Jump to content

Stargazers Lounge Uses Cookies

Like most websites, SGL uses cookies in order to deliver a secure, personalised service, to provide social media functions and to analyse our traffic. Continued use of SGL indicates your acceptance of our cookie policy.

sgl_imaging_challenge_banner_android_vs_ios_winners.thumb.jpg.803608cf7eedd5cfb31eedc3e3f357e9.jpg

Sign in to follow this  
Roysten

Polar alignment with Sharpcap tool

Recommended Posts

Over the last few months I've been trying my hand at imaging DSOs, in learning to do this I've found that you really can't get away with exposure times longer than a few seconds without motors and a solid polar alignment. Initially I tried this by eye using Polaris, the best I achieved was around 8 seconds of exposure before the inevitable drift of whatever is in my view. I thought about buying a polar scope or learning the drift technique, then I happened upon the polar alignment facility in Sharpcap 2.9 on this forum and gave it ago. Jut thought I'd write a few things about how that's gone for me.

Firstly I tried using the software with my 150P with a field of view of <1/2º, unsurprisingly this didn't work; I just couldn't see enough in the field of view for the software to lock onto anything. I calculated the field of view of my 70mm Travelscope with my cam and still found it to be shy of the 1º requirement. So I bought myself a cheap 0.5× reducer to achieve ~1.8º across field of view.

As most people have found the weather has been shocking with weeks of cloud at a time. Since I got the reducer at the beginning of Feb I've only been able to get out three times. It's only on the third time late last week I was able to finally achieve a polar alignment using the software! It took me over an hour; the light pollution in my subburban part of Hull is pretty bad and the software was rarely able to consistently detect the same dozen stars with every refresh of the image. I had to have my gain quite high with an exposure time of 5 seconds to get much out of my images. I was getting quite a bit of noise in the images but increasing the minimum star-size in Sharpcap from 1 to 2 pixels seemed to help. Quite often I'd get a lock for the first RA angle but not on the second, which was very frustrating. The times I got a lock from both angles were then hampered by the lack of the yellow line, which dictates which way to move the Alt-az and altitude on my mount, obviously in those instances I was very far away from the NCP. It isn't an overstatement when I say I was over the moon once it finally locked onto the correct point!

I achieved a PA less than 1" from the NCP, I have seen people who have achieved much better than that but this has been an order of magnitude better than anything I've done previously, where I was lucky to get within half a degree.

For those who struggle with PA I can definitely recommend the Sharpcap feature, it is fairly intuitive, easy to use and quick.

To show the difference it can make I've attached an image of everyone's favourite - M42. Multiple other problems with the image aside I'm pleased with the improvement of the image with good PA compared to the one by eye. Both images were taken as 30 second exposures on a motorised mount.

I hope this is helpful for those who have struggled like me and I'd be interested in hearing other people's experiences with PA.

Polar alignment comparison.jpg

  • Like 2

Share this post


Link to post
Share on other sites

Hey Roysten awesome picture of M42 well done :)

I am days away from having my first t-Ring and adapter and will be setting up for taking my first images. Is there no way of getting an accurate PA without using software? It seems every time I log into here to read the latest, there is always a new thing to learn.

  • Like 1

Share this post


Link to post
Share on other sites
2 hours ago, hornedreaper33 said:

Hey Roysten awesome picture of M42 well done :)

I am days away from having my first t-Ring and adapter and will be setting up for taking my first images. Is there no way of getting an accurate PA without using software? It seems every time I log into here to read the latest, there is always a new thing to learn.

Drift alignment as mentioned, is a possibility, as all it really needs is a camera capable of doing exposures of a few minutes. And it can be really accurate depending on how much effort you wanna put into it, and it takes software calculations (All Star Polar Allignment for example) out of the equation.

And to the original post, it sounds like an interesting feature. Are you just using your main imaging camera, or how do you go about it?

Share this post


Link to post
Share on other sites

Hi Roysten,

I tried using sharpcap to polar align from my city apartment, but the Light Pollution is simply too much and hardly any stars are visible in the region. What are the settings you use for PA with Sharpcap?

Share this post


Link to post
Share on other sites

Hey @hornedreaper33, that a Dungeon Keeper reference there? I think a lot of people generally use a polar scope, which you can pick up for ~£30 depending on your mount, FLO and 365Astronomy both sell one for my EQ3 mount. I've heard good things about Polar-Master's but they look very expensive! Before using Sharpcap I was just using my finderscope and aiming about 1º away from Polaris in the direction of Dubhe and Merak in the Plough, admittedly very crude!

