20 replies to this topic

[GIJoe50000]

I finally got around to doing a sequence in NINA tonight, my first target was M81 and it seemed to go OK. (I don't have a guide camera and scope yet, but they're on the way)

 

So after getting 100*60s images, I went to the Sombrero galaxy and said I'd do another 100*60s sequence.

 

But suddenly I started getting a lot of bad images, slight star trailing, as in the image below, you can see the number of stars changes after I went to M104.

 

 

The fact that this happened when I moved closer to the horizon suggests to me that this has something to do with it, but I'm not exactly sure why. My thoughts are that either the mount has to travel slightly further every second, compared to when it's near the NCP, or perhaps there's some play in the gears in the mount, or perhaps my polar alignment isn't perfect (but I think I got it very close).

 

Or, is this just par for the course if you don't have a guide camera and scope? There was no wind at all, and no vibrations from cars or anything like that.

 

Can anyone shed some light on this?

[Southstorm]

How low to the horizon were you?

Generally, I try to stay above 30 degrees from the horizon because the atmosphere conditions worsen as you approach the horizon.

[Bean614]

Which scope were you using? 

[GIJoe50000]

How low to the horizon were you?

Generally, I try to stay above 30 degrees from the horizon because the atmosphere conditions worsen as you approach the horizon.

Yea, I was fairly low alright,

Synscan tells me M104 is at: 164°, 25°, but I haven't quite gotten my hear around the coordinates yet, like whether that's a universal value, or whether it's relative to me.

 

But 25° sounds about right I'd say.

[GIJoe50000]

Which scope were you using? 

A Bresser 102s/600, with an ASI585MC camera, and a HEQ5 mount.

[David Boulanger]

You are also closer to the celestial equator with M104 than M81.  RA guiding errors will be "magnified " more the closer to 0 Dec you are.

[GIJoe50000]

You are also closer to the celestial equator with M104 than M81.  RA guiding errors will be "magnified " more the closer to 0 Dec you are.

Yea, that's along the lines of what I was thinking, since a degree near the horizon has a much larger arc length than a degree near the NCP.

 

It's my first time doing a "full" sequence, so I was expecting to run into a few issues, and to hopefully learn a little something from it..

[Jlex]

I've often wondered if load shift affects periodic error.  In my case, I have great balance, polar alignment and properly leveled rig but, I see the same as you on occaision.  I haven't taken mental notes on horizon position but I will include this going forward.  Like you, I'm also unguided.

[17.5Dob]

Besides the seeing effects, you were imaging at a very high northern declination on M81/82 where stars are barely moving. I love imaging towards the pole, my RMS errors are generally half of what I get near the equator. Makes me feel like I have a completely different mount.

[james7ca]

Polar alignment becomes more critical as you image nearer to the pole and when very close to the horizon you're also going to suffer from atmospheric refraction which will change the apparent rate of motion of the stars. Atmospheric refraction can be a pretty significant factor and just remember that when an object is on the horizon it is displaced by 30 arc MINUTES from it true location. Thus, when you image at the zenith there will be no refraction and by the time you reach the horizon it will seem that the object will have moved by 30 arc minutes from where it should be if it were moving at a true sidereal/tracking rate.

 

However, the vast majority of that movement happens within just a few degrees of the horizon, but even at an elevation of 30 degrees you're going to see a shift of a few arc minutes (although the rate of change at that elevation is fairly slow).

 

Below is something that I wrote about one year ago in a topic about unguided imaging:

 

This atmospheric refraction varies as you move closer to the horizon and when imaging at 30 degrees altitude and with a target near to the celestial equator you could see about one arc second of drift within one minute just from the change in the refraction as the target sets in the sky. However, below 30 degrees the situation can be worse, while above 30 degrees (like near to the zenith) it isn't much of an issue unless you are trying to take unguided images of fifteen, twenty, or thirty minutes (and here I'm taking about atmospheric refraction, no other factors). At twenty degrees altitude the drift becomes two arc seconds in one minute or about one arc second every thirty seconds.

 

And, here is a post with additional links to information concerning polar alignment and atmospheric refraction:

 

  https://www.cloudyni.../#entry12046078

 

All that said, I suspect that the majority of the changes you SEEMED to have are more about your mount (variability) and your polar alignment. Plus, the change in the number of detected stars could just be from focus, seeing, transparency, or the simple fact that there are fewer stars in one area of the sky than another.

Edited by james7ca, 31 March 2023 - 11:03 PM.

