Bad polar alignment, Shutter Shock/Shake or other?

[Enceladus Dan]

Hello all, 

I have been kicking myself each time I get out on a clear night as I never seem to get any longer than 30-45 seconds without star trails. On the 05th October it was a new moon and clear skies so I wanted to revisit Andromeda as my last try there was a 77% moon nearby. 
 

To give myself the best chance I tried out drift alignment. I set up as normal and pointed to a star in the south and somewhere near the celestial equator as hard with houses and street lights all around. Set my camera so the dec moved the star moved up and down. Put the star to the right side and started the exposure. 5 seconds in I moved the RA for 10 seconds so the star move left then back again ( I tried 30 seconds each way but the star left each side of the image). Repeated with a star to the East horizon ( more ENE as a tree was directly east). 
 

south

East

If I’m doing this right I should be good? But when I tried varying long test exposure in the time I had before the street lights go out I had trails at all lengths 1,2,3,4&5 mins. However I noticed the distance of the trail in 1 min makes it look like double stars next to each other and at 5 min it’s not much longer. If it was alignment shouldn’t the distance be 5x longer or thereabouts? 

1 min - from actual shoot after lights went out with good stars 35 of the 60 taken were like this. 

1 min - from test before with trails

5 mins - from before with trails

my other thoughts were ground shaking from me, so I WiFi remote controlled the Eos M3 from my phone indoors with the same result. This left me with wind? To me it seems pretty calm, or something in the camera.

I thought a mirrorless camera would have less shake from the shutter than a DSLR but on looking into how the shutter works ( I was clueless) I learned the shutter closes as the exposure starts then opens again, at the end of the exposure it closes again before opening ready for the next image. 
 

I watched a slow mo video on YouTube and it looks quite aggressive when it slams shut just as the exposure starts. Could this be my problem? I’ve read they have 1 electronic shutter and 1 mechanical and some newer models allow the electronic opening shutter to be held open for silent (shake free) images. Maybe the mechanical one slamming shut before the image is the cause. 
 

Or is there something else I should look into as loosing almost 50% of images is frustrating. 
 

thanks

Danny

 

Edited October 9, 2021 by DannyST

[ONIKKINEN]

Are you autoguiding?

All mounts experience something called periodic error in RA where the tracking speed oscillates a bit above and under the ideal rate. Unguided exposures are limited by your RA drift rate, which varies quite a lot between different mounts.

 

Below is an example i just found online, the red line is your DEC, which in this case was spot on as it hardly moves in the long term while the blue line is the RA which is constantly in use. The steep drops and cliffs of the graph are where you experience bad subexposures, but just by pure chance you could have your exposure at the relatively even top section and not notice trailing as much. I don't know what mount (or even if it was a mount) this image is from but it gets the point across. My EQM35-PRO experiences somewhere around 8 arcseconds RMS of periodic error during one worm cycle, which is 8minutes on my model. In theory that would limit a 1 as/pixel resolution image to just 30 seconds, but in reality there are these same steep cliffs and drops in the graph, making unguided exposures not worth the effort.

How much you can get away with will depend on your mounts periodic error and the resolution you're imaging in.

Edited October 9, 2021 by ONIKKINEN

[Enceladus Dan]

Has tracking motors been known to cause any intermittent shake? I have the EQ 5 deluxe with enhanced dual axis motors. 
 

thanks

Danny

[Enceladus Dan]

1 minute ago, ONIKKINEN said:

Are you autoguiding?

All mounts experience something called periodic error in RA where the tracking speed oscillates a bit above and under the ideal rate. Unguided exposures are limited by your RA drift rate, which varies quite a lot between different mounts.

 

Below is an example i just found online, the red line is your DEC, which in this case was spot on as it hardly moves in the long term while the blue line is the RA which is constantly in use. The steep drops and cliffs of the graph are where you experience bad subexposures, but just by pure chance you could have your exposure at the relatively even top section and not notice trailing as much. I don't know what mount (or even if it was a mount) this image is from but it gets the point across. My EQM35-PRO experiences somewhere around 8 arcseconds RMS of periodic error during one worm cycle, which is 8minutes on my model. In theory that would limit a 1 as/pixel resolution image to just 30 seconds, but in reality there are these same steep cliffs and drops in the graph, making unguided exposures not worth the effort.

How much you can get away with will depend on your mounts periodic error and the resolution you're imaging in.

I just thought of tracking and you found this wonderful info, thank you

[Dazzyt66]

Hey Danny, I must admit it sounds like alignment to me but I’ll let someone with greater experience guide you on that. 

