Rotating Stars

[robindurant]

Can anyonetell me what is causing this circuling, Im using a WO Z71 on a iOpton AZ Goto mount 3mins @iso 3200 , high I know but for testing. I have a Flattener fitted which is variable but have set it at 71. I have attached 2 images stretched to show the problem. the object was at appox 45 degrees altitude, low Moon well away from the Horsehead, the first image was with a Ha filter and the other without any. I know a alt az mount is not ideal for imaging but I have had reasonable results before

[dobblob]

Looks like a bad case of field rotation. This is an effect of using an ALT/AZ mount with long exposures. 3 minutes is far too long.

The star Alnitak is kept central in the field by the goto tracking but you can see the field rotating around it.

Typically most recommend not to go much above 15 sec exposures.

I would use shorter exposures and more of them.

 

[robindurant]

Thanks for replying. I know that AltAz mounts are prone to this rotation, however the stars are not elongated and using DeepSkyStacker this should handle this small amount of rotation that these 6 frames developed or maybe not

[tooth_dr]

I wonder if you dithered your exposures would this help reduce the circular noise. It looks like the walking noise you see with EQ mounts, but circular. 

[steppenwolf]

32 minutes ago, robindurant said:

The stars are not elongated and using DeepSkyStacker this should handle this small amount of rotation

Looking closely at the stars, they are in fact elongated in a radial pattern matching that of the rotating pattern that stands out most in the image. This pattern is caused by sensor noise that forms this shape after the stars have been ‘aligned’ when stacked.

This is field rotation caused by the altazimuth mount and a relatively long exposure.

[happy-kat]

Facing East or West at no more than 60 degrees attitude or lower is best to mini minimize field rotation but none the less 3 minutes is a very long exposure for an altaz mount which moves in tiny left right up down movements, so field rotation will show away from the centre though eventually even the centre will smear.

Your image has come out really well. Kappa sigma clipping in DSS can help tighten stars to some extent. There's an active No EQ challenge DSO imaging thread you might like.

[dobblob]

Dithering will help but that is a tough proposition with an ALT/AZ without some software which I don't think can connect easily to an I optron AZ goto via a laptop.

Others will know if this is possible.

I agree the noise looks like a typical result from a DSLR. Perhaps the high ISO of 3200 does not help. Try ISO 800.

An ALT/AZ when operating near the meridian and the celestial equator is at it's optimal position to minimise field rotation.

Fortunately Orion's Belt is perfectly positioned.

If the  ALT axis is at 90 degrees,( E_W) to the meridian (altitude swings due N-S in the rocker box) the need for large adjustments in ALT is reduced as the field rotates little as it passes across the meridian.

Edited January 4, 2020 by dobblob
altitude for azimuth

[robindurant]

Thanks chaps for your help. The Horsehead target was only 45 degrees and I have just processed some images taken last night using iso 1600 and 90 seconds again at the Horsehead and the images have the same rotation problem. I have tuned this mount and it does now produce good images at 90 seconds iso 1600 at even higher altitude than 45 degrees, the attached image is at those settings

[dobblob]

Excellent! Much better. Looks like a good candidate for the No EQ DSO challenge now.

[happy-kat]

The thread is like minded members sharing their images and questions, the title challenge is misleading and refers only too the challenges presented by the mount not being equatorial.

[michael8554]

Going back to the original images, the bottom left side of the coloured non-Ha image shows what is happening, the edges of the individual subs can be seen to be rotating about Alnitak, giving that circular noise pattern.

Michael

[robindurant]

9 hours ago, tooth_dr said:

I wonder if you dithered your exposures would this help reduce the circular noise. It looks like the walking noise you see with EQ mounts, but circular. 

Can you dither manually without software, say with your hanhset while exposing , if so how, thanks

[steppenwolf]

You *could* do this manually but it must be done in between exposures. Set the handset slew speed to minimum and just ‘brush’ the control pad direction keys in a random pattern.

What you are aiming for is a random position of the object that you are imaging with a minimal offset between each image.

[ollypenrice]

16 hours ago, robindurant said:

Thanks chaps for your help. The Horsehead target was only 45 degrees and I have just processed some images taken last night using iso 1600 and 90 seconds again at the Horsehead and the images have the same rotation problem. I have tuned this mount and it does now produce good images at 90 seconds iso 1600 at even higher altitude than 45 degrees, the attached image is at those settings

In this image I think you've black clipped it so much that none of the rotating noise of the first one would be visible anyway. Some field rotation remains visible in the stars, though. The only defence against field rotation with an alt-az mount is to shoot short subs. (Actually no, you do have the option of relocating to either the north or south pole... 😁) 

If you're feeling diligent and have Photoshop there is a simple software dodge for making stars round. Saved as an action it can work quickly but only on one star at once. Your action, saved to a function key, should record the following once the magic wand has been used to select a star:

-Select, modify, expand (try 5),

-Select, modify, feather (try 3). The chosen values depend on camera-scope specifics.

