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Egg-shape stars Bresser exos2 goto


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Hello

Not sure if I am in the right topic.

Equipment:

Bresser exos 2 goto

Skywatcher 150p

Canon eos 700d

guide scope 32mm

guide camera SvBony 305pro

Here is my problem. Since May when I bought the 2nd hand Bresser exos2 goto, I have a problem to get nice round stars. Polar alignment is good based on sharpcap tool.

For example two weeks back I was imagining the dumbbell nebula with 60s exposure unguided, from 55 frames, only 13 was ok with round stars. When I tried guiding, it was better, but I'm sure it can do much better.

The attached photos of stars are all unguided.

Last week I removed the RA motor, cleaned the worm gear and the RA gear, re-greased them and adjusted. I did same thing with motor gear and removed backlash between them. Last night I had first time to test the mount. I was hoping it will be doing better, but no. With 30s exposures the stars are little bloated, just not perfect. Is this the best what this mount can do?

I may try to tighten the worm gear with Ra gear, maybe it will help.

Thanks. 

egg shape stars.jpg

egg shape stars2.jpg

egg shape stars3.jpg

guiding02.20.png

Edited by chrobakx
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1 hour ago, scotty38 said:

My limited experience would say you need an exposure of 2-3 seconds and also to get calibration to complete.

Before this mount I had the eq3 with enhanced dual motor drive and I was able to have nice round stars with pore PA with 30 to 60 seconds exposures unguided. Let say from 50 subs, 45 had nice round stars. Now the opposite.

I think the problem is with the mount build quality.

Edited by chrobakx
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Your PHD2 screenshot is from last month, are you still using the same settings?

If so, I would go with Scotty38 and suggest increasing the exposure to 2 seconds, at 0.5 seconds you may just be chasing the Seeing (upper atmosphere air movement) and this won't give good guiding. Also your RA Agr setting seems very low at 15, I would expect to see that at around 60.

If you haven't done so, try resetting PHD2 to the default settings, make sure your guide camera focus is as good as you can get it, and try a calibration run. Once this is completed, then run the Guide Assistant (under the Tools menu) for about 5 minutes. This will tell you what the backlash is like and suggest the settings to use.

If there's a lot of backlash in the RA axis you could try going slightly counter weight heavy or scope heavy (depending on which side of the mount the weights are on) to try & eliminate most of the backlash. Think of it like you want to be pulling a small amount of weight up hill to keep the worm gear teeth meshed tightly together. ;)

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40 minutes ago, Budgie1 said:

Your PHD2 screenshot is from last month, are you still using the same settings?

If so, I would go with Scotty38 and suggest increasing the exposure to 2 seconds, at 0.5 seconds you may just be chasing the Seeing (upper atmosphere air movement) and this won't give good guiding. Also your RA Agr setting seems very low at 15, I would expect to see that at around 60.

If you haven't done so, try resetting PHD2 to the default settings, make sure your guide camera focus is as good as you can get it, and try a calibration run. Once this is completed, then run the Guide Assistant (under the Tools menu) for about 5 minutes. This will tell you what the backlash is like and suggest the settings to use.

If there's a lot of backlash in the RA axis you could try going slightly counter weight heavy or scope heavy (depending on which side of the mount the weights are on) to try & eliminate most of the backlash. Think of it like you want to be pulling a small amount of weight up hill to keep the worm gear teeth meshed tightly together. ;)

I'm trying first to sort things without the guiding. I don't know why the mount is doing what is doing. I have same problem as this guy from CN forum: https://www.cloudynights.com/topic/84084-is-this-periodic-error-lxd75/

"This evening I did an imaging run with my ZS66SD and LXD75 mount. My skyfog limit is a 1 minute exposure.

More than half of the exposures are just fine -- nice round stars. However, the rest have slighly elongated stars. I seemed to get a few good exposures, then a few bad ones...then repeat. Over the course of an hour, about half of them are bad.

The elongation is in one direction only -- RA.

No wind tonight after sunset, so that's not it....no vibrations -- I wasn't in the observatory during the imaging run. It's not polar alignment...I was drift-aligned. No movment for five minutes at 316x with reticle eyepiece.

Do you think this is periodic error?

I have not done the Train PEC on this mount. Actually, one question about that. I know I must park the scope in order for it to remember its PEC. After awaking the scope (unparking it), can I manually move the scope around by unclamping the RA and DEC lock-down levers? With this cause it to lose its PEC?".

 

 

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44 minutes ago, Budgie1 said:

Your PHD2 screenshot is from last month, are you still using the same settings?

