Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Celestron AS-GT: Alignment & GoTo issues.

  • Please log in to reply
10 replies to this topic

#1 Steve06

Steve06

    Lift Off

  • -----
  • topic starter
  • Posts: 5
  • Joined: 28 Dec 2020

Posted 07 April 2021 - 10:29 AM

Hi All,

 

Hopefully this is the right area to try and get some help with the AS-GT mount as I'm having a lot of trouble with the GoTo function being rather inaccurate and tracking issues that show up when taking photos.

 

I guess the 2 main things I need to know are (a) am I polar aligning correctly and (b) what my expectations should be for accuracy with GoTo.  The mount has been factory reset, is running 4.21 firmware, the settings on the controller (date/time/location are all correct), the mount is positioned correctly (level, right latitude, etc.) and I've checked the polar scope itself is aligned as it rotates around a fixed point so I feel like the basics are there.

 

I'm not sure if I'm doing the polar alignment correctly though (no manual for the polar scope).  I have the Celestron scope as below and have screwed it in tight to the mount but I'm unclear as to whether it should be facing a certain direction while the mount is in index position.  From here I have no idea what to do; I've got an app (polar scope align pro) which shows me an image of the view through the polar scope and I believe I'm supposed to loosen the mounts right ascension lock and rotate it until the polar scope looks like it does in the app (or so Cassiopeia and the Big Dipper are correctly positioned as they appear in the sky), then adjust the knobs to move Polaris into the small circle.  If this is correct then how accurate does the scope position need to be in terms of referencing the sky as it seems rather difficult to align in the dark. As a side note there's dials both next to the polar scope and the counterweight bar which spin but are marked with what I assume are angles (0-90) and 0-23 but can't see that they do anything when spun?

 

Secondly when I have the mount set up and try to begin the 2 star alignment process the scope will slew to the general direction but require significant adjustment, I'm unsure if this is normal and part of the mounts calibration process but once I've done this and try to slew to an easily observable target like the moon, planets, Orion nebula the scope will get me in the general area but still often need a sizeable nudge to locate it which works alright on objects I can see but being in a high bortle area it's seriously limiting what I can image.

 

I feel like the issue is definitely user error rather than problems with the mount as it has on occasion been very accurate, I just can't replicate this frequently so any help with where I might be going wrong would be appreciated.

 

 

Polar scope being used:

post-230365-0-13771000-1545595735.jpg



#2 randcpoll

randcpoll

    Messenger

  • *****
  • Posts: 426
  • Joined: 20 Jul 2013
  • Loc: New York

Posted 07 April 2021 - 10:57 AM

There can often be significant error in the gotos when you are initially aligning the mount. First make sure your Polar scope is correctly aligned with the mount if you haven't done that. Set up the polar scope to look at a distant terrestrial point and rotate the scope in RA. You will probably see drift. You need to carefully tweak the three adjustment screws on the polar scope until you have very little or no image shift as you rotate the scope.

 

Then for each session you need to level the tripod and get the scope pointed fairly close to Polaris using the scope and your app. I don't worry about rotating the scope, I just put Polaris near the point of the circle that the app says it should be. 

 

Then to get fairly accurate go-tos you will need to go through the two star alignment and then add a third star to get a pretty good pointing model.


  • Steve06 likes this

#3 wrvond

wrvond

    Mercury-Atlas

  • *****
  • Posts: 2,758
  • Joined: 25 Sep 2014
  • Loc: West Virginia

Posted 07 April 2021 - 10:57 AM

Yes, you rotate right ascension (sans telescope and counterweights) until the Big Dipper, etc are generally aligned, then you use the latitude adjustment screws and the azimuth alignment screws to get Polaris in the small circle. By the way, the tripod leg directly under the PA scope should be pointing north already. Now you can return everything to the home position and install the weights and scope. 

It's not necessary, for visual use, to perform a polar alignment. Simply pointing the tripod leg north (with the mount aligned on that leg) and setting the latitude on the latitude adjustment scale will get you there.