5 mins unguided is expecting a lot (I’m assuming you are unguided)? I have a HEQ5 with spot on polar alignment and I only go for around 60s exposures to be sure of no trails - with UK weather I don’t want to ruin what chances we have.

I also leave a 1s delay between exposures to allow for camera shake due to the mirror even though all my imaging is done remotely.

HTH

[Enceladus Dan]

7 minutes ago, Dazzyt66 said:

Hey Danny, I must admit it sounds like alignment to me but I’ll let someone with greater experience guide you on that. 

5 mins unguided is expecting a lot (I’m assuming you are unguided)? I have a HEQ5 with spot on polar alignment and I only go for around 60s exposures to be sure of no trails - with UK weather I don’t want to ruin what chances we have.

I also leave a 1s delay between exposures to allow for camera shake due to the mirror even though all my imaging is done remotely.

HTH

Hello, I am unguided at the moment as I’m new to astronomy, photography and astrophotography so all a big learning curve as just started in June this year. I have the EQ5 deluxe with enhanced motors, using a 150pds. 
 

I hoped it wasn’t the alignment as most 1 mins were ok, I aim to be able to reach 3 mins max but tested to 5 to try figure out a banding issue with the Hybrid AF sensor. I was thinking of getting a 760D which shares the same sensor as my M3 but uncertain if I’ll get lines so now thinking of a 77D. 
 

thanks 

Danny

Edited October 9, 2021 by DannyST

[Ohgodwherediditgo]

I think the HEQ5 has a worm period of about 8 mins, so that would rule out periodic error if you had a half hour with good tracking. The camera causing shake would cause it consistently ( i really doubt it's that ). Maybe a cable snag ? Or rough running RA bearing ?

[Enceladus Dan]

4 minutes ago, Ohgodwherediditgo said:

I think the HEQ5 has a worm period of about 8 mins, so that would rule out periodic error if you had a half hour with good tracking. The camera causing shake would cause it consistently ( i really doubt it's that ). Maybe a cable snag ? Or rough running RA bearing ?

Thanks, next time I’m out I’ll try disengage the motors and use the slow motion controls see if it feels smooth, hope so as it’s only a few months old. Cables I use a dummy battery and the cable drops to a power pack  not tied just hanging loose not touching anything. maybe the weight of the cable I can zip tie it up loose see if helps. 
 

thanks

Danny

[Ohgodwherediditgo]

1 minute ago, DannyST said:

Thanks, next time I’m out I’ll try disengage the motors and use the slow motion controls see if it feels smooth, hope so as it’s only a few months old. Cables I use a dummy battery and the cable drops to a power pack  not tied just hanging loose not touching anything. maybe the weight of the cable I can zip tie it up loose see if helps. 
 

thanks

Danny

I have started to loop my cables around one of the RA adjustment knobs used for polar alignment just to take the weight, with enough slack of course. I'll probably get flamed for that by someone with more experience.

[Enceladus Dan]

4 minutes ago, Ohgodwherediditgo said:

I have started to loop my cables around one of the RA adjustment knobs used for polar alignment just to take the weight, with enough slack of course. I'll probably get flamed for that by someone with more experience.

I’m planning to add an Aluminium bar to the top of the OTA rings for a dew heater controller and telrad to sit on. Might add a p clip to hook camera cable to and some Velcro somewhere on the Mount for the cables.  

[Enceladus Dan]

On my attempt at drift alignment, did I get the instructions right? Following a guide but was aimed at AZ mounts so I kind of just did what I could on my EQ Mount. 
 

thanks 

Danny

[rickwayne]

I should just know, after all the times I've looked at M31, which way is RA and which is DEC. And on my phone I can't analyze it. But you can. If the trailing is in the RA direction, you've got periodic error or drive train issues. Elongation in DEC, unless you're guiding,  means polar alignment problems. 

[rickwayne]

Oh,  and the drift alignment stuff sounds correct. One thing, though, change the slewing speed on your mount so you can run longer exposures. It actually doesn't matter if you go off the edge of the frame so long as you get the timing right. So long as the ends of the "V" wind up in frame and you can distinguish one star from another-- easy in the example you show-- it doesn't matter if the point of the "V" is way off the edge. Ideally you want to run the same length as your exposures, but in practice it can be shorter. 

Edited October 9, 2021 by rickwayne

[Enceladus Dan]

15 minutes ago, rickwayne said:

I should just know, after all the times I've looked at M31, which way is RA and which is DEC. And on my phone I can't analyze it. But you can. If the trailing is in the RA direction, you've got periodic error or drive train issues. Elongation in DEC, unless you're guiding,  means polar alignment problems. 