If so, I would go with Scotty38 and suggest increasing the exposure to 2 seconds, at 0.5 seconds you may just be chasing the Seeing (upper atmosphere air movement) and this won't give good guiding. Also your RA Agr setting seems very low at 15, I would expect to see that at around 60.

If you haven't done so, try resetting PHD2 to the default settings, make sure your guide camera focus is as good as you can get it, and try a calibration run. Once this is completed, then run the Guide Assistant (under the Tools menu) for about 5 minutes. This will tell you what the backlash is like and suggest the settings to use.

If there's a lot of backlash in the RA axis you could try going slightly counter weight heavy or scope heavy (depending on which side of the mount the weights are on) to try & eliminate most of the backlash. Think of it like you want to be pulling a small amount of weight up hill to keep the worm gear teeth meshed tightly together. ;)

Also I'll have to do new calibration in PHD because mount was disassembled and there is no backlash.

Thanks for help.

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12 hours ago, chrobakx said:

I'm trying first to sort things without the guiding. I don't know why the mount is doing what is doing. I have same problem as this guy from CN forum: https://www.cloudynights.com/topic/84084-is-this-periodic-error-lxd75/

If your symptoms are the same as his then it could be PE but there seems to be movement or an imbalance there somewhere.

When you had the mount apart, was the worm drive shaft rotating smoothly, no "notchyness" in some parts of the rotation? This is best checked with the shaft removed form the mount and worm gear.

When assembled, disconnect the motor from the RA and rotate the drive gear by hand for a complete rotation of the RA axis and check there are no really loose or tight spots.

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Your mount simply does not respond to guide pulses under your settings:

image.png.5d77981be6d32232789d8cd43c9a7244.png

RA error is much larger than DEC error and you have elongation in RA direction.

Set your guide speed at x0.5 sidereal or slower, use default settings ( too low aggressiveness in RA), calibrate near meridian and equator and use at least 2s guide exposures.

Make your setup east heavy - to eliminate any excessive backlash in RA

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6 hours ago, Budgie1 said:

If your symptoms are the same as his then it could be PE but there seems to be movement or an imbalance there somewhere.

When you had the mount apart, was the worm drive shaft rotating smoothly, no "notchyness" in some parts of the rotation? This is best checked with the shaft removed form the mount and worm gear.

When assembled, disconnect the motor from the RA and rotate the drive gear by hand for a complete rotation of the RA axis and check there are no really loose or tight spots.

I did that and I found tight spot actually. It was maybe on the on 1/4 of the RA drum gear, so it's not nice round anymore(who know if it ever was round from factory).

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5 hours ago, vlaiv said:

Your mount simply does not respond to guide pulses under your settings:

image.png.5d77981be6d32232789d8cd43c9a7244.png

RA error is much larger than DEC error and you have elongation in RA direction.

Set your guide speed at x0.5 sidereal or slower, use default settings ( too low aggressiveness in RA), calibrate near meridian and equator and use at least 2s guide exposures.

Make your setup east heavy - to eliminate any excessive backlash in RA

Hmm interesting, I never paid attention to this, I didn't know what it was doing. Thanks for pointing out. I'm always trying set up guiding speed to 0.25. I think the default is 0.5. Next time I'll set it to 0.125, that's the lowest speed in the controller. 

The thing with this mount is, even when I'm using Ascom driver, it's still sending signals just like when I'm using ST4 port, not pulsing signals. It's was proven. So the slower side speed I'll use, the more time will have the mount to react to signals from PHD2.

I've been imaging last night with guiding. I took the screen shot of it, but it's in my laptop. I'll check the RA error later tonight

Now I need clear sky.

Thanks

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4 hours ago, chrobakx said:

I did that and I found tight spot actually. It was maybe on the on 1/4 of the RA drum gear, so it's not nice round anymore(who know if it ever was round from factory).

As long as it runs smoothly over that part of the worm gear then it should be fine. Too tight and it will cause that axis to drag or start to stall the motor, so it's better to use this part of the worm gear to set the backlash.

With the guiding, what is the pulse guiding rate set too in the EQMOD control panel? The default is x0,1 on both axis, which is normally too low and the mount doesn't respond fast enough, so you end up chasing the seeing conditions, rather than correcting an error. If it's set too high (it goes up to x0.9) then it can over correct. So I would start with Vliav's suggestion of x0.5 and see how you get on, you can always change it after a few runs. ;)

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32 minutes ago, Budgie1 said:

As long as it runs smoothly over that part of the worm gear then it should be fine. Too tight and it will cause that axis to drag or start to stall the motor, so it's better to use this part of the worm gear to set the backlash.

