n00b question #3: Very uneven tracking with star adventurer

[GazK]

Hi all, n00b here again with another starter question.

Flush from managing to get a result with M42, I decided last night to try to capture the horsehead and flame. Yes, I know, don't run before you can etc etc. It was clear, I got carried away, what can I say.

I pushed the sub time to 45s with my WO Z71 on the star adventurer and took 1.25 hours of data. I'm now sitting here mid-process, and while I do have a head and it is horse-shaped, its a lot fainter than I'd hoped for.

The first thing that becomes very apparent looking at my subs is that, while about 50% have acceptably round stars, the other 50% show minor or significant trailing when zoomed in - so I had to reject 40 mins of data. Additionally, the star field is moving around somewhat between frames, and not linearly. My last imaging session (same rig, 30sec sub) had a much lower discard rate and none of the wandering. There was no wind on either night, and while I don't think the seeing was very good - lots of twinkly stars - I'm not sure that would account for it.

Can anyone help me diagnose the problem?

[Swillis]

Hi, whilst you waiting for someone who knows what their talking about to arrive...

The horsehead is much dimmer than Orion. So I imagine that 35min? (1.25h - 40min) would not be enough. I had I think nearly 3 hours of data and could barely see it (although with much worse optics). 

Could be that with the larger scope your getting closer to the limit of the equipment, and with the longer focal length it will be more demanding on a good polar alignment. 

Have you tried keeping all the subs? You can specify only the top xx% get stacked, and Siril will show you a graph of all the subs so you can guage the quality/variation across your session. If you've using a script, set cwd to process and search sequences. If you select r_pp_lights****, you should then be able to specify your parameters for stacking. (I remembered correctly that your using Siril? Sorry if not)

Sure someone else will be along promptly...

[michael8554]

With no Dec drive your PA needs to be very good for long exposures.

And RA may well be suffering from Periodic Error.

Michael

[900SL]

I had a Star Adventurer:

 

Needs a solid tripod and no slack in the wedge (no wobble at all, I had to shim mine)

Quality of manufacture was abysmal. Screws missing inside, poor machining, pot metal etc

However, it would track OK as long as you didn't push the focal length or exposure. 20-135mm was OK, most frames were keepers unless the wind was up at all

Needs balancing east or west heavy, can't remember which, to reduce the effect of backlash or slack in the gears

Needs accurate polar alignment with long lens or long exposure

I'd say a Z71 is pushing it on weight and focal length

I found it an exercise in frustration, after polar aligning it would go out of line if you moved the scope

Sold it and bought a Fornax lightrack, it just works. 100% keepers, tracks like it's on rails

 

Edited March 8, 2022 by 900SL

[GazK]

Thanks all for responding.

I went out last night and pointed the DSLR with my 70-300 @ 200mm at Orion, purely to do a tracking test. The lens is mostly plastic, so pretty light; DSLR + lens is under 2kg. Polar alignment was pretty accurate I thought, rechecked after the load was added. Balance was also checked in both axes. Also there was no wind and I'm inside a walled garden.

The following gif is a crop into one section of the resulting 2 hours worth of 60 second subs. E-W is roughly in the Y axis. Ignore the stars moving through the tree line and clouds in the later images. You can clearly see a periodic shift in azimuth, which I don't understand at all. It is the same behaviour I've seen on previous shifts. I don't think bad polar alignment could explain this could it? If not, then what is going on?

I wouldn't mind if it was just shift between subs, but 2/3 of these subs were useless because of star trailing in the same axis. Surely a 2/3 rejection rate when the mount is within its load envelope isn't right?

 

 

Edited March 21, 2022 by GazK

[Elp]

When I was using my modded dslr with my Z61 (f5.9) I found 2 mins was the absolute minimum on this target per sub at iso 1600 to resolve any form, 3 mins was better. This is from a bortle 7 zone. It's autoguiding territory unless you have a super quality unguided tracking mount. The longer your focal length the better the PA and guiding needs to be. I had an SA but would never use it with a scope.

[GazK]

11 minutes ago, Elp said:

When I was using my modded dslr with my Z61 (f5.9) I found 2 mins was the absolute minimum on this target per sub at iso 1600 to resolve any form, 3 mins was better. This is from a bortle 7 zone. It's autoguiding territory unless you have a super quality unguided tracking mount. The longer your focal length the better the PA and guiding needs to be. I had an SA but would never use it with a scope.