Wow, sounds so simple to diagnose when explained like that. So I opened stellarium on my phone and orientated so same as my image and moving the time moves andromeda in exactly the direction of the trail. So it’s periodic error or drive train if I’m understanding this right? 
 

this one will need to be permanently stored with other stuff learnt so far. Thank you this tip. 
 

thanks 

Danny

Edited October 9, 2021 by DannyST

[ONIKKINEN]

13 minutes ago, DannyST said:

On my attempt at drift alignment, did I get the instructions right? Following a guide but was aimed at AZ mounts so I kind of just did what I could on my EQ Mount. 
 

thanks 

Danny

The alignment is good. I don't see a reason to doubt polar alignment in this. I recommend you try shorter exposures and see where you have a good compromise between signal to noise ratio and failed exposures. The 60s sub in your post looks really good, actually unnecessarily good. If you can see the object in a single sub, then it is a really good sub. The 5 minute sub is actually overexposed, and you gain no useful data compared to taking a 1 minute sub 5 times. Something i would note is that 35 out of 60 subs being that good for unguided 60s subs with a newtonian is actually a decent result.

7 minutes ago, rickwayne said:

I should just know, after all the times I've looked at M31, which way is RA and which is DEC. And on my phone I can't analyze it. But you can. If the trailing is in the RA direction, you've got periodic error or drive train issues. Elongation in DEC, unless you're guiding,  means polar alignment problems. 

The elongation of stars is in the same direction as RA in this case, indicating that the tracking issue is entirely RA related.

[Enceladus Dan]

2 minutes ago, ONIKKINEN said:

The alignment is good. I don't see a reason to doubt polar alignment in this. I recommend you try shorter exposures and see where you have a good compromise between signal to noise ratio and failed exposures. The 60s sub in your post looks really good, actually unnecessarily good. If you can see the object in a single sub, then it is a really good sub. The 5 minute sub is actually overexposed, and you gain no useful data compared to taking a 1 minute sub 5 times. Something i would note is that 35 out of 60 subs being that good for unguided 60s subs with a newtonian is actually a decent result.

The elongation of stars is in the same direction as RA in this case, indicating that the tracking issue is entirely RA related.

Thank you, makes me feel better now. I thought I was loosing to many subs, eventually I’ll go guided but I want a better camera first then get it modified.

This was the reason for the 5 mins as my M3 has the hybrid AF sensor and when looking into cameras I saw lots of posts with strange dark lines caused by the hybrid AF. So I was trying to make my m3 do it to find out if it can be avoided. But I never got an image with the dark lines. 
 

now I’m looking for a 600D or a model after the 800D like the 77D etc. 
 

thanks

Danny

[ONIKKINEN]

2 minutes ago, DannyST said:

Thank you, makes me feel better now. I thought I was loosing to many subs, eventually I’ll go guided but I want a better camera first then get it modified.

This was the reason for the 5 mins as my M3 has the hybrid AF sensor and when looking into cameras I saw lots of posts with strange dark lines caused by the hybrid AF. So I was trying to make my m3 do it to find out if it can be avoided. But I never got an image with the dark lines. 
 

now I’m looking for a 600D or a model after the 800D like the 77D etc. 
 

thanks

Danny

If you're planning on getting a new camera and planning on guiding one day, consider not buying a camera for now and getting a dedicated astro cam with the guide setup? Especially true if you're planning on spending more than a couple hundred on a used model.

A cooled, low noise astro cam makes a world of difference compared to DSLRs. The only downsides are price and a bit of an annoyance setting everything up every time, but the results are well worth it. Since you will need some sort of computer to do the guiding, adding the astro cam is no extra effort.

[Enceladus Dan]

1 hour ago, ONIKKINEN said:

If you're planning on getting a new camera and planning on guiding one day, consider not buying a camera for now and getting a dedicated astro cam with the guide setup? Especially true if you're planning on spending more than a couple hundred on a used model.

A cooled, low noise astro cam makes a world of difference compared to DSLRs. The only downsides are price and a bit of an annoyance setting everything up every time, but the results are well worth it. Since you will need some sort of computer to do the guiding, adding the astro cam is no extra effort.

The thought had crossed my mind but I put a 4/3rds (based on a hypercam 294 pro camera) into the astronomy tools fov calculator and large dso objects like andromeda, orion etc don't fit. They just fit in my aps-c crop and astro cams with aps-c i found came in quite expensive to the hypercam. 

Worth their money I'm sure but too much for me now. 600D is almost straight trade for the M3 so just the modification cost. Or get some money off the 77D / 6D I'm tempted with. 

