Them pesky stars!

[Rustang]

I have a couple of images from recent imaging sessions, the big one is more of a 'how bright do you go' question, so how bright do you let you light frames go before its a wasted frame (loss of detail) !? This is only a 2min sub, low on the horizon, some moon shine which didnt help, Bortel 6 with the Optolong L-pro filter.

 

The cropped image is the star issues I'm getting. Its strange as they appear to do this across the frame but looking like they are falling down not spraying out diagonally towards the corners. You can see it in the bigger image. Mount is a HEQ5 pro, not guided (will be soon!),  pola alignment good and on concrete, 2 min subs. Its not happening on every frame either, is worse in some through out the session.

 

Edited September 2, 2020 by Rustang

[dark knight]

how good was the polar alignment? How did you polar align?

Edited September 2, 2020 by dark knight

[vlaiv]

7 minutes ago, Rustang said:

so how bright do you let you light frames go before its a wasted frame (loss of detail) !?

As long as you don't get saturation in signal from your target - you are good.

7 minutes ago, Rustang said:

Mount is a HEQ5 pro, not guided (will be soon!),  pola alignment good and on concrete, 2 min subs. Its not happening on every frame either, is worse in some through out the session.

Periodic error. Stock Heq5 pro can have around 30-40" P2P periodic error. Period is about 10 minutes, so in 5 minutes mount can travel 30-40 arc seconds. In two minutes that will be around 12"-16". What is your imaging resolution? If you are imaging at 2"/px - that will make 6px - 8px trails.

I'm reading about 9px trails in this particular sub. Of course, periodic error is not uniform - it is more sine like (if you are lucky, or very complex if you are unlucky), so some frames will have larger trailing and some smaller (if any).

There are several things that you can do:

- PEC training (EQMod might be required as I'm not sure stock HEQ5 supports this)

- Belt modding - this reduces P2P periodic error (to less than 10" or there about).

- Of course, guiding.

Just to test if this is indeed PE - look if elongation is oriented in RA direction.

[Rustang]

36 minutes ago, dark knight said:

how good was the polar alignment? How did you polar align?

Thanks for your response, I use the PolarFinder app and stick Polaris right where it shows on the outer circle.

Edited September 2, 2020 by Rustang

[Rustang]

32 minutes ago, vlaiv said:

As long as you don't get saturation in signal from your target - you are good.

Periodic error. Stock Heq5 pro can have around 30-40" P2P periodic error. Period is about 10 minutes, so in 5 minutes mount can travel 30-40 arc seconds. In two minutes that will be around 12"-16". What is your imaging resolution? If you are imaging at 2"/px - that will make 6px - 8px trails.

I'm reading about 9px trails in this particular sub. Of course, periodic error is not uniform - it is more sine like (if you are lucky, or very complex if you are unlucky), so some frames will have larger trailing and some smaller (if any).

There are several things that you can do:

- PEC training (EQMod might be required as I'm not sure stock HEQ5 supports this)

- Belt modding - this reduces P2P periodic error (to less than 10" or there about).

- Of course, guiding.

Just to test if this is indeed PE - look if elongation is oriented in RA direction.

Thanks for your response, Sorry to ask but can you clarify what you mean by 'saturation' in signal!?

 

How do you check the elongation is in the RA!? Its definitely random like you said, different across all images.

 

Is there something in the settings of PHD2 that can help with the issue then!? Nothing I can do (apart from belt mod) to the current set up to help!? I'm always east heavy!

 

I'm using an Astro modded Canon 600d 4.3 pixel size i believe.

Edited September 2, 2020 by Rustang

[vlaiv]

21 minutes ago, Rustang said:

Thanks for your response, Sorry to ask but can you clarify what you mean by 'saturation' in signal!?

Saturation is over exposure in regular photography - when some parts turn out white. In fact, you can have over exposure in astrophotography without signal going white - these will be border line cases - when one channel is saturated / over exposed and others are not (because they are not balanced in raw and one channel can be and usually is stronger than than others).

You can also recognize this as histogram clipping to the right, but this is severe case, you can have saturation in bright regions without histogram going all they way to the right.

For example this image:

core of M42 is very bright and easily saturated. It just turns white in post processing without any features.

24 minutes ago, Rustang said:

How do you check the elongation is in the RA!? Its definitely random like you said, different across all images.

You either need to be sure how you oriented your camera or you need to plate solve the image (astrometry.net for example).

If you have refractor and you place your camera "vertically" or "horizontally" when scope is at home position - looking at Polaris, then RA/DEC are aligned with the image - so RA is either horizontal or vertical. With Newtonian it's not that easy because of secondary mirror can be at any orientation and will add to rotation of FOV.

