposure due to the read-noise 'threshold' but you can risk over-exposure (classical Orion nebular problem) and you need >= 4 lights for the stacking with sigma-clipping to remove satellite trails. So I do most of my MW panos with 5x 30 sec.

4) If you have to choose between Fornax and Polarie: Select the Fornax if your main target is >200mm and select the Polarie if your main target is MW pano <100mm.

5) For my 'standard' 30 sec pano images with a 35mm lens I only use the 'peep-hole' of the Polarie for a rough polar alignment (probably plus-minus 5 deg). Make a quick test for verification. As you see from previous postings: With increasing focal length you also have to work more and more precisely.

Thank you all for responses.

I decided to get Polarie U.

I will possibly create a review for it with periodic error plot too.

So I have the Vixen Polarie U now and I have measured periodic error:

jtra said:

So I have the Vixen Polarie U now and I have measured periodic error:

Click to expand...

Looks good! Might be great for 35-70mm Milky Way pano shot!

zero214 said:

jtra said:

So I have the Vixen Polarie U now and I have measured periodic error:

Click to expand...

Looks good! Might be great for 35-70mm Milky Way pano shot!

Click to expand...

That calculation was done with a 135mm lens, so the error for a 35-70mm lens will be much smaller. I only use the 'peep-hole' of the Polarie U for my usual 30 sec exposures with a 35mm lens for panos and I haven't had any problem with it.

Thanks for the test! It demonstrates pretty well that theses small trackers are well suitable up to 135mm and - with certain limitations - a little bit above, but they are not really good for more extreme telephotos (< 200mm).