-Filter, blur, radial blur set to best quality and spin. Run the filter twice.

-Deselect. (Don't forget to record this.)

Once done you magic wand a star and hit the function key. Voila, round star.

Olly

 

[happy-kat]

That sounds a neat trick to try thanks Olly.

[ollypenrice]

1 hour ago, happy-kat said:

That sounds a neat trick to try thanks Olly.

If you want to optimize it you can save two versions with different expand-feather values for smaller stars and larger ones.

Olly

[robindurant]

15 hours ago, steppenwolf said:

You *could* do this manually but it must be done in between exposures. Set the handset slew speed to minimum and just ‘brush’ the control pad direction keys in a random pattern.

What you are aiming for is a random position of the object that you are imaging with a minimal offset between each image.

Many thanks Steve, I will try that

14 hours ago, ollypenrice said:

In this image I think you've black clipped it so much that none of the rotating noise of the first one would be visible anyway. Some field rotation remains visible in the stars, though. The only defence against field rotation with an alt-az mount is to shoot short subs. (Actually no, you do have the option of relocating to either the north or south pole... 😁) 

If you're feeling diligent and have Photoshop there is a simple software dodge for making stars round. Saved as an action it can work quickly but only on one star at once. Your action, saved to a function key, should record the following once the magic wand has been used to select a star:

-Select, modify, expand (try 5),

-Select, modify, feather (try 3). The chosen values depend on camera-scope specifics.

-Filter, blur, radial blur set to best quality and spin. Run the filter twice.

-Deselect. (Don't forget to record this.)

Once done you magic wand a star and hit the function key. Voila, round star.

Olly

 

Thanks Olly

 

[jgs001]

Robin, you can push to about 2 minutes, when the target is low in the east or west, but the higher it is, and the further North/South, the shorter the subs get to avoid the rotation. I don't recall the numbers now, but I bounced on this quite hard in my early days using a Nexstar SLT mount.

[carastro]

I presume you are not in a position to get an Equatorial mount.  I am afraid you are going to be stuck with these problems all the time you have an Alt/Az mount making your imaging life far more difficult than it need be.  

Carole 

[robindurant]

On 06/01/2020 at 09:59, carastro said:

I presume you are not in a position to get an Equatorial mount.  I am afraid you are going to be stuck with these problems all the time you have an Alt/Az mount making your imaging life far more difficult than it need be.  

Carole 

No Im in a situation where I can only veiw from SE - SW  but have produced reasonable images, this is new

[robindurant]

I have attached 2 images from last night using an H-Apha filter due to the Moon being up, however the target was well away from it and should according to the Filter specifacation not effect Ha imaging.

I reduced the exposure to 60 seconds as suggested by many and set iso at 3200 ( grainy but a good setting for accenting  the problem) 10 and 5 exposures, stacked them with Deepskystacker, part processed on PS .  As you will see it has made no difference whatsoever to the rotating marking or smearing effect

I have checked the optics in the Scope and the DSLR

Could this be a camera sensor fault or dodgy DSS software, although it stacks with no effort.

I have tried every settng on "Preference" on DSS inclusing the ones for Ha images, makes no difference

Im now at a loss as to what to try next

[happy-kat]

The effect is accentuated on the right hand side. Is that actually the top of the image when you took the images?

[robindurant]

42 minutes ago, happy-kat said:

The effect is accentuated on the right hand side. Is that actually the top of the image when you took the images?

The image has not been turned it is as you see

[happy-kat]

If you take an exposure on a static mount of say 5 seconds can you see the same elongation bias on one side, take the same again but no filter, do they compare in elongation bias. I guess aim true east or west 30• to give a control position.

How do you know it's not the optics?

Just thinking of what I'd do to see what was contributing.

[robindurant]

13 hours ago, happy-kat said:

If you take an exposure on a static mount of say 5 seconds can you see the same elongation bias on one side, take the same again but no filter, do they compare in elongation bias. I guess aim true east or west 30• to give a control position.

How do you know it's not the optics?

Just thinking of what I'd do to see what was contributing.

Hi single shots even at 30 secs are OK and Ive checked and cleaned all the optics