You can also check this when at the scope - if when slewing telescope in one axis (either RA or DEC) stars and objects move horizontally or vertically - you have aligned your camera and RA is either horizontal or vertical (which ever way things move when you slew in RA direction).

28 minutes ago, Rustang said:

Is there something in the settings of PHD2 that can help with the issue then!? Nothing I can do (apart from belt mod) to the current set up to help!? I'm always east heavy!

PHD2 will solve this if you are guiding, but I believe you said you are not guiding yet. Once you start guiding - both this and polar align error are fixed by guiding - that is the main point of guiding - to fix these two issues.

If you are still not guiding nor intend to belt mod your mount - you should try Periodic Error Correction.

I have HEQ5 mount as well, but have Syntrek version, not Synscan. My hand controller is simple one and does not support PEC (Periodic error correction), however I use my mount via EQMod and computer and EQMod supports PEC, so I use it like that. I'm not 100% sure, but you hand controller could support PEC - so you need to check your manual for PEC training section.

Let me see if I can google that for you.

https://inter-static.skywatcher.com/downloads/Synscan_V4_Hand_Control_Manual_SSHCV4-F-161208V1-EN.pdf

If your controller is V4 - then take a look at manual at page 42.

You'll need high power eyepiece - preferably one with cross hair and some time at your scope. Procedure is rather simple, just set scope to track and then issue corrections via hand controller for some time. After you'll have periodic error correction programmed.

Do be warned - you might need to park your scope after each session to retain this PEC information or you'll need to record it again next time (read the manual for details - I know this from my HEQ5 and EQMod and PEC there).

[Rustang]

I have the V4 Controller by the looks of it but I think I will continue to get my guiding set up and running as that sounds like a mission and Ive already been through enough get the guiding set up, I'm nearly there just needs a couple of clear nights

My camera was Horizontal in those above images if that helps.

I see what you mean by Saturation, I know it as blown highlights, a confusion of terms. I'm guessing I can push it a little futhur then.

Edited September 2, 2020 by Rustang

[vlaiv]

Just now, Rustang said:

My camera was Horizontal in those above images if that helps.

Then it is simple - RA direction is "up/down" and DEC direction is left right. Star elongation is in RA direction - definitively periodic error.

If you are already close to guiding - just go with it and it will sort these problems.

Another hint - since you are in light polluted area - don't shoot things near horizon. Shoot what is directly above you at zenith or at least high in the sky (like 60 degrees of altitude and above).

This is the darkest part of the sky and also there is atmosphere the least thick so it will block the least amount of light from target. This will give you best images. Leave targets that are low down at horizon - either for trips to dark locations or even better - trips to abroad. Shoot targets that you can from your location.

[Rustang]

Really appreciate all the info mate, it all helps, at least its something that can hopefully get sorted.

Its a shame to have to leave targets lower on the horizon, feels limiting, but trying to capture M33  which the above sub is of ,proves its getting washed out in that area. Ive always wanted to go on a weeks holiday somewhere nice and dark but how on earth do you predict the English weather!? 🤔😄

[vlaiv]

19 minutes ago, Rustang said:

Its a shame to have to leave targets lower on the horizon, feels limiting, but trying to capture M33  which the above sub is of ,proves its getting washed out in that area. Ive always wanted to go on a weeks holiday somewhere nice and dark but how on earth do you predict the English weather!? 🤔

M33 is going to be better placed later on.

I'm a bit more south at 45N but just for argument sake - at the moment M33 is at a 48 degrees altitude at midnight. If I wait another one and a half months before I go for it - it will be at 75 degrees altitude at midnight - that is only 15 degrees shy of zenith (24th of October).

Stellarium is your friend - take your regular imaging time - split it in two - this will be your marker (for example if you image between 10pm and 2am - it will be midnight, but it can be earlier, especially in winter when it gets dark really early, however - do bare in mind that light pollution changes with time as well - people turn off lights and shops close after some time in the evening).

Plan to image target that is crossing meridian very near your "marker" time. This will give you time when the target is highest in the sky. Of course, adjust for Moon cycle and weather - if you have window before or after - move your "marker" accordingly - no reason not to start at 9pm and finish by 1am of if you can stay up late - start at 11pm and finish at 3am - in either case, you get where this is going .

This can also be a guide for selecting targets - plan to image tonight? Again go to Stellarium and see what objects cross meridian around mid of your regular session - choose one of them that you like and then "adjust" session time so that target crosses meridian half way thru the session.

[Rustang]

👍