Thanks, I get that. But right now I'm back at square one, trying to work out why I'm getting star trails in most of my images in 1 minute subs at half that focal length. See my last post.

[alacant]

42 minutes ago, GazK said:

The lens is mostly plastic

Hi

Assuming the drive is meshing correctly and you do not plan to employ autoguiding...

 What did the processed image look like? Not sure what you're expecting.

Guess: the stars remain in the same position from one frame to the next? You really don't want that. Do you have the mirror locked up? Unless you have a very solid mounting, at 200mm, the return of the reflex mirror with a cheepo lens on a DSLR is enough to shift a sa as your gif suggests. Free dither:) Otherwise, soft ground, wind, inadequate mounting, something loose, poor balance...

Maybe a photo of your setup along single acceptable and trailed frames would help us if you want to diagnose further.

Cheers

 

[Elp]

Is the setup balanced, from the animation it looks like the mounts trying to move one way and something is counteracting the motion.

[GazK]

26 minutes ago, alacant said:

Hi

Assuming the drive is meshing correctly and you do not plan to employ autoguiding...

 What did the processed image look like? Not sure what you're expecting.

Guess: the stars remain in the same position from one frame to the next? You really don't want that. Do you have the mirror locked up? Unless you have a very solid mounting, at 200mm, the return of the reflex mirror with a cheepo lens on a DSLR is enough to shift a sa as your gif suggests. Free dither:) Otherwise, soft ground, wind, inadequate mounting, something loose, poor balance...

Maybe a photo of your setup along single acceptable and trailed frames would help us if you want to diagnose further.

Cheers

 

Thanks. I don't know what I was expecting really, I'm a n00b! I suppose I'm just surprised at a) the amount of field shift between subs and b) the ratio of sharp subs.

I'll set the rig up in the next couple of days and post a pic. Basically a Benro video tripod, fully extended, stock SA pro on top of that, standard SA Dec bracket with counterweight, camera mount as close as possible to RA axis without fouling the reticle illuminator, canon 80d and 70-300 mounted on a rack and pinion bracket to allow balance (this could be a weak point for one-ff shifts, the 1/4" fixing doesn't have as much grab as I'd like, but the lens doesn't have a tripod collar).

Example good and bad frames attached, also a very rough process of the final image (I wasn't aiming for an actual image, hence the trees!)

 

Edited March 21, 2022 by GazK

[GazK]

10 minutes ago, Elp said:

Is the setup balanced, from the animation it looks like the mounts trying to move one way and something is counteracting the motion.

Yes it is balanced in both axes. I checked after moving the camera onto the target.

[happy-kat]

With reference to my experience with an altaz mount imaging to the South and North in the NH is most demanding and star trails show quicker. Orion and family are very much in the South now, if you setup to image to the East with the same equipment would you get 1 minute exposures you were OK with?

I realise you are using an EQ mount but thought it might still be worth commenting.

Edited March 21, 2022 by happy-kat

[alacant]

20 hours ago, GazK said:

Example good and bad frames

Hi

Given the zoom lens' astigmatism and that you are not guiding, the processed image looks good.

If you want more good frames, the next step would be to dismantle clean, lubricate and adjust the mechanisms. It's quite simple.

HTH

[GazK]

On 21/03/2022 at 20:05, happy-kat said:

With reference to my experience with an altaz mount imaging to the South and North in the NH is most demanding and star trails show quicker. Orion and family are very much in the South now, if you setup to image to the East with the same equipment would you get 1 minute exposures you were OK with?

I realise you are using an EQ mount but thought it might still be worth commenting.

thanks, that's a good point. Going to point it at Leo next chance I get, see how that goes.

[GazK]

On 22/03/2022 at 16:13, alacant said:

Hi

Given the zoom lens' astigmatism and that you are not guiding, the processed image looks good.

If you want more good frames, the next step would be to dismantle clean, lubricate and adjust the mechanisms. It's quite simple.

HTH

Thanks, that's very reassuring. It's just annoying that I'm imaging for 2x the amount of data I get.

Thanks for the link, I'll work through it.

[900SL]

It looks like periodic error, if it is cycling back and forth. 

My SA was junk. Even after stripping, meshing the worm, shimming the wedge, it was still a cheap POS. Buy a better mount.