EQ2 tracking going zig-zag

[Vulisha]

Hi guys, 

I have EQ2 mount motorised wirh custom arduino software and stepper motor, but I have issues wirh tracking since mount goes one frame left one frame right, as you can see on star trails in gif bellow. 

 

Exposures are almost 2 minutes, pictured with 18-55 at ~25mm f6.3

 

I know EQ2 is nothing special but it is only camera with almost no zoom. Do you have any idea what could be the problem? Is this normal for this mount and can it be fixed? 

Edited February 2, 2020 by Vulisha

[vlaiv]

Hi, yes that sort of thing can happen and in principle you can fix it it to some extent.

Zig-zag type of pattern in images will happen if two things happen at the same time - first one is polar alignment error.

With polar alignment error there is constant drift in DEC - which means that your images will have constant offset in one direction. If there are no other contributions it will just show up as streaks and shifts in that direction.

In your video that would be upward motion of stars - if you compare any of your images to star chart you will see that Up/Down direction is DEC direction of the mount.

Second part of this motion is periodic error. This happens because gears used to driver the mount and/or worm gear of the mount are not perfect circles. They are a bit egg-shaped or elliptical. In any case- just irregular and not perfect circles. This means that at some points in time mount will trail behind the stars (slower rotation - ellipsis major axis), but at another time mount will lead in front of the stars (faster rotation then sidereal - ellipsis minor axis) - and these will repeat in cycle.

This is called periodic error of the mount and it will repeat with worm period. It is often not simple sine or cosine wave because there are multiple gears in the system and each of them will be somewhat irregular. True periodic error is superposition of these smaller periods of all of gears involved.

There are two ways to deal with this - one is to disassemble the mount and make sure that everything fits nicely together and change the bearings and tighten everything up - it's called "tuning" the mount. Maybe even replace some gears with belt system if you see that largest oscillations are with that period.

Other would be to do Periodic error correction. It might even be possible for you to do since you have custom stepper and arduino controller. You need to figure out main oscillations and program arduion to actually move stepper faster and slower to compensate for periodic error. It will sure be tricky thing to do, but in principle you should be able to do it.

You can see similar thing happening here - its an animated gif of one of my recordings (refresh page if animation stops - I did not make it loop for some reason):

there is DEC drift due to PA error in left direction and there is wobble up and down - that is RA periodic error (a bit complex because it contains multiple harmonics). It is not fast enough to cause serious trailing, but in some frames stars are elongated.

HTH

 

[Vulisha]

Thank you very much on such detailed answer! 

 

Yes first issue of flowing up is due to Polar alignment error, I didn't try so much with alignment since it is not so big zoom but should do that better as well.  Regarding mount zigzag, i will definitely first try to disassemble and fix mount as much as  I can and then I will maybe try to add some feature for fixing zigzag into Arduino code but it is not as easy task since it has to have calibration option so if I change something I can re calibrate. 

But first step is fixing mount, I tried to grease up worm gear, hoped it would fix it at least a bit, but it did not.  I have googled there is some content on tuning EQ2  so that could help as well. 

 Yes I can see you had? similar issues did you fix it?

[vlaiv]

In order to see if polar alignment error or periodic error is going to have an effect on your image - focal length is not enough, you really need to consider sampling rate - in arc seconds per pixel and compare that with drift in arc seconds per second and exposure duration.

For example if you sample at 10"/px and you have drift of about 0.5"/s, then you can do 20s exposure before drift becomes 1px in your image - simple math but getting accurate drift rates is not easy - you need to measure those.

I had this issue with my HEQ5 from the start and had several enhancements done to it:

- I started guiding

- found out that I had one cracked bearing that needed replacing, so I replaced all bearings, tightened things up and re lubricated the mount

- did belt modification (replaced gears with belts)

- changed a few more things on my mount (saddle plate, tripod and such)

and now I'm finally happy with the mount to the extent one can be happy with HEQ5 class mount - it guides with RMS of 0.5" on a good night.

I guess that above does not mean much to you - but greatest single enhancement I did was that I started using auto guiding. I'm not sure that it means much to you since you have arduino driver and EQ2 and you won't be doing out guiding.