@The-MathMog, I used my ASI120MC with a 0.5× reducer attached to my Travelscope 70 to get the alignment then switched to my 150P and DSLR to capture the main images. Probably be better if I could mount my smaller scope ontop of my 150P but I don't really have all the necessary attachments.

Hi @astrosathya, the light pollution is made near me too! To get many stars at all I was having to use an exposure time of 5 seconds and gain at near 100% to get a lock. Seeing was pretty fair, better than the other time I tried it. Will be interesting to get a little into the country and see if it's any easier.

Share this post


Link to post
Share on other sites
3 hours ago, Roysten said:

Hey @hornedreaper33, that a Dungeon Keeper reference there? I think a lot of people generally use a polar scope, which you can pick up for ~£30 depending on your mount, FLO and 365Astronomy both sell one for my EQ3 mount. I've heard good things about Polar-Master's but they look very expensive! Before using Sharpcap I was just using my finderscope and aiming about 1º away from Polaris in the direction of Dubhe and Merak in the Plough, admittedly very crude!

@Roysten you got it. I loved that game and when I first got the internet in the late 90's it was the first thing that came to mind.

I was reading the follwing link:

https://www.cloudynights.com/articles/cat/articles/darv-drift-alignment-by-robert-vice-r2760

Once I finally got my head around the process I think I should be able to align somewhere near. Good luck with your imaging.

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Sign in to follow this  

  • Recently Browsing   0 members

    No registered users viewing this page.

  • Similar Content

    • By MarsG76
      The Orion Nebula imaged in RGB through a Celestron 8" SCT at F10 (2032mm FL) using a full spectrum modded and cooled Canon 40D. Tracked using a Celestron CGEM mount. Total exposure time was 1 hour and 24 minutes.
    • By MarsG76
      Hello All,
      I was wondering whether it's possible to image a DSO and capture any depth. Every 3D astro image online is faked so at the start of the year, I decided to image M42 six months apart.
      Back in March I posted a image of M42 imaged at f10, 2032mm FL through my 8SE on 28th February 2019. Than on 3rd September (setup and captured 15 second subs on 1 September) I captured M42 at the same focal length, same orientation and very similar subs for a total exposure of 1 hr 24 minutes. This was almost to the day exactly 6 months between the two images, so the earth was 300 million km away from the original position on the other side of the sun, furthest I could hope for imaging a 3D stereo pair.
      First attached is the image from September...

       
      I color matched the above image with the image from February, aligned them and below is the end result....

      As you can see there is no detectable 3D effect... There was a 3Dish effect but this was most likely due to the differences in processing of the two stacks and when I SCALE and rotate the two images to align them, and hence no 3D effect.
      Of course the stars and nebula are certainly not on a flat plain so I believe that the reason for the lack of any discernable depth is simply due to the distance of M42 resulting in  a very small angular shift in the stars, so small in fact, that it’s beyond the sensitivity of my 8” SCT, camera pixel resolution and tracking accuracy of the CGEM.
      Calculation of the expected motion of any parallax shift when the Orion Nebula is 1344 lightyears away and the distance of Earth being 149,600,000km from the Sun:
      1344LY = 1.2715e+16km
      Θ° = Tan-1(149.6e+6/1.2715e+16)
      Parallax Shift Θ” = 2 x 3600 x Θ
      Parallax Shift Θ” = 0.0048536712567150
      An angular motion of 0.005” was not picked up by my system that tracks with an average accuracy of about 1” RMS, with a camera sensor that has a resolution of 1.16”/pixel at 2032mm focal length with a 8” SCT. Even if I could get consistent tracking at the best accuracy that I have ever seen with my gear, 0.38” RMS, this is still well above 0.005” and well beyond the 40D sensor pixel resolution, and all this is without considering atmospheric distortion, obviously my setup is not even close to sensitive enough.
      This was a good project but unfortunately the distances of objects in the universe are too great, even objects classed as in our celestial “backyard”. If I didn’t try this experiment than I would be always wondering and curiosity would most likely make me try it eventually.
       
