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Old 26-01-2021, 03:35 PM
Craig_
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Help with star shape?

Hi all,

I picked up an Esprit 120 recently and unfortunately it's been a bit of a frustrating time with it so far. I am really, really struggling to get the nice round stars I'm accustomed to on the Esprit 80.

Imaging on a 533MC Pro, my pixel scale is about 0.92" per pixel on the 120 so definitely a lot more challenging than on the Esprit 80 and that's fine. But I'm kind of at a loss as to what I need to do to improve matters, short of spending obscene money on a better mount. My mount is an EQ6-R Pro currently.

First light with the 120 was earlier this month and unfortunately most subs showed star elongation. I put this down to guiding quality as my guiding wasn't amazing that night and I was just using a small findershoe guide scope (which has served me fine on the Esprit 80, perhaps not on the 120.) I replaced this with a much better, more rigidly mounted guide scope but unfortunately my next session still showed elongation issues

One other variable aside from the guide rig that changed between sessions is that for the first session I used the regular Skywatcher field flattener that ships with the scope. The 2nd session I used the newish Skywatcher 0.77 reducer/flattener designed for the Esprit 120 (sold seperately.) Now I could be wrong (I'd appreciate any expert opinions here) but to my eye the elongation "pattern" is actually different from session 1 to session 2 - ie potentially, maybe, two different problems I am dealing with.

I'm assuming guiding is the driving factor behind the elongation with the standard field flattener, in part because the elongation seems to all point in the same direction across the entire frame, and in part because I did manage to capture a few (not many, but a few) subs that look just fine with this flattener. With the reducer/flattener though, the stars seem to point in different directions depending on where in the frame you look.

Here is a link to some .fit files showing the problem, as well as some guide logs. I've included some sample .fit files using both the standard flattener and the reducer/flattener, and have included a few that, to my eye, look OK as well.

I'd very much appreciate any opinions people have after looking at the files about whether I am dealing with two different issues here, and what the likely causes are? I've posted a similar thread elsewhere but also keen to hear from users here about what they reckon the issues could be? Right now the theory is guiding for the standard flattener, and perhaps backspace for the reducer/flattener. Overall, it has not been a fun step up to the 120 unfortunately, with very little usable data gathered and much frustration trying to figure this out.

Cheers
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  #2  
Old 26-01-2021, 04:29 PM
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Camelopardalis (Dunk)
Drifting from the pole

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Craig, just considering the stock flattener shot for a minute, there certainly looks like there is some drift going on.

The reason I say that is that it is all going the same direction across the field.

Tackle that one first, and it'll make it a lot easier to decipher any other spacing issues.
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Old 26-01-2021, 04:51 PM
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Quote:
Originally Posted by Camelopardalis View Post
Craig, just considering the stock flattener shot for a minute, there certainly looks like there is some drift going on.

The reason I say that is that it is all going the same direction across the field.

Tackle that one first, and it'll make it a lot easier to decipher any other spacing issues.
Thanks, yes I had noticed that in the stock flattener shots the stars do seem to go all in the same direction. (Whereas with the reducer this doesn't seem to be the case.) Is 'drift' a guiding issue or is there something else causing that?

Cheers
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Old 26-01-2021, 09:47 PM
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It doesn't have to be a guiding issue, but if the drift happens to be in RA then it's either ineffective guiding or incorrect tracking speed...at least, those would be the first things to check.
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Old 27-01-2021, 01:25 PM
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Originally Posted by Camelopardalis View Post
It doesn't have to be a guiding issue, but if the drift happens to be in RA then it's either ineffective guiding or incorrect tracking speed...at least, those would be the first things to check.
Thanks. If I've platesolved an image to check the direction of drift, how do I best interpret it to determine if it's RA or Dec?
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Old 27-01-2021, 06:40 PM
chooch65 (Amir)
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hi all i need your help
i have a saxon 127mm refractor and an asi1600mm cooled camera. i have purchased a saxon 0.7 reducer flatner. i can get focus with 55mm and 56mm but the stars on the outer edge are kind of stretched. i can NOT get the flatner to work for me. any advice will be much appreciate it
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Old 27-01-2021, 08:37 PM
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Quote:
Originally Posted by Craig_ View Post
Thanks. If I've platesolved an image to check the direction of drift, how do I best interpret it to determine if it's RA or Dec?
Might be worth checking the star chart of your choice...
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Old 28-01-2021, 10:46 AM
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Quote:
Originally Posted by Camelopardalis View Post
Might be worth checking the star chart of your choice...
Right, so validating my assumption here, it looks to me like it's drifting on RA?

Attached: Stellarium SS of Rosette on the night & time the sub was captured, the full sub and a zoomed in portion of it.

If I am reading it right the drift seems to be roughly along the East grid line from Stellarium, setting aside the fact that the camera sensor is rotated differently. Am I on the right track here?

