skwatcher AZ-GTe photo star trails

[keen]

Hi all. Been using AZ-GTe with Startravel 102mm.

No trouble aligning and finding objects. But a photo of even 30sec leaves star trails. The mount is moving, but not quite enough. After 30 min or so, the object is obviously no longer centered.

If I do a 1 star alignment, and then just watch that object, I get the same thing. 30 sec exposure and I can just start to see star trails.

Is this as good as it should be? I was expecting I could do a 2 min exposure

For setup, all I do is get the level as close as I can by eye. Then I do the 2 star alignment using stars close to what i want to look at. I am under the impression the software can figure everything out from that.

Do I need any other physical alignment? 

thanks for any advice.

[Astro Noodles]

Hi Keen.

I'm not an expert on this, but are you using a wedge? It looks like the AZ-GTE is an alt-az mount. Without an equatorial wedge, stars will start to trail around the periphery of the image quite quickly.

Make sure everything is fully tightened and that nothing is slipping. This has happened to me.

It doesn't look like you can move the centre of balance on this scope, so with a big camera hanging off the back, this might be putting a bit of strain on the motor.

Also, the mount is rated for 5kg (although I can't find out whether it is for optical or imaging). The StarTravel 102 OTA is 3kg, and the camera + accessories probably weigh another KG which will put you close to the rated capacity. This could potentially be putting strain on the motor which would cause the stars to trail.

Also, make sure your mount and tripod are properly levelled before you start your imaging session.

As I said, I'm not an expert on this, but these are a few suggestions. I hope it is one of them rather than a fault with the mount. 🙂

[PEMS]

I suspect that your expectations are a little high. As Noodles has said the mount itself is an Alt-Az and so you need a good wedge for it. I will assume that you have one, if not 30 seconds is good before trailing on an Alt-Az mount.

Next is that even with a wedge you can align the wedge to North, but what guarantee is there that the mount sat on it is then equally aligned? If the wedge was perfect and there was say 2 degree angular offset between the wedge and the mount then your wedge may be polar aligned but your mount is not polar aligned. I suppose basically never assume that because one component or item is correct or right that this makes all components equally correct. Trust nothing!

Sounds like you are up at the upper range of the mount capacity. And also reads that you are not guiding. If you were then I would say you are over the mounts capacity. If you read around then a dedicated equitorial mount, say EQ5 variety, when unguided will have comments like expect 30-60 seconds for exposures with good polar alignment. The Az Gte is not in the performance of an EQ5. So expect possibly similar but likely less exposure duration. Certainly not 2 minutes. The only place I read that are Youtube videos and I would not believe Youtube if they said that sun rises in the East and sets in the West.

Have seen people say that Skywatcher state the mount is not for Astrophotography. The rather simple answer is if you want to do astrophotography then get a proper dedicated equitorial mount. You have posted this in the Equipment section, and the Az Gte is in effect the wrong equipment for any serious astrophotography. Apologies.

Edited May 15, 2021 by PEMS

[Dean Hale]

Alignment is key as mentioned, the wedge will really help too.

The AZ-GTI is an excellent place to start and many do. Don't think you need to spend £0000's to get fantastic results. Just manage your expectations then spend away if you feel the need. 

Some good results here
https://www.astrobin.com/search/?q=az-gte

Edited May 15, 2021 by Dean Hale
edited the link

[keen]

thanks for the replies.

I've seen some notes on using a wedge to align north. But isn't the point of a computerized mount like this, with the 2 star alignment procedure, that you don't need that physical alignment? 

It shouldn't matter what the physical alignment is. Once the software alignment procedure is complete, it should be able to track.

But, like you say, maybe it's a matter of my expectations. The software is obviously tracking, but maybe it's just not good enough for a 30 sec exp.

I'd like to know what the expected error is. I've never seen that from the manufacturer. Maybe I'm not centering my alignment stars accurately enough?

[Astro Noodles]

3 minutes ago, keen said:

thanks for the replies.

I've seen some notes on using a wedge to align north. But isn't the point of a computerized mount like this, with the 2 star alignment procedure, that you don't need that physical alignment? 

