PA with a dobsonian

Using SharpCap's Polar Alignment feature
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admin
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Re: PA with a dobsonian

Post by admin »

Hi,

glad to hear that the angle limit change helped.

It really doesn't matter whether the platform (or the mount if you were using a mount) is tracking - the rotation amount so close to the pole is tiny and will not affect the results.

cheers,

Robin
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Tiago Ferreira
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Re: PA with a dobsonian

Post by Tiago Ferreira »

admin wrote: Tue Jun 22, 2021 2:02 pm Hi,

glad to hear that the angle limit change helped.

It really doesn't matter whether the platform (or the mount if you were using a mount) is tracking - the rotation amount so close to the pole is tiny and will not affect the results.

cheers,

Robin
Should i point to polaris to start? Or near?
Youtube channel: https://www.youtube.com/channel/UCgR3Ha ... Vmw/videos

GSO 12" 1500 f/5 Dobson on Geoptik Eartha Eq Platform

ZWO ASI224MC + ZWO ASI294MC
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Re: PA with a dobsonian

Post by admin »

Somewhere within about 4-5 degrees of the pole. If polaris is in view it's fine, but not essential.

thanks,

Robin
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Tiago Ferreira
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Re: PA with a dobsonian

Post by Tiago Ferreira »

Well, finally the wind yesterday gave me a break and as the moon is so bright i took the opportunity to test again.
It works!!!
I can do it without problems but it's like adjusting an elephant position ahahah what a challenge.
To make each fine adjustment i have to lift all the set up (almost 50 kg/110 lbs) in the dark with 1 hand while adjust the equatorial platform legs with the other hand and keeping the bubble level precise which implies loosing fine precision. Crazy. But i have a plan and some ideas and i now will work on different techniques to help specially older people to do it without any effort. Patience and time will be the way to go. Also a dedicated fast technique for stressful / urban / young fellows that can't wait eheheh.
First i will play with this for a while because i suspect that for heavy set ups like mine, harder to fine tune, may be better to have a "fair" result on average rather to try reach a "good". I don't know yet. As said, it's like fine tuning an elephant. So i will test, test, test and test until i get a firm conclusion about the best methods and results.
In my case since i upgraded my equatorial platform from 3 to 5 legs a few days ago, i then had to realign it so i was already close with the drift method but was an opportunity to try sharpcap polar align feature. I was on "fair" already and after 1 or 2 "elephant tweaks" i kept on "fair" but a much better tuned "fair". I left it like that last night, as picture below.
The most important is that now we can do it. Finally a Dobsonian telescope can be polar aligned with sharpcap!!
Now i will keep working on this and then i will record a video and post here and everywhere, to allow dobsonian users to benefit from this feature in a smoothest way possible.
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Youtube channel: https://www.youtube.com/channel/UCgR3Ha ... Vmw/videos

GSO 12" 1500 f/5 Dobson on Geoptik Eartha Eq Platform

ZWO ASI224MC + ZWO ASI294MC
timh
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Re: PA with a dobsonian

Post by timh »

Hi,
Can you explain in very simple terms what it is that you have done here? I also have a Dobsonian mounted scope but I don't understand what it would mean to polar align such a set up other than by changing the mount so that it was turned into an Equatorial? Is it that you have tilted and clamped the base so that the Zenith position is now aligned with the pole. In my case - to achieve something similar - I removed the scope entirely from its Dobsonian mount, put tube rings and a dovetail on an remounted it on an Equatorial mount. Thanks..it seems intriguing.

I couldn't get the link to work?

TimH

PS Apologies for the question..I just found the Geoptik platform on google. How ingenious!
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Tiago Ferreira
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Re: PA with a dobsonian

Post by Tiago Ferreira »

timh wrote: Sun Jun 27, 2021 9:56 pm Hi,
Can you explain in very simple terms what it is that you have done here? I also have a Dobsonian mounted scope but I don't understand what it would mean to polar align such a set up other than by changing the mount so that it was turned into an Equatorial? Is it that you have tilted and clamped the base so that the Zenith position is now aligned with the pole. In my case - to achieve something similar - I removed the scope entirely from its Dobsonian mount, put tube rings and a dovetail on an remounted it on an Equatorial mount. Thanks..it seems intriguing.

I couldn't get the link to work?

TimH

PS Apologies for the question..I just found the Geoptik platform on google. How ingenious!
Hi Tim,

No, no, not transform in eq mount. It's polar align a dobsonian in an equatorial platform with sharpcap for the first time.
Don't worry that i will record a video with simple explanation how to do it but it will take a while because i want to keep testing it first and only after share the experience. But if you use an eq mount with a newtonian you have videos explaining that on youtube, just have to search. Mine is a different approach. Just subscribe this topic to stay tuned. Cheers!
Youtube channel: https://www.youtube.com/channel/UCgR3Ha ... Vmw/videos

GSO 12" 1500 f/5 Dobson on Geoptik Eartha Eq Platform

ZWO ASI224MC + ZWO ASI294MC
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Tiago Ferreira
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Re: PA with a dobsonian

Post by Tiago Ferreira »

Follow up:

I observe almost all nights and i keep making readings with this PA feature. I make at least 2 readings each session. Lots of readings so far.
At the beginning i was having every night results around FAIR / GOOD / EXCELLENT but since i've changed my 12" dobsonian weight (lighter now) i'm having trouble to have consistent results with the software although it keeps aligned as i can confirm by the images and drift patterns. I will need more time to figure what happened. Unfortunately i can't shout victory yet with this feature and my set up. The consistency fails only on declination readings and make no sense because it can give an error to the UP side and after one hour an opposite error to DOWN side without any latitude changes in the platform, of course. My bet is that when i reset the platform now, it changes altitude position because of the tube moving. I may try with the springs more tight to avoid the heavy tube to shake.
Youtube channel: https://www.youtube.com/channel/UCgR3Ha ... Vmw/videos

GSO 12" 1500 f/5 Dobson on Geoptik Eartha Eq Platform

ZWO ASI224MC + ZWO ASI294MC
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Re: PA with a dobsonian

Post by admin »

Hi,

the most common cause of variation in the PA readings is any flex between the camera/scope being used for the alignment and the 'RA Axis' - loose dangling cables are quite enough to affect the reading. Worth considering this as a cause.

cheers,

Robin
timh
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Re: PA with a dobsonian

Post by timh »

Just as a general and somewhat philosophical query does anyone ever get fully consistent PA alignment?

With my CEM70 equatorial mount set up I often start out with what appears to be near spot on PA according to the ipolar camera. However this perfect PA alignment only accurately applies to the RA setting I started with -- As I rotate the telescope and counterweight bar around the axis PA always appears to need adjusting again. My assumption is that the shift in weight (totaling tens of kgs) is always going to cause enough flexure to shift things and cause the axis to precess slightly ?

Then when I run PHD2 that often tells me that PA is further out than I imagined. In the end I have just taken the view that close enough is good enough and as long as the guiding and subs work in practice then all is well?

Tim
DanK
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Re: PA with a dobsonian

Post by DanK »

This is great Tiago and Robin! Exactly what I needed

Dan Kahraman
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