If I think sensibly, I should get the 600d for little actual cost and use it to learn more before diving in with a more expensive camera be it dslr or dedicated. But I've not been that sensible since I took this up in June lol.  

Edited October 9, 2021 by DannyST

[Richard Wesson]

7 hours ago, Ohgodwherediditgo said:

I think the HEQ5 has a worm period of about 8 mins, so that would rule out periodic error if you had a half hour with good tracking. The camera causing shake would cause it consistently ( i really doubt it's that ). Maybe a cable snag ? Or rough running RA bearing ?

He is using the EQ5 - not HEQ5 so the worm period will be different. 

[Richard Wesson]

7 hours ago, DannyST said:

Hello, I am unguided at the moment as I’m new to astronomy, photography and astrophotography so all a big learning curve as just started in June this year. I have the EQ5 deluxe with enhanced motors, using a 150pds. 
 

I hoped it wasn’t the alignment as most 1 mins were ok, I aim to be able to reach 3 mins max but tested to 5 to try figure out a banding issue with the Hybrid AF sensor. I was thinking of getting a 760D which shares the same sensor as my M3 but uncertain if I’ll get lines so now thinking of a 77D. 
 

thanks 

Danny

The 150PDS is really pushing it on an EQ5 for imaging and many would say is too heavy. This will have an impact on longer subs, esoecially unguided. 

[Enceladus Dan]

5 hours ago, Richard Wesson said:

The 150PDS is really pushing it on an EQ5 for imaging and many would say is too heavy. This will have an impact on longer subs, esoecially unguided. 

Thank you, I think this was another error when I was first looking into what I should get. When I planned my set up I looked up the weight capacity on skywatchers website as 9kg and the OTA was 5.5 so I upgraded the EQ3 to the EQ5. Now looking again I see there are posts saying for AP we should not go over 6.5kg. 
 

so this will need addressing, I’ll keep plodding on with my journey and keep an eye out for a HEQ5 maybe second hand but at the moment the ones I see are only just cheaper than new. Could be because of waiting times for new. 
 

thanks

Danny

[ONIKKINEN]

9 hours ago, DannyST said:

Thank you, I think this was another error when I was first looking into what I should get. When I planned my set up I looked up the weight capacity on skywatchers website as 9kg and the OTA was 5.5 so I upgraded the EQ3 to the EQ5. Now looking again I see there are posts saying for AP we should not go over 6.5kg. 
 

so this will need addressing, I’ll keep plodding on with my journey and keep an eye out for a HEQ5 maybe second hand but at the moment the ones I see are only just cheaper than new. Could be because of waiting times for new. 
 

thanks

Danny

I am imaging with an EQM35-PRO (basically an EQ3 with extra marketing and an extra counterweight) and an 8 inch newtonian that weighs in at around 9kg with guiding and cameras on top. While it is painful sometimes and windy days are by default no-go it is far from impossible. Shorter subs and leaving DEC unguided are proving to be effective measures for me. I can get around 2/3rds of my subs under 1 arcsec RMS (generally regarded as good) if the conditions are average.

 

The payload and mount capability conversation has gotten really out of hand IMO. What do you suppose is "good performance" for the payload? Eternal sub arcsec guiding, no effects from wind, never loses a single sub? Looking at 10k mounts there. A more reasonable approach would be: More than half of the subs are good or usable and this you can already achieve unguided, so adding guiding will improve things drastically. In my opinion don't stress about this, its easy to get into a spiral of spending because of general opinions. The EQ5, especially when guided will be quite capable of handling your 150PDS. Wind will keep ruining your exposures but that is a problem that newtonian users just have to live with, regardless of mount used.

 

The "mount limited" concept is also in my opinion outdated and not quite true. Really there is no such thing as being mount limited, the system is being photon count limited and a better mount allowing longer subs will bring in more photons per exposure. But why not go the other way around and make sure that all photons count? Using modern dedicated CMOS astrocameras you can use exposures as short as 10s and stack 6 of them to get a result that is pretty much exactly the same as a single 60s exposure. I was in the same boat as you, looking for a mount replacement because i thought i had to take longer subs, but changing to a dedicated astro cam changed this completely. I am now looking for more storage space for my computer to allow processing thousands of subs. This applies to DSLRs as well, but you probably cant get quite as short in the exposures, but IMO 30s should be just fine.

 

Now having said all that i do agree that cheap mass produced mounts should probably not go over 50% of their rated payloads to get consistently good results, but since money is limited for a lot of people i just choose not to follow this argument.

