Tracking problem

[Parky Superman]

I'm new to this and this is my first post. I've attached a pic of my setup. Last night I had my Sigma 150-600mm lens zoomed into 400mm. I had the intervalometer set to take 1 minute exposures continuously (with a ~10s pause in between). The tracking was pretty good surprisingly but only about 1 out of 3 or 4 pictures didn't have star streaking. As I flip through the sequence of photos (in adobe bridge) I can clearly see that the stars appear to oscillate. They move up for a couple of frames, pause, move down for a couple of frames, pause, repeat. The movement is tiny so just slight streaking. I had stabilization off on the lens. Everything on the mount was tightened up as far as I could tell. The gear mesh adjustment on the Star Adventurer seems ok as I can't discern any slop. The clutches were tight. It was not windy. I've learned to not even touch the tripod while taking photos and I did not do that. I think the balance was fine and slightly heavier towards the east (though I'm not sure how much heavier it should be). What caused the oscillation? Am I just at the limit for this mount? Prior to this lens I was using an 85mm and tracking was a lot easier which I guess isn't surprising. I did manage to get the Pleiaides a few nights ago at 300mm and successfully stacked ~75 light frames, ~20 darks, ~20 bias, ~20 flats in DSS. Looked good after postprocessing in Photoshop. Anyway, having fun with this. Let me know what you think about my tracking problem!

Parky Superman

[KathyNS]

Hi, Parky Superman, welcome to TSS.

Unfortunately, you picture didn’t show up. Can you tell if the oscillation is in the RA direction or the Dec direction? “Up” and “down” don’t have much meaning in space. If the oscillation is in RA, it is likely caused by periodic error in the mount. All mounts have it to some degree. Some mounts can correct for it. Guiding will also correct for it.

[Parky Superman]

The streaking appears to be about the RA. What kind of guiding setup work work with my setup?

Also here are some samples of the imagery. I considered this one decent (though a bit streaked)

This one is definitely a little streaked:

There's some kind of weird thing to the left of the star (on both pics) that I think is an artifact of the bright star, but I'm not sure. I'm using an APC-C Optolong L-pro fiilter.

[SkyHiker]

Going above 200 mm will be problematic on most star trackers. Since your star trails happen occasionally, I wonder if you have a balancing problem. It looks like the way the camera is positioned, the lens moves the center of mass off center. If that is true try moving the camera back. I also see a lot of dangling cables, try tidying that up. At the rear I see what looks like a lace attached to the base and the mount, what is that, could it drag down RA?

[Parky Superman]

I think you might see the tail of a zip tie that is providing strain relief for the 5V input cable to the tracker. I need to trim that off. For the balance, to clarify are you saying that the payload should be balanced fore and aft? This is in addition to the balance about the RA I'm assuming. I guess a fore and aft balance should be at the attachment point of the payload to the tracker?

[yobbo89]

oscillation hmm ,worm float,even balanced scope (<---it's probably this),wind,chasing the seeing ,flexure,walking around on the concrete xd

[Parky Superman]

How uneven should the balance be to avoid problems?

[Kerry C.]

I would remove the neck strap from the camera. The slightest movement of it can cause problems. With it removed you at least remove that from the equation possibilities.
Kerry

[Greenman]

I would remove the neck strap from the camera. The slightest movement of it can cause problems. With it removed you at least remove that from the equation possibilities.
Kerry

I found that when using a DSLR too…

[Parky Superman]

Thanks everybody for the suggestions. I removed the camera strap and tidied up the cables. It did improve my tracking. Last night I got Orion's Nebula at 400mm. I took about 65 2 min exposures and and enough were good to stack about 50 of them. I got about 50 darks, 50 bias, and 50 flats and stacked it all is Deep Sky Stacker.It was very windy though so maybe that's why I still had some problems?

BTW, the ONLY way I would have ever known how to post process in Photoshop is I bought the book, "The Deep-Sky Imaging Primer" by Charles Bracken.

[JayTee]

Hi Parky,

That's an excellent image that you've achieved there. Please don't hesitate to also post it in the Astrophotography Deep Sky sub-forum. We would love to see it over there too. Don't forget to also give us the image acquisition details like you did here.

Cheers

[OzEclipse]

Thanks everybody for the suggestions. I removed the camera strap and tidied up the cables. It did improve my tracking. Last night I got Orion's Nebula at 400mm. I took about 65 2 min exposures and and enough were good to stack about 50 of them. I got about 50 darks, 50 bias, and 50 flats and stacked it all is Deep Sky Stacker.It was very windy though so maybe that's why I still had some problems?

BTW, the ONLY way I would have ever known how to post process in Photoshop is I bought the book, "The Deep-Sky Imaging Primer" by Charles Bracken.

If wind was the cause, the movement is unlikely to have been only in RA. Could be balance/rock and roll on the worm or flexure of the RA shaft. It could also be periodic error.

When you do short duration "lucky" imaging, some exposures fall in the flat spots on the top and bottom of the periodic error curve while others get maximum shift during the transition from tops to bottoms. For 60s and 120s subs, I have placed arbitrary ticks and crosses for whether a sub is good or bad using approx 15 arc seconds (4.5 micron pixels at 60mm focal length) as my limit.

In practice, the amount of drift that's acceptable in one sub for your equipment is related to the focal length and the pixel size. The table below shows the arc second per pixel for a variety of pixel sizes and focal lengths.

[Parky Superman]

If I'm reading that right, the T4i has a pixel size of 4.3 um so at 400mm focal length that means each pixel represents 2 arc seconds of angle. With that periodic error in the chart there is a 30 arc second or 15 pixel swing to each side. That doesn't seem like a whole lot. Is that a typical periodic error? Is there a way to correct it? Would autoguiding help?

[JayTee]

If I'm reading that right, the T4i has a pixel size of 4.3 um so at 400mm focal length that means each pixel represents 2 arc seconds of angle. With that periodic error in the chart there is a 30 arc second or 15 pixel swing to each side. That doesn't seem like a whole lot. Is that a typical periodic error? Is there a way to correct it? Would autoguiding help?

Please ask this question over in the AP Equipment forum. This knowledge level definitely exceeds this beginner's forum and you'll get better visibility over there to get the most correct answer.

JT

[OzEclipse]

If I'm reading that right, the T4i has a pixel size of 4.3 um so at 400mm focal length that means each pixel represents 2 arc seconds of angle. With that periodic error in the chart there is a 30 arc second or 15 pixel swing to each side. That doesn't seem like a whole lot. Is that a typical periodic error? Is there a way to correct it? Would autoguiding help?

Yes, you are reading and interpreting correctly. I copied the PE curve off the internet but from what I have read in reviews and seen elsewhere, it is typical for a Star Adventurer.

This thread started out because you were concerned about the streaks in RA. ±15 pixels of oscillation is more than enough to explain those streaks. So I am puzzled why you now state, "That doesn't seem like a whole lot." Yes it can be guided out however that will add weight to the mount which may increase other problems with flexure, balance, and overloading.

My opinion is that a SW Star Adventurer was never designed to carry big loads. Perhaps a 50-100mm focal length lens with a finderscope autoguider. An autoguided 400mm lens probably deserves a full equatorial mounting.

regards

Joe