18 replies to this topic

[Simon Pepper]

Hi Guys 

 

Hopefully a quick one here. I have moved over to imaging at a greater focal length usually around 250mm to 1000mm. On the face of it the image seems fine but upon zooming in the stars are elongated quite considerably. This is all over the image all corners and centre so backspacing is not the issue here. What I am struggling to understand is why is this affecting the stars further away the bigger more prominent stars appear to be fine? Please see image at normal size and then zoomed at the centre. I am pretty sure this is guiding, but if someone can clarify please? I noticed that even after 10 seconds I see this trailing, but it doesn't appear to get that much worse all the way up to 180 seconds which is what these were shot at. Total RMS was roughly 1.2. Also I have found a way round this by shooting 5 seconds Lum images and then using Starnet and Pixel math, but this is not what I want to do... Thanks 

 

 

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[bobzeq25]

Probably.  It would be better if you provided all the detail you can.  Mount, optics...

[Simon Pepper]

Probably.  It would be better if you provided all the detail you can.  Mount, optics...

Hi Bobzeq25,

 

Apologies that may help. 

 

Explore Scientific 127mm FCD100, HEQ5 Pro, ASI1600mm, WO uniguide 120mm and ZWO 120mm mini.

 

Thanks 

[bobzeq25]

I'd suspect guiding.  That's a big long scope for that mount.

[OregonAstronomer]

Hello Simon,

 

I have had similar issues if I do a meridian flip and forget to adjust the counterweight to kepp the east side heavier. I'll see periodic swings in the RA graph in PHD2 while the Dec doesn't change. Does your guiding program provide a graph of guide star movement in RA and corrections being made? That data could potentially be quite useful.

 

Arnie

[David Boulanger]

A guide graph would be helpfull.  Oscillation in the RA axis?  Backlash or balance issue maybe.

[Simon Pepper]

Hi all

 

I use an AAP so I can grab the logs the RA is causing me some pain though it often jumps up and down. I thought this was a guiding issue but what was throwing me off here was the closer stars there were no issues, but I think you all have confirmed my suspicions. I have tried being a little east heavy, I am not sure how much heavier it should be but I can try going a little more. The RA is probably backlash, so I could try an perform the Rowan belt mod to see if that helps. The OTA is probably 18lbs and the limit is 24, so I am still well under here, but as Bobzeq25 has stated its quite a large scope for the mount it maybe time to upgrade!

[Simon Pepper]

See attached. I have never looked at this before so not sure I understand it...

Attached Files

•  PHD2_GuideLog_2022-01-16_173950.txt   354.39KB   7 downloads

[David Boulanger]

RA axis is over correcting it looks like.  Have you tried running the guide assistant with PHD2 and use its recomendations?

Edited by David Boulanger, 24 January 2022 - 05:48 PM.

[Simon Pepper]

hmmm I am not sure there is a guide assistant in the Asi Air Pro?

[David Boulanger]

PHD2 has the guiding assistant.  It's one of the tools in the program.

[Oort Cloud]

Hi Guys
What I am struggling to understand is why is this affecting the stars further away the bigger more prominent stars appear to be fine?

This is normal. Say your guiding error is 2 pixels...an error of 2 pixels will be more apparent on a star that is 2 px wide, where 2px error would be 100%, than on a star that is 50px wide, where 2px error would only be 4%

[jonnybravo0311]

PHD2 has the guiding assistant.  It's one of the tools in the program.

The ASIAir does not include a guiding assistant.

[David Boulanger]

That sucks.  It looks like his min move, aggression and something else was outside of normal values that I have in countered.

[jonnybravo0311]

Yeah, it's one of the limitations of the ASIAir. It uses a limited version of PHD under the covers.

[imtl]

You are under-mounted for this scope and focal length. Your RMS values are quite dominating your image scale. This looks like a combo of several things. Mount not up for the job, diff flexure, high rms, overcorrection, RA/DEC ratio is high etc.

[michael8554]

Hi Simon

 

From your GuideLog:

 

Calibration was okay with no error message.

 

Dec guiding was okay, Dec = 0.77arcsecs, very few corrections required due to good PA.

 

RA was RA = 1.10arcsecs, nearly 50% more than Dec = elongated stars.

 

Here's how RA looked, it was like this throughout the 2h 25min session:

 

 

 

RA Guiding is continuously being over-corrected, and sometimes takes 3 or more pulses to correct.

 

As this is ASIAir, I believe all you have available to adjust is the RA Aggression, currently Aggression = 75%.

 

Try lowering that in 10% steps, allow guiding to settle for a minute in-between each adjustment.

 

However, three pulses to correct may be a sign of stiffness (stiction) in RA

 

If after lowering Aggro RA still takes 3 or more corrections pulses to start RA moving, that would point to stiction in RA.

[Simon Pepper]

Hi all,

 

Thanks so much this is really useful information and I will try some of these suggestions. I think IMTL is correct its right on the limit of the HEQ5 Pro at my image scale. I think the first thing I will try is to add the Rowan belt mod and attempt a tuning with new bearings and some grease. If this still cant get the guiding < 1 total RMS I think its time for an upgrade to the EQ6r. Also thanks Oort Cloud for explaining why smaller further away starts are effected more than the closer larger ones when you think about it its actually quite obvious! Thanks all again here though really valuable info and always learning new stuff in this hobby! 

Edited by Simon Pepper, 28 January 2022 - 03:24 PM.

[KenS]

What you are seeing is the high frequency PE (13.6 seconds) caused by the gear teeth meshing. This can happen anywhere in the drive train so a belt drive may or may not help as it does not eliminate all the gear interfaces. The usual culprit is the motor end of things and can be helped by adjusting the motor position. If you remove the motor cover you will see the motor is held in its mounting bracket with screws in slotted holes that can be used to adjust the mesh with the first spur gear. And other screws that hold the mounting bracket in the mount body that can be used to adjust the mesh with the second spur gear.

Pic from Astrobabys guide at https://www.astro-ba...ld/heq5-we1.htm