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I need a help with guiding: PHD2 @ Astroberry @ ATM fork mount


Vroobel
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Good evening,

I decided to create a separate thread from the one about my ATM fork mount. The mount is ready enough to begin astrophotography.

I have taken an opportunity of several clear nights to set up my guiding, but after a failure every time I have no choice and have to ask for help. My guider is based on the SW 8(9)x50 180mm finderscope and the ASI120MM mini camera attached to the finderscope with an adapter designed and printed by me. It's OK. Recently I used a dew heater designed by me for the Baader Zoom 8-24 Mark III - it matches perfectly. I decided to go towards the Astroberry and Kstars/Ekos. I tried to use the built-in guiding software, but finally I chose the PHD2. Both with the built-in guiding software and the PHD2 I have never obtained satisfaction. Look please on the charts from last night (on the bottom).

First of all I don't know whether the problem is caused by my lack of knowledge about the guiding or by the unknown fault of the OnStep @ MKS Gen-L v2.1 cooperating with the Astroberry. I have some bad experiences with the OnStep or the Ekos, but I hope that is another issue. Let's assume that everything is OK with the hardware/firmware. The Astroberry works on the Raspberry Pi 4 8GB RAM. A base of the mount is leveled properly. A wedge and fork are aimed excellently, originally using the SharpCap Pro, but recently using the modern Polar Alignment Tool from the Ekos. Both ways give an accuracy about several arc seconds in total. I know that people obtain arcminutes and the guiding can do its job. The wedge is stiff and the less than 20kg OTA with accesories are nothing for the mount as I hanged on it with my 70kg with no bigger stress, siriusly. I didn't weld the fork yet, it's aluminium, so I have to order the proper service. But the only bent I expect is towards its sides (left or right), which can be important, when the fork is rotated more than 60* left or right, never when the OTA is aimed near the Zenith. I found out that the OnStep may have problems driving the stepper motors when their wires are longer, so I changed the relevant parameter and compiled. Additionally I reduced the accuracy from 0.5 arcsec to 1 arcsec. I was advised to also consider a cone error, but still with no good result.

As you can see in the pictures, the guider escapes towards North West (Is that because I live in Merseyside? 😁) In the second picture is an amazing concentration of the marks in the top left quarter of the Target chart.

 

What did I try?

- leveling with two different spirit levels,

- changing algorithms, 

- changing an aggression around defaults,

- binning 1 and 2 with relevant values of the MiMos (I understand that if MiMo=0.2 for binning 1 than it's 0.1 for binning 2, but maybe I'm wrong),

- focusing the guider with the Bahtinov mask,

- attaching the Baader UHC-S 1.25" filter on the camera for eliminating an IR; I have no 1.25" UV/IR cut filter,

- Guiding Assistant and its suggestions,

- Drift Polar Alignment (I damaged the excellent alignment this way, so I had to fix it in the classic way),

- a Dec backlash analysis and attempts of use its results,

- guiding with and without a calibration,

- guiding using one and more stars,

- manual and automatic choice of star(s),

- and of course the exposure time between 0.5s and 6s.

 

The guiding floats are usually in range (-2",+2"), but sometimes reach much more, like 8". 

I found out that the guider should be screwed using a more stable bracket with two rings, but it's not heavy, it's not 80mm APO. The recent nights have been windless and I am surrounded by fences, shed, house and trees, so no wind is in my garden. 

Because of unexpected backlash in the Dec axis the OTA is not balanced, so it's bottom goes down itselfs. As I understand, the only problem with the backlash can be visible during the GoTo, not during the tracking when the Dec stepper motor shouldn't work apart the guiding.

Is it possible that the dev heater around the guider cell affects the air over the guider? I can try to guide without the heater tomorrow.

I can also try to connect the OnStep controller to my laptop instead of the RPi. 

Maybe the 180mm focal length is wrong?

I have a plan to make a kind of shelf which I will hang on the fork instead of the OTA. I will screw a spare finderscope shoe to use the guider as the only OTA on the fork. My idea is to check if the over 17kg load generates the problem somehow. BTW, I will use the shelf for another, smaller OTA or simply for my DSLR with a Sigma Zoom lens.

 

Is there anyone who experienced a similar problem and sorted it?

 

PHD2_1.jpg

PHD2_2.jpg

Edited by Vroobel
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Hi

To make a start, perhaps best to first...

...use the EKOS internal guider with SEP multistar, polar align, calibrate and then let it guide (anywhere in Cygnus would be fine) for at least 30 minutes. Take defaults throughout. Adjust nothing.

Post -Iinks to-  both the guide and kstars logs.

Cheers

Edited by alacant
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Need to see your GuideLogs, but I can see from your screenshots:

Dec Minimum Move is only correcting spikes bigger than about 2 arcsecs, much too high !

And the really large Dec excursion of 8 arcsecs is taking 12 x 6sec exposure = 24 seconds to correct.

That large excursion is the result of Dec Backlash, the 24 secs to correct could be due to too-low guiderate and/or Dec Stiction.

And corrections only every 6 secs doesn't help, try 2 second exposures.

But it all starts with the PHD2 Calibration, if that isn't good, guiding will suffer.

Seems to me you're focusing on the fine detail when your basics aren't right.

Read the Help and How To guides via the PHD2 Help menu, start here:

https://openphdguiding.org/phd2-best-practices/

Michael

 

 

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  • 2 weeks later...

Hi,

The night 7-8th September was great, I waited for so good weather!

As you advised, I began the guide with a 2 sec exposure. The misalignment wasn't too big, only 1', but I wanted to check how it behaves with different misalignment. Next time I will try 5' and 10' to be able to choose the best setting. I found guiding settings giving me spread of +/- 1", which is almost perfect in comparison to the previous results. As the chart is not a goal itself, I can present two single pics of M13 and M57, 60s and ISO400, taken by my Sony Nex-6. I forgot to attach my IDAS LPS-D2 filter, so the black is light gray. Taking into account that my mount drives a 10" Newtonian telescope with 1270mm focal length and the guiding isn't yet set up perfectly, I must say that the stars are very point. Not everywhere in the field of course, I should buy a coma corrector in future. Apart from "improving" the misalignment next time I'll try to use a Barlow lens (2x 2" ED) and swap settings of the camera with 4:3 ratio. Let's see what the future holds. :)

Thanks again for your support!

PHD2_3.jpg

PHD2_4.jpg

20210908_M13_1.jpg

20210908_M57_1.jpg

Edited by Vroobel
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That's more like it 😆

14 hours ago, Vroobel said:

The misalignment wasn't too big, only 1', but I wanted to check how it behaves with different misalignment. Next time I will try 5' and 10' to be able to choose the best setting

What misalignment are you referring to ?

Your guidecam focus could be better, aim for HFD of 3 to 4 on the Star Profile instead of over 5.

Michael

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11 hours ago, michael8554 said:

Your guidecam focus could be better, aim for HFD of 3 to 4 on the Star Profile instead of over 5.

Michael

I have read that the UV/IR cut filter may help, as the IR generates a halo around the star, so I ordered SvBONY 1.25" filter. Should be soon. 

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