 

Here's another method from the owner's manual:

Polar Align- The Advanced GT has a polar alignment function that will help you
polar align your telescope for increased tracking precision and astrophotography.
After performing an Auto Alignment, the telescope will slew to where Polaris should
be. By using the equatorial head to center Polaris in the eyepiece, the mount will then
be pointed towards the actual North Celestial Pole. Once Polar Align is complete,
you must re-align your telescope again using any of the alignment methods described
earlier. To polar align the mount in the Northern Hemisphere:
1. With the telescope set up and roughly positioned towards Polaris, align the
mount using the Auto Align or Auto Three Star method.
2. Select Polar Align from the Utilities menu and press Enter.
Based on your current alignment, the telescope will slew to where it thinks Polaris
should be. Use the equatorial head latitude and azimuth adjustments to place Polaris in the center of the eyepiece.
Do not use the direction buttons to position Polaris. Once Polaris is centered in the eyepiece press ENTER; the
polar axis should then be pointed towards the North Celestial Pole.

Alignment Procedures
In order for the telescope to accurately point to objects in the sky, it must first be aligned to three known positions
(stars) in the sky. With this information, the telescope can create a model of the sky, which it uses to locate any
object with known coordinates. There are many ways to align your telescope with the sky depending on what
information the user is able to provide: Auto Align allows the telescope to select three stars and uses the entered
time/location information to align the telescope; Auto Three Star Align involves the same process as Auto Align,
however it allows the user to select which star to use to align the telescope. Quick-Align will ask you to input all the
same information as you would for the Auto Align procedure. However, instead of slewing to the alignment stars
for centering and alignment, the telescope bypasses this step and simply models the sky based on the information
given. Finally, Last Alignment restores your last saved star alignment and switch position. Last Alignment also
serves as a good safeguard in case the telescope should lose power.


  • Steve06 likes this

#4 spereira

spereira

    Fly Me to the Moon

  • *****
  • Moderators
  • Posts: 5,430
  • Joined: 21 Apr 2017
  • Loc: Bedford, NH

Posted 07 April 2021 - 11:24 AM

Moving to Beginning Deep Sky Imaging.

 

smp


  • Steve06 likes this

#5 Chris Johnson

Chris Johnson

    Viking 1

  • *****
  • Posts: 502
  • Joined: 08 Sep 2004
  • Loc: Stockholm, NJ

Posted 07 April 2021 - 11:27 AM

If you are setting the mounts latitude using the scale on the mount,I suggest that you check it against a digital angle gauge. The factory scale is notoriously inaccurate. When making the final altitude and azimuth adjustments you use the appropriate bolts on the base of the mount. See page 12 of the instruction manual. 
 

If you are just using the scope visually I suggest you use the “QuickAlign” option at the bottom of the alignment list. That requires inputting time, location and date. Once that is done you will see Align Success” on the screen. You can then pick alignment stars, 2 in the east and 1 in the west. Then most objects will be in the field of a medium power eyepiece. The first star for alignment will be pretty far off, the second closer and the third very close if not In the medium power eyepiece. This is the Celestron alignment routine. My SE, ASGT & AVX mounts all have the same oddity, 1st star way off, 2nd closer & 3rd pretty much right on the mark. That is why I use a red dot not an optical finder as I find it easier for locating the 1st and second stars.When aligning, I use either a reticle eyepiece with a Barlow lens or a defocused star image at very high power to center the star in the eyepiece.


  • Steve06 likes this

#6 ecuador

ecuador

    Messenger

  • -----
  • Vendors
  • Posts: 425
  • Joined: 15 Aug 2014
  • Loc: Manchester, UK

Posted 07 April 2021 - 03:17 PM

If you want to use your polar scope, you first have to make sure it is aligned with the RA axis as noted above - a process usually done during the day, and then, you correctly surmised you are to rotate the RA to get the reticle oriented like it should, and the app actually gives you options of doing this precisely instead of just eyeballing if you'd like to go the extra mile. Both these procedures are described in the in-app help, if you go to the Settings screen and click on Reticle Instructions.