Let's see what else I can offer as advice that might help

- balance mount as best as you can

- EQ2 mount is light so avoid any wind (although you don't have much on that mount - mount itself will catch some wind)

- Maybe use faster lens and shorter exposure lengths (but get more of them)? This will minimize trailing per single exposure

[Vulisha]

Well weekend has come, and I relubed mount, tried to disassemble and fix it even more with this guide:

 

But my all of my socket wrenches weren't able to go inside nut hole, so that failed. Then I tried again to polar align my telescope with "drift align" technique using 130P but that did not help since it drifts differently all the time, there was no wind at all but results were better but not. good enough. As you said probably mount gears are eggy and probably simplest solution would be to get some more serious mount that can accept proper guiding. So I am already looking at some (maybe used) EQ5 + OnStep and some guide cam and scope. Wife will be mad 😂

 

 

Only solution that could work is shorter expo, and then stack them. 

Edited February 9, 2020 by Vulisha

[vlaiv]

What sort of light pollution do you have?

If you have stronger light pollution then you can actually use shorter exposures without loosing much in the end result. Only difference between short and long exposures for same total imaging time is in read noise, and only when read noise is significant component. You are using DSLR without cooling so you already have dark current noise that can be as much as read noise in short exposure. If you add light pollution noise to that, you could be able to achieve similar results with only 30s exposures.

It's certainly worth a try - just remember to do same total integration time and not same number of subs. If you imaged for example for one hour with 2 minute exposures and made 30 of them. If you go for 30s exposures - do one hour again and do 120 exposures.

[Alan64]

1 hour ago, Vulisha said:

But my all of my socket wrenches weren't able to go inside nut hole, so that failed. 

Yes, I had to use needle-nosed pliers, the heavy-duty type.  A pair of those will make it a breeze.  The lock-nut is not difficult to turn.

[Vulisha]

On 09/02/2020 at 22:44, vlaiv said:

What sort of light pollution do you have?

If you have stronger light pollution then you can actually use shorter exposures without loosing much in the end result. Only difference between short and long exposures for same total imaging time is in read noise, and only when read noise is significant component. You are using DSLR without cooling so you already have dark current noise that can be as much as read noise in short exposure. If you add light pollution noise to that, you could be able to achieve similar results with only 30s exposures.

It's certainly worth a try - just remember to do same total integration time and not same number of subs. If you imaged for example for one hour with 2 minute exposures and made 30 of them. If you go for 30s exposures - do one hour again and do 120 exposures.

Hi vlaiv I actually have very big light polution issues, so i took your advice, set EOS M on 130p and took 80 6 second exposures @ 3200 ISO of Orion nebula, and result was great! Just stacked in sequator with some options, no post processing. 

On 09/02/2020 at 23:45, Alan64 said:

Yes, I had to use needle-nosed pliers, the heavy-duty type.  A pair of those will make it a breeze.  The lock-nut is not difficult to turn.

Oh, I don't have heavy duty, only small ones, tried and gave up as I am not sure will it even fix whiplash.  I also noticed as you have that 90 degrees is not in line with RA axis, did you ever found an answer is that suposed to be? 

[Alan64]

3 hours ago, Vulisha said:

Oh, I don't have heavy duty, only small ones, tried and gave up as I am not sure will it even fix whiplash.  I also noticed as you have that 90 degrees is not in line with RA axis, did you ever found an answer is that suposed to be? 

If I'm not mistaken, you're referring to the latitude-scale...

If so, I carefully lifted it off, and then re-glued it on as true and square as I could manage.  I used what I call a squaring-jig, and of my own design...

I made it out of narrow strips of thin plywood.  It's not that crucial to correct that, however.  Now, in so far as the setting-circles, for the RA and DEC, they might as well have been printed with clown faces...

Aside from those aspects of the mount, and of far more importance, if the RA-axis is stiff, bound up, or loose and sloppy, you will have to figure out a way of adjusting its lock-nut.  You don't have to take the axis apart, but you should at least be able to adjust it.

I can't help but think that there is a set of sockets that have thin walls, the kind that would crack and shatter if you used them for automobile repairs.  Such a set would be made in China, of course,  The set should also be dirt-cheap, and found at discount-houses or other.  Back in the late 20th-century, here in the U.S., I used to run across sets like that all the time.  If that fails, and if you desire to defeat the manufacturer in that, then you will need the tool that I had described previously.

Edited February 17, 2020 by Alan64

[Vulisha]

8 hours ago, Alan64 said:

If I'm not mistaken, you're referring to the latitude-scale...

If so, I carefully lifted it off, and then re-glued it on as true and square as I could manage.  I used what I call a squaring-jig, and of my own design...

I made it out of narrow strips of thin plywood.  It's not that crucial to correct that, however.  Now, in so far as the setting-circles, for the RA and DEC, they might as well have been printed with clown faces...

Aside from those aspects of the mount, and of far more importance, if the RA-axis is stiff, bound up, or loose and sloppy, you will have to figure out a way of adjusting its lock-nut.  You don't have to take the axis apart, but you should at least be able to adjust it.

I can't help but think that there is a set of sockets that have thin walls, the kind that would crack and shatter if you used them for automobile repairs.  Such a set would be made in China, of course,  The set should also be dirt-cheap, and found at discount-houses or other.  Back in the late 20th-century, here in the U.S., I used to run across sets like that all the time.  If that fails, and if you desire to defeat the manufacturer in that, then you will need the tool that I had described previously.

Oh sorry I ment on this part about 85° :

 

"The DEC setting circle is fixed, and with screws, but look at where it points, and at its home-position...

...at about 85°.  You can't rotate the circle independently of the axis.  Is that correct, or does it even matter?  I know that I won't be using either circle, in a practical manner, but I want it to be correct nonetheless."

 

But latitude is wrong as well in my, but i do not bother so much with that one. 

So that circle is wrong? it should be  90° in the middle ,and i should not rely on that circle as well?

[Alan64]

5 minutes ago, Vulisha said:

Oh sorry I ment on this part about 85° :

 

"The DEC setting circle is fixed, and with screws, but look at where it points, and at its home-position...

...at about 85°.  You can't rotate the circle independently of the axis.  Is that correct, or does it even matter?  I know that I won't be using either circle, in a practical manner, but I want it to be correct nonetheless."

 

But latitude is wrong as well in my, but i do not bother so much with that one. 

So that circle is wrong? it should be  90° in the middle ,and i should not rely on that circle as well?

Again, I'm not certain, as I no longer have my Japanese-made EQ-2 from the early-1990s with which to compare, therefore I only suspect, and strongly, that that's incorrect.

[Vulisha]

Well I will then try to fix something, I like portability of this mount, but probably will have to go to something like EQ5

Although, using some good tips like good polar alignment, balance and especially shorter expos i did manage to get some fun results in post processing with mobile phone(lightroom and snapseed):

if I could fix mount enough to get 30s @  f5 it would be more than enough for me. 10 is max now then star trails get noticable.

Thank you both very much on great tips and advices and guides! 

[Alan64]

An EQ5-class mount would be an excellent choice, and the sweet-spot among the equatorial mounts.  They do get ponderous at an EQ-3 and up.  Also, the best EQ-3 is an EQ-5.  An EQ-5 weighs only a little more than an EQ-3, yet is much more versatile in the range of telescopes that can be supported.

[Vulisha]

Yes I was thinking that too, eq5 could be great upgrade. It has full motion RA and DEX slow knobs, it has polarscope option, and in steel variant should be really sturdy

 

 

Here is one more, leo tripplet 173x30s iso 3200(originally over 350 frames, more than half removed because of zig zag) and managed to perfect polar align 

[vlaiv]

49 minutes ago, Vulisha said:

Yes I was thinking that too, eq5 could be great upgrade. It has full motion RA and DEX slow knobs, it has polarscope option, and in steel variant should be really sturdy

Until you get EQ5 - which you can motorize / goto enable in DIY manner - look at this project: https://github.com/TCWORLD/AstroEQ

here is another tip for reducing star trails from RA periodic error - shoot targets that have high DEC.

RA error is most pronounced when you are tracking targets at equator while it is minimized when you track target near pole (in fact - when tracking exactly at pole - FOV does not move - it "rotates"). Choosing targets at high declination will lessen effects of trailing. Try imaging M81/M82 for example and maybe you will be even able to do 1 minute subs without trailing. If not, at least you will be able to keep most of 30 second subs.

 

[Vulisha]

I was planing on using OnStep as it is massively supported and has a lot of features.  But I will look into this one as well as I am having some issues with OnStep. 

 

I tried m81/82 but they were too high for my balcony, so i went to M101 Pinwheel, and indeed I got around 70% of good frames for 30 sec, I will try to go longer in upcoming  days. But definitely targeting pole targets reduces zig zag a lot

73 frames,UHC, 30 sec, iso 6400(bit high) just stacked, no post processing. Vignetting  is due cardboard "dew shield" that I made for use as Street lamp light polution protection, and  EF-M to C adapter, but light pollution is reduced massively. 