Cheers
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Old 28-01-2021, 08:24 PM
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Yeah that’s what I’d say
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Old 29-01-2021, 10:10 AM
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Hi Craig,

I had a look at your flattener elongated stars image.

My first impression is its not bad at all. Its only subtle elongation. In a stacked image I doubt that would show through at the end.

But we all want nice round stars to the corners.

An 80mm scope with a camera with larger pixels could easily hide that small amount of elongation.

Do you have PEC enabled on your mount? How old is the PEC file? It may pay to do a fresh one as a new scope, heavier, may cause changes in the PE and PE can change over time as gears wear in more etc.

How old is the mount? What sort of RMS guide errors do you get?
Have you upgraded the mount? Some of these mounts have upgrade kits. Not sure about the 6R.

What settings are you using for your autoguiding? Exposure length, guide rate, aggressiveness, min/max move?

What software are you using for guiding? PHD2 is very good. Sky X is very good.

What was the seeing like?

Greg.
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Old 30-01-2021, 03:42 PM
Craig_
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Quote:
Originally Posted by Camelopardalis View Post
Yeah that’s what I’d say
Cheers!

Quote:
Originally Posted by gregbradley View Post
Hi Craig,

I had a look at your flattener elongated stars image.

My first impression is its not bad at all. Its only subtle elongation. In a stacked image I doubt that would show through at the end.

But we all want nice round stars to the corners.

An 80mm scope with a camera with larger pixels could easily hide that small amount of elongation.

Do you have PEC enabled on your mount? How old is the PEC file? It may pay to do a fresh one as a new scope, heavier, may cause changes in the PE and PE can change over time as gears wear in more etc.

How old is the mount? What sort of RMS guide errors do you get?
Have you upgraded the mount? Some of these mounts have upgrade kits. Not sure about the 6R.

What settings are you using for your autoguiding? Exposure length, guide rate, aggressiveness, min/max move?

What software are you using for guiding? PHD2 is very good. Sky X is very good.

What was the seeing like?

Greg.
Hi Greg,
Thanks for the reply.

Unfortunately my perfectionist nature when it came to landscape photography carried over into astrophotography as well - I can't deal with any visible elongation I must admit that in the one set of data I did stack from it, the elongation in the final stack was hard to see, but as long as it remains I will just have a nagging feeling that I need to fix it.

Re: PEC on mount - if I am honest, no idea. I will need to look at this next clear night. Soon I will be able to image with the EQ6-R + Esprit 80 simultaneously with another EQ6-R and Esprit 120 which should help my patience with fixing the issues I have on the 120, as I won't be wasting good imaging time on the 80.

Mount is only about 8 months old - no upgrades. I'm not aware of any upgrade kit for it but could be wrong (since it already uses a belt.)

Autoguiding - mostly default settings actually. Those have served me fine on the Esprit 80 but clearly not so fine on the 120, but unfortunately I am a bit lost as to what to adjust, when, and by how much. PHD2 is what I am using. Any good resources you know of on how to fine tune your guiding via settings? I mean the manual explains each setting but I guess it's the effect each setting has on other settings and such that leaves me a bit lost when stepping outside the default.

Seeing was pretty good that night from memory but I am no expert in judging it either.
Cheers for the help!
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Old 30-01-2021, 10:17 PM
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One thing we didn't talk about is your guider setup. Are you using an offaxis guider or a guidescope? Guidescopes are subject to differential flexure. Offaxis guiders are usually way superior.


Here are fairly common settings:

1. The better the mount and the better the polar alignment then you tend to use longer exposures on the guide camera and shorter exposures if all is not good. By longer I mean 6 seconds which is what I find works best on the better mounts. What works for yours is trial and error. I used to use 1 second with a Tak NJP mount.

2. Guide rate .5X. Some mounts have some backlash so you can put in say a 1 second delay between corrections to help with that. Some software lets you set the aggressiveness separately for the 2 axes, others don't.

3. Aggressiveness. Similar to the above. The better tracking the less aggressive it needs to be. Again a bit of trial and error and watch the guiding errors. But 6 is setting I use a lot. You can raise that if the seeing is good.

If the seeing is bad you don't want to be chasing the seeing so watch the corrections and if they go from - to + a lot then you are chasing the seeing and the mount is correcting some of the last correction instead of the seeing.
4. Min/Max: Min: something low like .1 Max 2 to 3. If your polar alignment is good and your tracking is good then you don't want a correction for an error of 3 - its probably PE or a wind gust so set it at 2 so larger "errors" are ignored.

5. I find with PEC the reported errors can get high for a little bit and then lower and that is the PEC kicking in and correcting the larger wave of errors the worm cycle is producing.

6. One of the first things I do if I get too large errors is select another star.
You don't want too bright a star. A nice semi bright star is ideal especially if some light cloud goes over.