[Alex McConahay]

In addition to all the other posts, I think  you need to consider seeing, which is worse the closer you get to the horizon. 

 

Alex

[Jeffmar]

I think Alex is right! It has also been my experience that imaging below 30 degrees is more difficult. Guiding is usually worse due to all the extra atmosphere the light is going through. Seeing is worse with all of those thermals moving air through that long path the light is taking. Even if your guiding isn’t a huge issue, at low angles, the quality of your images can be worse for the same reason. 

[acrh2]

I finally got around to doing a sequence in NINA tonight, my first target was M81 and it seemed to go OK. (I don't have a guide camera and scope yet, but they're on the way)

 

So after getting 100*60s images, I went to the Sombrero galaxy and said I'd do another 100*60s sequence.

 

But suddenly I started getting a lot of bad images, slight star trailing, as in the image below, you can see the number of stars changes after I went to M104.

 

Screenshot (4) (Lsfgarge).png

 

The fact that this happened when I moved closer to the horizon suggests to me that this has something to do with it, but I'm not exactly sure why. My thoughts are that either the mount has to travel slightly further every second, compared to when it's near the NCP, or perhaps there's some play in the gears in the mount, or perhaps my polar alignment isn't perfect (but I think I got it very close).

 

Or, is this just par for the course if you don't have a guide camera and scope? There was no wind at all, and no vibrations from cars or anything like that.

 

Can anyone shed some light on this?

RA errors are minimized with increasing declination:  RAd = RAe x cos(Dec), where RAe and RAd are RA errors at the celestial equator and at the declination of Dec, respectively. 

M81 is at 69 degrees, Sombrero galaxy is at -11 degrees. The errors in RA tracking/guiding are going to be different by a factor of cos(-11) / cos(69) = 2.7.

There could be other factors as well.

Edited by acrh2, 31 March 2023 - 10:53 PM.

[james7ca]

I think Alex is right! It has also been my experience that imaging below 30 degrees is more difficult. Guiding is usually worse due to all the extra atmosphere the light is going through. Seeing is worse with all of those thermals moving air through that long path the light is taking. Even if your guiding isn’t a huge issue, at low angles, the quality of your images can be worse for the same reason. 

What you say is true, but the OP wasn't using any guiding.

 

Also, while the rate of tracking error from the mount's polar axis drive decreases when you move toward the pole (greater declination) the rate of tracking error from polar alignment error actually increases as you get closer to the pole. So, to some extent these are offsetting factors. That said, you generally will get better tracking when imaging nearer to one of the poles, but you still need good polar alignment.

Edited by james7ca, 31 March 2023 - 11:08 PM.

[Alex McConahay]

Guiding or not, I think the seeing is worse near the horizon. 

 

Also, if you want to explore it, check out "King Rate."

 

Sidereal rate in our typical mounts follows the stars so that they move the same no matter where they are in the sky.

 

But, because of atmospheric refraction, the stars travel at a different speed near the horizons (that at or near zenith). 

 

King rate compensates for this. (So does guiding.) 

 

But, mostly stars and tracking are poopy near the horizon because of seeing . 

 

Alex

[james7ca]

Alex, I wasn't contradicting what you were saying, but Jeffmar's response seemed to focus mainly on guiding.

 

As for the King Rate, yes, that's fully explained in the link I gave earlier.

Edited by james7ca, 31 March 2023 - 11:14 PM.

[ButterFly]

The number of stars changing after the 100 mark is nothing more than you having slewed to a different part of the sky.  There is nothing else you can read into that really.  Different sky, different number of stars.

 

Around 20 and 60, you probably got a cloud.  The rest is fairly stable at that part of the sky,  The increased variation after the 100 mark is probably because of the lower altitude.  It could just as easily have been lots of clouds passing by.  Hard to tell without more.  The decreasing trend of the number of stars is pure extinction.

 

Guiding, when you use it, is a lot harder near the horizon.  As james7ca mentions, the rates are actually changing.  But, the directions are also changing along with those rates.  Guiding programs calibrate to find out which direction is which, so it knows how to make a change in RA and Dec.  Refraction near the horizon always acts upward.  How much RA and Dec move upward depends on the azimuth of the horizon you are shooting near.  East and West, upward is essentially all RA.  North and South, upward is all Dec.  Unguided, it is entirely up to your mount to make corrections, if any.  Most will not unless refraction is accounted for in its tracking model.  Then, when it tracks in RA and Dec, it will do so based on the measured (as refracted) RA and Dec.

 

Sometimes you just have to shoot near the horizon.  The highest it gets can still be quite low.  If guiding, calibrate near where you will be shooting.  If unguided, take shorter subs and recenter with dithers.

[GIJoe50000]

The number of stars changing after the 100 mark is nothing more than you having slewed to a different part of the sky.  There is nothing else you can read into that really.  Different sky, different number of stars.

 

Around 20 and 60, you probably got a cloud.  The rest is fairly stable at that part of the sky,  The increased variation after the 100 mark is probably because of the lower altitude.  It could just as easily have been lots of clouds passing by.  Hard to tell without more.  The decreasing trend of the number of stars is pure extinction.

 

Guiding, when you use it, is a lot harder near the horizon.  As james7ca mentions, the rates are actually changing.  But, the directions are also changing along with those rates.  Guiding programs calibrate to find out which direction is which, so it knows how to make a change in RA and Dec.  Refraction near the horizon always acts upward.  How much RA and Dec move upward depends on the azimuth of the horizon you are shooting near.  East and West, upward is essentially all RA.  North and South, upward is all Dec.  Unguided, it is entirely up to your mount to make corrections, if any.  Most will not unless refraction is accounted for in its tracking model.  Then, when it tracks in RA and Dec, it will do so based on the measured (as refracted) RA and Dec.

 

Sometimes you just have to shoot near the horizon.  The highest it gets can still be quite low.  If guiding, calibrate near where you will be shooting.  If unguided, take shorter subs and recenter with dithers.

 

Yea, I immediately disregarded the "amount of star" because I moved to a different target, so I knew that was irrelevant. But indeed, I'd imagine turbulence and refraction would cause problems with guiding alright near the horizon, with the stars appearing to move up, down, and backwards, and as the guiding corrects it, the stars move somewhere else a few seconds later..

 

It'd probably be like chasing chickens for the poor guide program!

[GIJoe50000]

Thanks for the replies guys, I definitely learned that tracking at the horizon is problematic, and it's something I never heard about before (or never took notice of!).

 

I think I'll put it down to bad seeing and errors in the mount at low declination, and I'll know to expect it again in the future, and I'll either take extra exposures, or wait a few months for summer to come around.

[Oort Cloud]

The closer you get to the horizon, the more air you're looking through. Air is what causes seeing, the bane of astronomers worldwide, except in select locales. More air = worse seeing, more blur, and more separation of color (atmospheric dispersion).

Image targets near the zenith when possible, or near the meridian when not.

[james7ca]

All good replies, but I want to emphasize again that seeing really has no effect on tracking and thus given the title of this thread it's not really a factor. However, seeing definitely will have an effect on your image quality, the smallness of your stars, their number, and to some extent their shape. Plus, seeing will definitely impact GUIDING and for that reason it's always harder to guide when the seeing is poor.

 

Thus, we have two different questions. Does seeing affect your tracking? The answer is no. Does seeing affect the quality of your results? The answer is yes.

 

Note that the effects of seeing are kind of independent of your target's altitude. Thus, on a night will really poor seeing your results when imaging near to the zenith could be just as bad as when imaging near to the horizon on a night with very good seeing. So, it's the quality of the seeing and not necessarily the target's altitude that is the major factor in overall image quality (if your ignore atmospheric refraction and dispersion). Of course, on any give night (or hour) your seeing will always be worse when imaging near to the horizon but again that has no real effect on your tracking (just as it would have no effect on the results when tracking near to the zenith).

 

The only factors that affect tracking are the quality of your mount, your polar alignment, and atmospheric refraction and all three of these factors will depend on where in the sky you are imaging (both in altitude and direction).

 

All that said, seeing does play a MAJOR role in the quality of your results so for that reason alone it's usually best to avoid imaging when near to the horizon. Similarly, depending on the quality of you mount and the length of your exposures it may actually be a good idea to disable guiding when the seeing is really poor because your tracking might be better than your guiding.

 

However, unless you have a premium mount with absolute encoders and a well-calibrated sky pointing model I wouldn't advise doing any exposures longer than about 30 seconds (but this depends upon your image scale -- your focal length and pixel size). Is it possible to go longer than 30 seconds when NOT guiding? I'd say yes but that would require a fairly modest image scale with a "decent" mount, a good polar alignment, and when imaging fairly high in the sky.

 

In any case, the results reported by the OP are probably the result of both tracking and seeing where seeing would certainly have the potential for the cause of most of their quality issues.

Edited by james7ca, 01 April 2023 - 10:34 PM.