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Tracking error


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Good Day,

Can someone supply me with an answer? Last night I took this image of M31 (Andromeda galaxy) using the following,

Camera - Pentax K70

Lens - Pentax DA* 300mm

Mount - Star Aventurer 2i

iso - 200

f - 4.8 using a 62mm step-down ring

Light subs - 60 x 90 seconds

Dark subs - 30 x 90 seconds

Bias subs - 30 x 1/6000 sec

Flat subs - 30 x 1/200 sec

399881531_Andromeda0908202201.thumb.jpg.b5f5e4c8a9a6f3e6af36ace881907cde.jpg 

As can be seen there is a tracking error, at the start of the session M31 was in the centre of the viewfinder. The final light sub had moved up and right in the frame.

Stacking in Siril rejected 31 light subs.

To find the Hour Angle of Polaris I use a Nautical Almanac, last night at 11pm (U.K.) the Hour Angle of Polaris was 4hrs 9mins.

If I had mis-read the clock face of the Star Adventurer and placed Polaris at 4hrs 20mins, would this cause the tracking to appear to be slow as shown by M31 moving up and right during the session.

Does this make sense?

Thanks for reading

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There are usually two types of errors that appear when tracking.

One is related to actual tracking rate - how well speed of tracker matches earth's rotation.

Other is related to how well you polar aligned.

First one causes drift in RA exclusively, second one causes drift in DEC exclusively.

In your image you appear to have drift in both - in RA and DEC - which means that you have mixture of the two.

DEC drift is handled easily - just make sure you are well polar aligned.

RA drift can be caused by number of reasons.

1) Selecting wrong tracking rate. Mounts track at different tracking rates - Sidereal, Lunar, Solar. For these kind of images - you need to select Sidereal tracking rate - other two will cause trailinf

2) Mount simply lags or trails actual earth's motion - this is generally almost never the case. While it is 100% guaranteed to happen - effects are only visible after long periods of time - like whole day, so it is never a problem during single exposure, but it can sometimes happen that something is wrong with unit and that this error is much larger than it should be.

3) There is periodic error of the mount. This is related to mechanical build of mount. Circles are never perfectly round and machined parts are never 100% correct in their specification - there is always some tolerance. This causes some variation in tracking speed - sometimes mount trails and sometimes it leads compared to earth. This is called periodic error and it causes either small elongation of stars or streaks - depending on how big this error is.

This error is repetitive / cyclic in nature (much like wave / ripple - sometime it is higher than average surface sometimes it is lower - but overall there is average surface height) and there are few ways to deal with it:

- shorten your exposure length. Shortening exposure length will create less streaking and less subs will be affected by this

- use PEC, or periodic error correction if available for your mount (it is usually present in more expensive units /  computer controlled units), not sure if star tracker such as SA has this capability.

- guide. This is primary reason why people guide their mounts - to eliminate this error

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@vlaiv pretty much hit this on the head. From the gear you've listed, I didn't see an auto guider. Without an auto guider, you are limited by how well you have polar aligned and the precision of the Star Adventurer (SA) mount. You mentioned that your polar alignment was out slightly which may explain the elongated stars. 

When I started out with my SA, I was using a focal length of 250mm. Without guiding, I would stick with exposures of 60s and I sometimes saw some star trailing. After I implemented an auto guider, I no longer had star trailing and it allowed me to use a longer exposure time. 

So I concur with Vlaiv, either reduce the exposure time until you don't see any star trailing, or see if you can introduce an auto guider for your setup. 

PS nice image of M31! 

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2 hours ago, Baldy45 said:

To find the Hour Angle of Polaris I use a Nautical Almanac, last night at 11pm (U.K.) the Hour Angle of Polaris was 4hrs 9mins.

I am not sure we are both talking the same units....At 11pm the hour angle is around 10pm on a 12hr scale.

Great image for only 60 x 90s

Edited by AstroMuni
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Thanks for the replies, this gives me something to think about.

Richard_ and vlaiv, I got the tracker in June and have been experimenting with different camera settings, this was the first time I have used an exposure length greater than 60 seconds.

So it looks like with my current set-up 60 secs is my limit.

AstroMuni, I think I know what you mean, here is how I work out the Hour Angle, this is for 11pm yesterday. Find attached how I workout the Hour Angle of Polaris.

Thank you for the kind comments, and will have another go tonight.

 

 

 

Hour Angle.pdf

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5 minutes ago, Baldy45 said:

Thanks for the replies, this gives me something to think about.

Richard_ and vlaiv, I got the tracker in June and have been experimenting with different camera settings, this was the first time I have used an exposure length greater than 60 seconds.

So it looks like with my current set-up 60 secs is my limit.

AstroMuni, I think I know what you mean, here is how I work out the Hour Angle, this is for 11pm yesterday. Find attached how I workout the Hour Angle of Polaris.

Thank you for the kind comments, and will have another go tonight.

Hour Angle.pdf 79.06 kB · 2 downloads

Good luck Baldy, this is an interative hobby :) You'll no doubt learn something new each time you set up!

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27 minutes ago, Baldy45 said:

AstroMuni, I think I know what you mean, here is how I work out the Hour Angle, this is for 11pm yesterday. Find attached how I workout the Hour Angle of Polaris.

Maybe try some of the free apps that tell you where you should place polaris in your polar scope - like this one:

https://apps.apple.com/us/app/polar-scope-align/id970157965

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