It shouldn't matter what the physical alignment is. Once the software alignment procedure is complete, it should be able to track.

But, like you say, maybe it's a matter of my expectations. The software is obviously tracking, but maybe it's just not good enough for a 30 sec exp.

I'd like to know what the expected error is. I've never seen that from the manufacturer. Maybe I'm not centering my alignment stars accurately enough?

Hi Keen

Without the equatorial wedge, the telescope won't rotate to track the sky correctly. Celestial objects describe an arc across the sky. Without a wedge you would doing very well to get 20 sec subs with a 500mm focal length.

This may help explain it better than I can

What is an equatorial mount and how does it work? - Space Oddities

There is also a very long thread called 'No EQ challenge' or something on here which will show you what is possible without an equatorial mount.

 

[vlaiv]

40 minutes ago, keen said:

thanks for the replies.

I've seen some notes on using a wedge to align north. But isn't the point of a computerized mount like this, with the 2 star alignment procedure, that you don't need that physical alignment? 

It shouldn't matter what the physical alignment is. Once the software alignment procedure is complete, it should be able to track.

But, like you say, maybe it's a matter of my expectations. The software is obviously tracking, but maybe it's just not good enough for a 30 sec exp.

I'd like to know what the expected error is. I've never seen that from the manufacturer. Maybe I'm not centering my alignment stars accurately enough?

Mount like AzGti/Gte is not astrophotography mount.

It can be turned into astrophotography mount if you turn it into EQ mount and load appropriate firmware onto it (even then SkyWatcher warns that they are not responsible for how mount performs). One part of turning it into EQ type mount is adding appropriate wedge so that mount can be polar aligned.

Difference between AZ type mount and EQ type mount is among other things - in the way mount tracks - or how motors work.

With EQ mount, only one motor needs to be working for mount to track - RA axis motor - and that motor needs to move at constant speed. With AZ mount - both motors need to be continually working and they are always working at different speed - not only different speed to each other - but speed that changes every second.

Just to give you an example - if you are tracking object just rising in the east or setting in the west - most of motion of object will be in vertical direction. This means that Alt axis motor will do most of the work - and Az axis motor only needs to do small corrections. In contrast to this - think of what is happening to object crossing meridian due south. Here object is moving almost exclusively in horizontal direction - Az axis motor here is doing most of the work and Alt motor - only corrections.

Speed of each motor changes continuously between these two "extremes" as object is tracked across the sky.

Mount like AzGti/Gte is powered by stepper motors - and those have finite precision. It also has finite precision in measuring time intervals. To be able to track object accurately in alt az mode - you need good precision of both of these things.

Ok, this may have been too harsh on little AzGti - it has enough precision to track object for visual applications - but not enough for very demanding applications like astrophotography.

On top of that all - mounts are not perfect and they have machining errors / tolerances in their parts so they don't track perfectly even when having enough resolution / precision in timing and stepping. This is called periodic error associated with gears. Most mounts regardless of the type suffer from it.

If you want to do photography with your AzGte mount - look into switching it into EQ mode - that will help with tracking, but even then, if you want to have exposures of decent length - you'll need to guide your mount.

Btw, if you want to try EQ mode - you can do it fairly easily, here is my setup before I purchased dedicated wedge - I used DIY counter weight / counter weight shaft and ball head as Equatorial wedge (yes, polar alignment was bit of a pain - but it was doable):

With this setup, I was able to do up to 60 seconds with short focal lengths - up to 100mm or so. With 500mm focal length - well, that is going to be tough on little mount and I don't think you'll be able to pull much longer than maybe 20-30s without use of guiding.

[Astro Noodles]

8 minutes ago, vlaiv said:

Mount like AzGti/Gte is not astrophotography mount.

It can be turned into astrophotography mount if you turn it into EQ mount and load appropriate firmware onto it (even then SkyWatcher warns that they are not responsible for how mount performs). One part of turning it into EQ type mount is adding appropriate wedge so that mount can be polar aligned.

Difference between AZ type mount and EQ type mount is among other things - in the way mount tracks - or how motors work.