  • Steve06 likes this

#7 Steve06

Steve06

    Lift Off

  • -----
  • topic starter
  • Posts: 5
  • Joined: 28 Dec 2020

Posted 07 April 2021 - 06:17 PM

Thanks for all of the help, seems like I'm nearly there but haven't been putting it all together as one yet. 

 

The 3 star alignment being more accurate makes sense as thinking back the times I've had more luck were probably the same times I did a 3 star alignment. 



#8 Astrolamb

Astrolamb

    Vostok 1

  • -----
  • Posts: 129
  • Joined: 26 Feb 2020
  • Loc: Arlington, Texas

Posted 07 April 2021 - 07:34 PM

Since you are taking photos its obviously the biggest thing to get as close as possible to perfect, until you get into guiding.

 

Do what randcpoll suggested first and make sure your polar scope is accurate, its really easy to check its accuracy and it can be a pain to adjust but it's worth checking that first. 

 

Next do not use the bubble level on the mount. Its poorly glued in place and it is horribly inaccurate because of it. You can polar align without it being perfect, but the result is that any adjustment to azimuth will effect the altitude and vice versa. If you get a cheap level with 2 bubble levels that you can take apart you can place one on the mount head opposite of the built in bubble level and the other on the north side behind the counterweight bar on top of the bracket that holds the second altitude adjustment in place. Those were the best spots I found to get an accurate reading from the level.

 

After you do those two things your polar alignment from the polar scope should be within a few arcmins. If you want to do better than this you will need to use a software that can assist you in polar alignment.

 

For a rough estimate, depending on your camera and scope combination you will need to be within 10arcmins of the CNP in both the altitude and azimuth to achieve a 1 minute unguided sub. 

 

 

If you need any help with troubleshooting your ASGT feel free to ask me, I recently worked with mine a lot to get it perfect for my needs, so it's all really fresh in my head...

Goodluck and let us all know how it goes! 

 

BTW you should definitely consider platesolving, its the best thing I decided to do and I'll never go back to using the hand controller. 


Edited by Astrolamb, 07 April 2021 - 07:41 PM.


#9 wrvond

wrvond

    Mercury-Atlas

  • *****
  • Posts: 2,758
  • Joined: 25 Sep 2014
  • Loc: West Virginia

Posted 08 April 2021 - 04:42 AM

Hi All,

 

<snip>

  The mount has been factory reset, is running 4.21 firmware, the settings on the controller (date/time/location are all correct), the mount is positioned correctly (level, right latitude, etc.) and I've checked the polar scope itself is aligned as it rotates around a fixed point so I feel like the basics are there.

 

<snip>

Can you people not read?  fingertap.gif


  • Astrolamb likes this

#10 michael8554

michael8554

    Viking 1

  • -----
  • Posts: 691
  • Joined: 19 Aug 2020
  • Loc: Wiltshire UK

Posted 08 April 2021 - 09:34 AM

For visual use your described PA with the polar scope would be adequate.

 

And for guided imaging that might be good enough too.

 

But for unguided imaging, PA needs to be as close to perfect as possible, within a reasonable amount of time spent adjusting.

 

Drift Align with a high powered cross-hair eyepiece.

 

First Alignment star may well be a long way out.

 

Once that's centred and ENTERed, the mount's sky map will have been dragged into sync with reality, and further stars ought to be much closer.

 

If you're imaging only one target per night, carry out a One Star Alignment on a star close to your target, and then a GoTo the target should be in the FOV.



#11 Astrolamb

Astrolamb

    Vostok 1

  • -----
  • Posts: 129
  • Joined: 26 Feb 2020
  • Loc: Arlington, Texas

Posted 08 April 2021 - 09:56 AM

Can you people not read?  fingertap.gif

Apparently not! scratchhead2.gif

I've really gotta stop skimming through stuff lol


  • wrvond likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics