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How stable can you normally center a planet?

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#1 controlpower29

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Posted 28 September 2020 - 12:15 AM

I read quite a lot of articles about how to align a telescope. However, sometimes I still feel hard to center a planet with my Celestron 8SE.

Sorry that I cannot upload too long videos, so let me use the screenshots to describe the issue.

 

Example 1: Jupiter

We can see the Jupiter's position slowly moved to the lower right in 5 mins.

 

Example 2: Saturn

The movement of Saturn in this example is more complex. It moved to the right from 00:00:00 to 00:01:30, then to the top from 00:01:30 to 00:03:20, finally to the top right from 00:03:20 to 00:05:20.

 

The movement is slow, so that if the video is in a few minutes, I can bear with it. But if I make consecutive videos, for example, take 5 videos for Saturn with each 3 mins long, the planet could go out of the scope. Notice that the direction of the movement is really unpredictable, it could go anywhere and could make turns, the moving speed is also quite random. If I am lucky sometimes, the planet could stay without any movement for a few minutes. It could stay for quite a long time and then start to move as well.

 

Is this movement normal or did I make something wrong?

I read through this article and was aware of some alignment tips: https://stargazerslo...g-how-to-align/

  • I did OTA balancing instructed in the article
  • I am sure the tripod is leveled
  • I made the final movement up and right to reduce backlash
  • I tried 3-star and 2-star alignment with and without using planets
  • Power supply shouldn't be an issue since I connected to the outlet at home, adapter is 12v 5a
  • I am using city database and time should be accurate

I've racked my brain and cannot come up with another idea to make the telescope more stable. Could anyone help?

 

 



#2 Jinux

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Posted 28 September 2020 - 12:39 AM

This can be solved by simple thing, so called 'auto guiding' which is used for DSO photography.

Guiding quality doesn't matter at all, because only thing you need is centering the object while capturing short video.

I used 480mm refractor for guiding while I was taking video for Jupiter and Saturn with C11 at F10 and they stay right in the middle as supposed so. 

I believe any guide camera on any scope will do this job.



#3 Kevin Thurman

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Posted 28 September 2020 - 12:51 AM

Sounds like the mount simply has a lot of periodic error. It's probably worsened by having two axes of motion. Sounds like you'll need to guide somehow either manually or with a camera.



#4 ButterFly

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Posted 28 September 2020 - 01:21 AM

Firecapture has a built in autoguider.  No need for a separate camera.  Set up ASCOM on our computer and be able to control the scope.  Read through this post once your computer can control the scope.


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#5 sg6

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Posted 28 September 2020 - 03:11 AM

 would say it is just a function of the scope. The 8SE has long said to be at the top of the mount capacity and many say it is over the capabilities of the mount. The mount is decent, but not really high precision and there will be drift. Why do astronomers expect absolute precision? The 8SE itself with a long focal length will not help. Neither I suppose do the optics - you have 2 reflective surfaces and each will double any incidence angle errors. That likely makes things dance around a lot.

 

Doubt that Saturn nor Jupiter move at sidereal rate will make much difference nor that I think both are moving "backwards" at this time. But both are something to throw into the pot.

 

Guiding seems overkill for what you want to get out of the scope. Which I suppose only leaves the set up and balance and weighing. Reads that you have tried those aspects. How well ?

 

Does Up and Right help? I would have thought that although it might that it was more for the initial centering of an object, once the normal movement has started then backlash would be taken up. So you are really taking backlash movement out yourself for the start, not for 60 seconds in when the system will have done it.

 

Some will be atmosphere, as ever both planets are low, that may account for the movement - there is the odd fact that the sun drops fully below the horizon at about the time we see the lower edge touch the horizon. So light from either planets is already bent by the atmosphere by the time we aim a scope at it.

 

Other then check all aspects of set up - balance and having say tail slightly heavy, maybe the Up and Right - just doubt that Up+Right accounts for movement. How accurate is your data. Is Long+Lat accurate to the best minute?

 

How old is the scope? Wear and tear on the gearing and drive train. And as I have said they are assembled in a factory somewhere. By some person who is paid to do 200 a day of their bit then pass it to the next person.

 

Really will be a case of drop the motor and gear assembly in, tighten up, apply power, do they rotate, pass to next person.

 

Other then a bit more care and accuracy in setting up I cannot really think of a great deal you will manage to get out. Some may be trial and error. But I suspect the scope and mount are at/beyond their limits for the accuracy you want, and the rather poor altitude of the planets.

 

Have you tried a goto Altair, centering that and seeing how fast it drifts? If significantly less then likely the atmosphere and angle are your problem. Wonder how well the software handles the refraction of the light from low objects, they are basically not where they appear to be. That could I suppose be the big one. You see the planet from a higher apparent position then it is but the scope movement is based on the real position.



#6 gfstallin

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Posted 28 September 2020 - 03:14 AM

I've always found there is some movement, particularly when imaging at 5 or 6 meters in focal length (a barlowed C11, for example). Sometimes there is more (with my AVX), sometimes there is less (generally with the CGX). For me, it isn't bothersome if there is a little movement of the planet within the region of interest. I find the lack of drift during imaging leads to...mental drift. I need a little excitement at 3:00 am, something to ensure that I'm paying attention to what I'm doing. Something to keep an eye on - keeping the planet within the ROI - when I'm tired keeps me sharper than I might normally be. Of course, as others have mentioned, you can have FireCapture track the planet and I've done this before. It does work and you might find this handy. I like having the capability, but I've only employed it a few times. 