7. Roland Christen from AP recommends calibrating your autoguider near where the celestial pole and the meridian intersect.

8. AP has some other informative things about tracking. Like don't have all your counterweights at the bottom of the shaft. Its better to have more weight and have it near the top of the shaft and one weight down near the end of the shaft to get balance as there is less resistance to movement.

Ideal balance is not perfectly balanced but slightly biased to mesh the gears in east/west, I think its to bias towards the west and similarly bias the weights towards the camera. RA and Dec autoguide differently.

There is an article about these points on the AP website I think its under "technical".

Pulse guiding through Ascom is supposed to be superior to ST 4 relay guiding.

I use both an SBIG STi guider and an ASI290 which is very clean and very very sensitive so getting a guide star is easier.

Greg.



Quote:
Originally Posted by Craig_ View Post
Cheers!



Hi Greg,
Thanks for the reply.

Unfortunately my perfectionist nature when it came to landscape photography carried over into astrophotography as well - I can't deal with any visible elongation I must admit that in the one set of data I did stack from it, the elongation in the final stack was hard to see, but as long as it remains I will just have a nagging feeling that I need to fix it.

Re: PEC on mount - if I am honest, no idea. I will need to look at this next clear night. Soon I will be able to image with the EQ6-R + Esprit 80 simultaneously with another EQ6-R and Esprit 120 which should help my patience with fixing the issues I have on the 120, as I won't be wasting good imaging time on the 80.

Mount is only about 8 months old - no upgrades. I'm not aware of any upgrade kit for it but could be wrong (since it already uses a belt.)

Autoguiding - mostly default settings actually. Those have served me fine on the Esprit 80 but clearly not so fine on the 120, but unfortunately I am a bit lost as to what to adjust, when, and by how much. PHD2 is what I am using. Any good resources you know of on how to fine tune your guiding via settings? I mean the manual explains each setting but I guess it's the effect each setting has on other settings and such that leaves me a bit lost when stepping outside the default.

Seeing was pretty good that night from memory but I am no expert in judging it either.
Cheers for the help!
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Old 31-01-2021, 10:39 AM
Craig_
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Quote:
Originally Posted by gregbradley View Post
One thing we didn't talk about is your guider setup. Are you using an offaxis guider or a guidescope? Guidescopes are subject to differential flexure. Offaxis guiders are usually way superior.


Here are fairly common settings:

1. The better the mount and the better the polar alignment then you tend to use longer exposures on the guide camera and shorter exposures if all is not good. By longer I mean 6 seconds which is what I find works best on the better mounts. What works for yours is trial and error. I used to use 1 second with a Tak NJP mount.

2. Guide rate .5X. Some mounts have some backlash so you can put in say a 1 second delay between corrections to help with that. Some software lets you set the aggressiveness separately for the 2 axes, others don't.

3. Aggressiveness. Similar to the above. The better tracking the less aggressive it needs to be. Again a bit of trial and error and watch the guiding errors. But 6 is setting I use a lot. You can raise that if the seeing is good.

If the seeing is bad you don't want to be chasing the seeing so watch the corrections and if they go from - to + a lot then you are chasing the seeing and the mount is correcting some of the last correction instead of the seeing.
4. Min/Max: Min: something low like .1 Max 2 to 3. If your polar alignment is good and your tracking is good then you don't want a correction for an error of 3 - its probably PE or a wind gust so set it at 2 so larger "errors" are ignored.

5. I find with PEC the reported errors can get high for a little bit and then lower and that is the PEC kicking in and correcting the larger wave of errors the worm cycle is producing.

6. One of the first things I do if I get too large errors is select another star.
You don't want too bright a star. A nice semi bright star is ideal especially if some light cloud goes over.

7. Roland Christen from AP recommends calibrating your autoguider near where the celestial pole and the meridian intersect.

8. AP has some other informative things about tracking. Like don't have all your counterweights at the bottom of the shaft. Its better to have more weight and have it near the top of the shaft and one weight down near the end of the shaft to get balance as there is less resistance to movement.

Ideal balance is not perfectly balanced but slightly biased to mesh the gears in east/west, I think its to bias towards the west and similarly bias the weights towards the camera. RA and Dec autoguide differently.

There is an article about these points on the AP website I think its under "technical".

Pulse guiding through Ascom is supposed to be superior to ST 4 relay guiding.

I use both an SBIG STi guider and an ASI290 which is very clean and very very sensitive so getting a guide star is easier.

Greg.
Thanks for the tips - greatly appreciated. Definitely some things for me to try next clear night! One point I did note, around the optimal location to calibrate the guider. I had heard similar and so tried doing this one day, but got awful results compared to just calibrating in the part of the sky I am imaging. Maybe I did something wrong? Will have to try again when the clouds part.
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