Edited October 10, 2021 by ONIKKINEN

[Enceladus Dan]

12 hours ago, ONIKKINEN said:

I am imaging with an EQM35-PRO (basically an EQ3 with extra marketing and an extra counterweight) and an 8 inch newtonian that weighs in at around 9kg with guiding and cameras on top. While it is painful sometimes and windy days are by default no-go it is far from impossible. Shorter subs and leaving DEC unguided are proving to be effective measures for me. I can get around 2/3rds of my subs under 1 arcsec RMS (generally regarded as good) if the conditions are average.

 

The payload and mount capability conversation has gotten really out of hand IMO. What do you suppose is "good performance" for the payload? Eternal sub arcsec guiding, no effects from wind, never loses a single sub? Looking at 10k mounts there. A more reasonable approach would be: More than half of the subs are good or usable and this you can already achieve unguided, so adding guiding will improve things drastically. In my opinion don't stress about this, its easy to get into a spiral of spending because of general opinions. The EQ5, especially when guided will be quite capable of handling your 150PDS. Wind will keep ruining your exposures but that is a problem that newtonian users just have to live with, regardless of mount used.

 

The "mount limited" concept is also in my opinion outdated and not quite true. Really there is no such thing as being mount limited, the system is being photon count limited and a better mount allowing longer subs will bring in more photons per exposure. But why not go the other way around and make sure that all photons count? Using modern dedicated CMOS astrocameras you can use exposures as short as 10s and stack 6 of them to get a result that is pretty much exactly the same as a single 60s exposure. I was in the same boat as you, looking for a mount replacement because i thought i had to take longer subs, but changing to a dedicated astro cam changed this completely. I am now looking for more storage space for my computer to allow processing thousands of subs. This applies to DSLRs as well, but you probably cant get quite as short in the exposures, but IMO 30s should be just fine.

 

Now having said all that i do agree that cheap mass produced mounts should probably not go over 50% of their rated payloads to get consistently good results, but since money is limited for a lot of people i just choose not to follow this argument.

Thank you, some good food for thought. I have much to think about now, but most importantly I know it’s not the alignment or camera. Now I learn to work with it and I don’t need to spend hours retrying polar alignment over and over. 
 

first I’ll trade the camera for the 600D and go guided, give myself some time to learn and see how much my laptop drains my power pack. At home I can use an extension but I’m limited to 3 hours from 1am to 4am due to street lights. I’m waiting on permission to go to some private land in a bottle 4 out of the city with no artificial lights around. 
 

next will be the leap to dedicated Astro cam, I’ll spend my time with the 600D looking at astro cams and if dropping to 4/3rds sensor size will be ok or save for APS-C sized one. Another subject I have much to learn about.
 

thankyou everyone, much appreciated

Danny 

[ollypenrice]

I'm not sure your drift procedure was correct. These are the key steps:

1) Orientate the camera along RA and Dec. To do this, take a 5 second sub while using a slow slew speed. This will produce star trails. Rotate the camera and repeat until your trails are parallel with one side of the chip. It doesn't matter which.

2) Find out which way your image is appearing in terms of north, south, east, west.  (Point south and turn off the tracking.  Take a few short shots. The stars will move west between subs so now you know east-west. Lower the angle of the scope till you see the horizon appear. It will appear on the southern edge of the image first so now you know N-S. Mark this on a piece of paper!

3) Observe a star close to the equator and in the south. If it drifts south, move the mount west. If it drifts north, move the mount east. Ignore movement N-S.

4) Observe a star around 20 degrees up in the east, or as close as you can get to that. If it drifts south, raise the mount. If it drifts north, lower the mount.

They key thing is that you need to be clear about N-S-E-W in the image you're using.

In analyzing mis-shaped stars, anything with two blobs close together, or overlapping each other, is likely to be produced by backlash in the drives. What's happening is that the star is being imaged on one side of the backlash then on the other but is flopping quickly from one side to the other, leaving little or no trace of its journey. If one blob is bigger than the other, it means the mount is spending longer sitting on that side of backlash than on the the other.  If you have evenly thick trails which get longer as the subs get longer then you have a problem with periodic error.

Olly

Edited October 11, 2021 by ollypenrice

[rickwayne]

Olly, forgive me if you already knew this but he was using the DARV technique, where you slew in RA during an exposure, out and back, to make a deliberate star trail. If the mount is perfectly aligned, there will be no DEC drift during the exposure and so the incoming trail will be the same as the outgoing, leading to just a single line. If there is DEC drift, the two trails will form a "V" shape. 

D.A.R.V (Drift Alignment by Robert Vice) - Articles - Articles - Articles - Cloudy Nights

All that you say about aligning the camera axis and figuring out which direction is which on the sensor applies, of course. Obviously, DARV doesn't give you any information about how your RA axis is doing.