Edited February 21, 2020 by Vulisha

[vlaiv]

29 minutes ago, Vulisha said:

Vignetting  is due cardboard "dew shield" that I made for use as Street lamp light polution protection, and  EF-M to C adapter, but light pollution is reduced massively. 

Could be due to EF-M / C adapter, but it is almost certainly not due to dew shield / LP shield.

Field of view is at most a degree or two wide here and there is no chance that this amount of vignetting is due to 1 degree light cone being clipped on side. Odds are that dew shield is larger than ota diameter and lens/mirror is certainly less than ota diameter - so even that one degree is questionable. Here is diagram of what is happening:

Let's assume that mirror is exactly the same diameter as dew shield:

If a star is one degree of axis, then parallel rays coming down at aperture will create very small shadow on primary - marked with arrow - rest of the mirror will receive light. At most, only very small fraction of the mirror - less than 1% will be in shadow. You should not be able to detect that in the image, or it would be very very small effect. Usually mirror is not the same diameter but smaller a bit - just enough to still be fully illuminated.

You are free to use dew shield - LP shield. If you are using newtonian scope - it will improve contrast even if there is no street lights around - It is often said that extension in front of focuser should be at least x1.5 diameter of tube, and today's scopes often have very small section of tube in front of focuser.

Marked section of scope needs to be at least 1.5 x 130mm = about 200mm or 20cm and it is clearly not that much so yes, dew shield is a good thing to add contrast with newtonian. Refractors have focuser on the other side of the tube so they provide very good contrast by design, especially if they have tube baffles as many do.

[vlaiv]

If you are using lens - then just ignore my comment above

Just realized that you might be using regular lens with camera - not sure about that and dew shield - could be that it is vignetting because of that

[happy-kat]

Member @bobro from what i recall fitted stepper motors to his fettled eq2 mount, there might be a thread on here on it.

[bobro]

24 minutes ago, happy-kat said:

Member @bobro from what i recall fitted stepper motors to his fettled eq2 mount

My EQ2 has dual axis guiding (but not goto). As guiding controls the position of the mount by making small adjustments to the speed of the simple DC RA motor or turning on/off the DEC motor, stepper motors are not necessary (they are necessary for a goto mount or a non-guided mount for positional accuracy).

Note: I haven't used the EQ2 for some time as I now use an EQ5 (stepper motors) with AstroEQ - a very good solution though care has to be taken with choosing the stepper motors.

Here is a link to more info on the guided EQ2: http://guiding.web.fastmail.co.uk/

[Vulisha]

 

On 21/02/2020 at 15:03, vlaiv said:

If you are using lens - then just ignore my comment above

Just realized that you might be using regular lens with camera - not sure about that and dew shield - could be that it is vignetting because of that

No no I am using both, sometimes lens, but mostly Heritage 130P, but I have set dew shield from inside, not outside since heritage has elliptical tube and this is my "dew shield", don't laugh  https://imgur.com/ZFXcuFa 

I made it in 5 minutes because I noticed that if my body blocks light pollution well, and when I added tube I was frankly stunned with contrast difference  as H130p has very short section in front. 

Vignetting could also possibly be due to "shadow cone" but it is definitely partially due to C adapter as well since C has diameter of 21.7mm and sensor is bit bigger(27.8mm in diagonal). But had to choose this adapter since it has smallest profile. 

I will update dew shield in time due with velcro and dark paper as I did with tube.  It is good to know what size it should be, thanks! 

On 21/02/2020 at 16:47, bobro said:

My EQ2 has dual axis guiding (but not goto). As guiding controls the position of the mount by making small adjustments to the speed of the simple DC RA motor or turning on/off the DEC motor, stepper motors are not necessary (they are necessary for a goto mount or a non-guided mount for positional accuracy).

Note: I haven't used the EQ2 for some time as I now use an EQ5 (stepper motors) with AstroEQ - a very good solution though care has to be taken with choosing the stepper motors.

Here is a link to more info on the guided EQ2: http://guiding.web.fastmail.co.uk/

So I am on same upgrade path as you are  I will look into this but do not plan to put much more effort to EQ2, if it had free motion DEC, then maybe I would, but like this I think EQ5 is better futureproof path. Although since I am in middle of home renovation, funds are not keen towards astronomy, so EQ5 might wait some more time, unless some really good offer comes by, and EQ2 will still zigzag, although much less thanks to you guys  

 

Here is one more work:

Edited February 23, 2020 by Vulisha