With EQ mount, only one motor needs to be working for mount to track - RA axis motor - and that motor needs to move at constant speed. With AZ mount - both motors need to be continually working and they are always working at different speed - not only different speed to each other - but speed that changes every second.

Just to give you an example - if you are tracking object just rising in the east or setting in the west - most of motion of object will be in vertical direction. This means that Alt axis motor will do most of the work - and Az axis motor only needs to do small corrections. In contrast to this - think of what is happening to object crossing meridian due south. Here object is moving almost exclusively in horizontal direction - Az axis motor here is doing most of the work and Alt motor - only corrections.

Speed of each motor changes continuously between these two "extremes" as object is tracked across the sky.

Mount like AzGti/Gte is powered by stepper motors - and those have finite precision. It also has finite precision in measuring time intervals. To be able to track object accurately in alt az mode - you need good precision of both of these things.

Ok, this may have been too harsh on little AzGti - it has enough precision to track object for visual applications - but not enough for very demanding applications like astrophotography.

On top of that all - mounts are not perfect and they have machining errors / tolerances in their parts so they don't track perfectly even when having enough resolution / precision in timing and stepping. This is called periodic error associated with gears. Most mounts regardless of the type suffer from it.

If you want to do photography with your AzGte mount - look into switching it into EQ mode - that will help with tracking, but even then, if you want to have exposures of decent length - you'll need to guide your mount.

Btw, if you want to try EQ mode - you can do it fairly easily, here is my setup before I purchased dedicated wedge - I used DIY counter weight / counter weight shaft and ball head as Equatorial wedge (yes, polar alignment was bit of a pain - but it was doable):

With this setup, I was able to do up to 60 seconds with short focal lengths - up to 100mm or so. With 500mm focal length - well, that is going to be tough on little mount and I don't think you'll be able to pull much longer than maybe 20-30s without use of guiding.

I knew you would explain it better than I could. 🙂

[keen]

thanks all.

Vlaiv, with that setup, did you modify the mount firmware? Once in this orientation, you still did a 2 star calibration? So what you did was make the motion profile simpler. But depending on the software, it may not make it better. Were you able to compare how much better it was?

I get how the motion profile is a arc. But thought the software would be better. It's just math that should be well understood. I'll try to balance better. Maybe Noodles is right and I have too much weight and I'm losing motor steps

Are there any computerized GOTO mounts that are better? (ie that will let you do a 60 sec exposure at 500mm?) . Other than the long term accuracy, I really like how this mount works (so easy to setup and find stuff).

[vlaiv]

5 minutes ago, keen said:

Vlaiv, with that setup, did you modify the mount firmware? Once in this orientation, you still did a 2 star calibration? So what you did was make the motion profile simpler. But depending on the software, it may not make it better. Were you able to compare how much better it was?

Yes, I flashed firmware that supports EQ mode of operation.

Once you switch to EQ mode - you still need to do alignment if you want goto to works for you.

Indeed, motion profile is uniform in EQ mode - no change in tempo and only one motor running continuously. That makes for smoother ride - simpler tracking.

I did not compare - I did not try to image in Az mode. In Az mode, besides all of this - you have additional thing to worry about and that is field rotation. Target slowly rotates in field of view when you track in AltAz mode. I took some mosaic images of the moon (using lucky imaging technique) and in that half an hour or so - it was really visible. I had trouble composing final image out of panels as panels were rotated between themselves (not oriented the same). This does not happen in EQ mode.

9 minutes ago, keen said:

Are there any computerized GOTO mounts that are better? (ie that will let you do a 60 sec exposure at 500mm?) . Other than the long term accuracy, I really like how this mount works (so easy to setup and find stuff).

Of course there are - but are much more expensive.

There are mounts that have high precision encoders that can track 10-20 minute without being guided. Such mounts for example cost $8000+ (look at 10Micron GM 1000 HPS mount).

I have much heavier and more precise mount - HEQ5 and I still need to guide that mount if I want to do 60 second exposure at around 500mm. If you want tight round stars - you'll have to guide on almost all mounts - except for those few very expensive mounts that have exceptional precision and encoders.