With the guiding, what is the pulse guiding rate set too in the EQMOD control panel? The default is x0,1 on both axis, which is normally too low and the mount doesn't respond fast enough, so you end up chasing the seeing conditions, rather than correcting an error. If it's set too high (it goes up to x0.9) then it can over correct. So I would start with Vliav's suggestion of x0.5 and see how you get on, you can always change it after a few runs. ;)

I think it's on default speed, I tried to change it before, but unsuccessful, I don't know why. I'll check all the settings again.

Lots of good suggestions.

Edit: I realised, I'm not using EQMOD. I thought you was talking about settings in PHD. I know there is a option to change the side rail speed.

Edited by chrobakx
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37 minutes ago, chrobakx said:

Edit: I realised, I'm not using EQMOD. I thought you was talking about settings in PHD. I know there is a option to change the side rail speed.

How are you connecting your mount to PHD2, are you using the ST4 cable from the camera to the mount and a USB cable from the camera to the PC/laptop with PHD2?

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8 hours ago, Budgie1 said:

How are you connecting your mount to PHD2, are you using the ST4 cable from the camera to the mount and a USB cable from the camera to the PC/laptop with PHD2?

Not ST4. USB to camera and usb to hand controller via Ascom( very basic version).

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15 hours ago, chrobakx said:

I'm always trying set up guiding speed to 0.25. I think the default is 0.5. Next time I'll set it to 0.125, that's the lowest speed in the controller. 

As Martin alluded to, your Guide Rate/Guide Speed setting in the mount menus is too low.

You need to set it higher, not lower.

Try at least 0.5 / 50% / 7.5arcsec/sec, depending on your mount's settings.

Guide stars are way out of focus, HFD = 5.48 in the Star Profile.

Alyer focus to get the lowest reading.

 

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28 minutes ago, michael8554 said:

As Martin alluded to, your Guide Rate/Guide Speed setting in the mount menus is too low.

You need to set it higher, not lower.

Try at least 0.5 / 50% / 7.5arcsec/sec, depending on your mount's settings.

Guide stars are way out of focus, HFD = 5.48 in the Star Profile.

Alyer focus to get the lowest reading.

 

Why do you think it should be higher rather than lower?

High guide rate works good with mechanically sound / stiff mounts that respond well to sudden changes in speed.

Entry level mounts that have backlash, are lighter in comparison to weight they are carrying and are in general wobbly - don't cope well with acceleration and jerk.

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49 minutes ago, michael8554 said:

As Martin alluded to, your Guide Rate/Guide Speed setting in the mount menus is too low.

You need to set it higher, not lower.

Try at least 0.5 / 50% / 7.5arcsec/sec, depending on your mount's settings.

Guide stars are way out of focus, HFD = 5.48 in the Star Profile.

Alyer focus to get the lowest reading.

 

I agree with the focus, I'll fix it next time. But I disagree with the faster speed. PHD2 is sending two signals to RA motor. First the beginning of the move, then the end of it. It's not pulsing guiding despite the using Ascom driver. It's very basic goto mount.

If I'll increase the side rail speed, mount will be chasing the signals from PHD2 and guiding will be even worse. It won't be able to keep up.

 

We can talk theory as much as we can, but I need the clear sky to test all suggestions.

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I don't think you're giving your mount enough credit.

It looks very similar to the Sky Watcher EQ5 but looking at the specs for the Bresser EXOS 2 goto, it has a larger payload and both axis have bearings fitted (the EQ5 has no bearings on the DEC axis).

My EQ5 runs happily on x0.6 pulse guiding using EQMOD & Astro Photography Tool (APT) with an Evostar 100ED, guide scope and camera attached. Sure, your 150P is a lot larger and more likely to catch the wind but it looks like a good entry level mount for astrophotography. ;)

As you've said, the biggest issue is lack of clear nights to try things out to make some headway. You've got plenty of suggestions, see what works best when the skies clear. :D

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Quick update. 

While I'm still waiting for the clear sky, I had an idea to swap DEC and RA worm gear housings to see if it will make any difference, because in theory DEC axis isn't moving so much and thus should be less worn out than RA axis.

When I removed DEC worm housing I noticed the bearings wasn't moving freely, even after cleaning and adjustment, it wasn't as smooth as it should be. So I ordered 4 new bearings mr126 ZZ for both axis.

I'm hoping this will make significant change in guided and unguided sessions.

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  • chrobakx changed the title to Egg-shape stars Bresser exos2 goto
  • 4 weeks later...

Alright. Finally I had the time to test  everything. 

Great news is there is no more star trail on unguided tracking. 60s subs are fine with nice round stars.

Guiding is improved as well, but it still required fine adjustments. New bearings definitely work better than old ones.

 

13.1.22.2.17.png

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