 

George


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#7 happylimpet

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Posted 28 September 2020 - 05:03 AM

Firecapture has a built in autoguider.  No need for a separate camera.  Set up ASCOM on our computer and be able to control the scope.  Read through this post once your computer can control the scope.

This.

 

No scope will be perfect at tracking (well, for big bucks). I use firecapture autoguiding using ST4 (for some reason the newest firecapture wont guide using ASCOM with my EQ8 mount, as previous versions did. Yours will probably be fine).

 

It works brilliantly. check it out.


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#8 ryanha

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Posted 28 September 2020 - 09:01 AM

+1 to FireCapture's autoguiding which works without an external guide camera. You connect your mount to FireCapture and it senses when the image is drifting out of center and gives small corrections to your mount to get it back to center (much like you would do manually with hand controller while looking at the live capture).

 

Please note that it is not as simple as turning it on.  It can be frustrating to learn how to get the settings right.  You have to make sure your camera is aligned and you have to tune FC so that it does the right amount of correction in the right direction and you also may have to set the backlash.

 

But, once you learn how to dial it in it will keep the planet in a small ROI indefinitely.

 

--Ryan


Edited by ryanha, 28 September 2020 - 09:03 AM.


#9 happylimpet

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Posted 28 September 2020 - 09:17 AM

+1 to FireCapture's autoguiding which works without an external guide camera. You connect your mount to FireCapture and it senses when the image is drifting out of center and gives small corrections to your mount to get it back to center (much like you would do manually with hand controller while looking at the live capture).

 

Please note that it is not as simple as turning it on.  It can be frustrating to learn how to get the settings right.  You have to make sure your camera is aligned and you have to tune FC so that it does the right amount of correction in the right direction and you also may have to set the backlash.

 

But, once you learn how to dial it in it will keep the planet in a small ROI indefinitely.

 

--Ryan

You dont really need to set backlash, it'll just issue corrections until its 'soaked up'. Its not as exact a science as autoguiding for DSO imaging as a small amount of drift is fine, and even desirable to dither out sensor variations.

 

The camera needs to be roughly aligned so east-west is left right, and then you just need to set a tolerance (i use around 10 pixels) and tick the boxes (or not) for ra and dec to ensure it corrects in the right direction. Took me less than 2 minutes to get it working.



#10 ryanha

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Posted 28 September 2020 - 10:29 AM

You dont really need to set backlash, it'll just issue corrections until its 'soaked up'. Its not as exact a science as autoguiding for DSO imaging as a small amount of drift is fine, and even desirable to dither out sensor variations.

Fair point that not all situations require this.  Depends on mount, FL, etc.

 

With small ROI and 2x Barlow I found that in some cases if I don't set backlash that the planet would leave the frame before the backlash was soaked up.  The other option would be to lower the value for how frequently adjustment commands are set, but I find that creates too much movement.  Plus when I have done that I find that it can sometimes oscillate if it over compensates then re-compensates. 

 

The meta-point is that you should not expect it to just work right away (though it may), it sometimes (often?) takes some fiddling with but once you get it right it should work for as long a capture session as you want.

 

--Ryan


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#11 Jkaiser3000

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Posted 28 September 2020 - 12:07 PM

The auto-align feature may be very helpful in this situation. You’d still need to use the hand controller from time to time to “recenter” the align box but the planet stays centered constantly. I normally forgo polar alignment altogether (GEM mount) and this feature is quite handy. I need to recenter the box constantly but at least it keeps me on my toes, or is it on my fingertips? lol.gif


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#12 GeorgeInDallas

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Posted 28 September 2020 - 12:32 PM

For  me, using the auto-align with the auto guiding keeps the plane rock solid in a small ROI after a little fiddling with the correction direction and correction amount settings. It eliminates constant corrections with the hand controller. That all works with a very rough polar alignment using my iPhone compass. One of FireCaptures best features.

 

George


Edited by GeorgeInDallas, 28 September 2020 - 12:35 PM.


#13 happylimpet

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Posted 02 October 2020 - 10:19 AM

I recommend using a small-ish ROI and a tight 'cut-out' to save on storage space and maximise frame rate.


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#14 controlpower29

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Posted 08 October 2020 - 01:44 AM

Thank you guys. I am trying to use FireCapture Autoguide feature. Posted another topic in https://www.cloudyni...oguide-feature/.



#15 dcaponeii

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Posted 08 October 2020 - 08:32 AM

I shot my Mars images the other night with an ROI of something like 500 x 400 (my notes are at home) and only needed to make one or two slight corrections during the 180s capture using Guide rate on my Autostar keypad.  Probably could have not needed any.  When I chose to not use the cut out feature in Firecapture to get the frame rate up to over 600fps then I needed to guide a bit more actively but it was still no trouble keeping Mars centered in the about 270 x 250 ROI.




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