      Clear Skies,
      MG
       
       
       
    • By antariksha
      Abstract :
      The topic of Polar Alignment is not at all new. Lot of approaches, automation tools are available. Yet, some aspects in all the current approaches drove me towards doing some more work. The key aspects of this approach are as follows.
      Ability to do the Polar alignment without polaris sited Relatively less complexity than drift alignment Ability to address to a good extent the atmospheric refraction to finally locate correct NCP / SCP position A good starting point for amateurs who wish to graduate towards sophisticated tools and techniques Ability to quickly verify if the polar alignment is intact after one object photographed or viewed, and the equipment is being pointed to another object. This point is mentioned in light of the fact that sometimes the polar alignment gets disturbed and the next object photographed shows star trails. This is especially true if payload is tweaked for next photo imaging. What is required?
      One should have a good understanding of the sky and ability to identify stars upto Mag 4.5 using star maps and basic concepts of RA and Dec. One should have Equatorial mount with ability to fine tune Azimuth and Alt adjustments. Availability of cross hair eye piece for the ability to locate the star exactly at the cross hair point. It is good to have finder scope attached and the finder cross hair is aligned with the main telescope eye piece cross hair. Please note this technique is not for the GoTo mounts which many times have Alt-Az mounts fitted with tracking motors. The GoTo alignment is done using 3 Star method.
      However, there are a few mounts which are equatorial design and also have GoTo tracking capabilities with RA and Dec motors. For these mounts, it is preferred to carry out polar alignment. The only point about these mounts, is that GoTo should have ability to start the RA motor ( tracking) without doing the 3 Star alignment, in other words, bypassing the steps for 3 Star alignment.
      The technique is based on the mathematics around the stellar current positions precisely computed. The technique suggests NCP or SCP alignment using specific pointing stars.
      Method
      The technique relies upon pairs of stars identified such that pair has same RA or same Dec. The details about finding such pairs, are given in the next section ( Mathematics).
      Step 1
      Try to do a very coarse polar alignment using your latitude and pointing the equatorial axis approximately towards a possible Polaris direction. This is only to reduce the iterations in the method below. There is no dependency to visually site Polaris. Step 2
      Select the pair of stars of the same RA from the table 1 below. Now, while choosing the pair, please select that pair which is closest to the zenith. This will reduce the error due to atmospheric refraction of siting those stars. Choosing such pair, will get better alignment. Note that the NCP and SCP lie on the same RA contour of the pair, you have just chosen. Locate the first star of the pair in the cross hair eye piece. Loosen the DEC knob of your Eq mount. Let the RA axis not to be loosened. Further, please start the RA motor and hence the tracking. In case of GoTo capability, please ensure the tracking is On, while the 3 Star alignment being bypassed. Rotate the telescope around DEC axis such that the second star of the pair is in the Cross hair eye piece. In the first attempt, the second star almost certainly will not be at the center of cross hair eye piece. And you need corrections. At this point, take the help of the finder with its wider field of view. Identify the position of the second star whether it is below or above the finder cross hair. Adjust the Azimuth of the mount through the coarse or fine depending on how off the second star has been. o   Tip : In case, someone is facing difficulty in finding which direction to move Azimuth of the mount for correction, the following tips may be of use. A simple way to determine is to locate where the second star lies with respect to cross hair. Assume it is on the lower side of cross hair. Then the correction in the Azimuth of the mount should be such that the star is moved upward. It may be noted that your finder can be either inverting or non-inverting. Now, to determine the movement, please hold the finger on the lower side in front of the primary of the finder. And slowly lift the finger towards the center of the primary to obstruct it and continue moving upward. While doing so, please observe from the eyepiece. The blackish ghost image of finger will be seen moving. If movement is lower to upward, the optics is non-inverting. If ghost image moves from up to down, it is inverting. With this small trick, you would know how to apply correction. Once the correction is done, please point the finder to the first and then second star alternately simply by rotating around Dec axis of the mount. Both stars will be seen at the cross hair. At this point, coarse polar alignment is done. Now, please use the main telescope cross hair to locate the first and then second star using Dec axis movement. If required, please carry out the necessary Azimuth correction. Again, please use the above small trick to find out more on how to apply correction. At this point, please note that at the telescope’s high power ( with cross hair eyepiece), the Dec axis is correctly tracing two stars in your pair. Note that NCP/SCP lie on the same Dec axis. The Azimuth alignment of NCP/SCP is achieved. No more touching of azimuth knob of your Equatorial mount now. Step 3
      Site the pair of stars of the same Dec from the table 2 below. Now, while choosing the pair, please identify roughly the midpoint of them. Now, select that pair whose midpoint is relatively closest to the Zenith. With this, one star is relatively East ward and other one almost at a same distance but Westward. This will reduce the error due to atmospheric refraction of siting those stars. Choosing such pair, will get better alignment In case you are unable to select a pair, please read Step 4. Note that the NCP and SCP lie on the centre of the Dec circle which the above pair inscribes. Locate the first star in the cross hair of finder. To locate the second star, please lock Dec axis. But loosen the Eq axis and rotate the telescope around Eq axis. Please carry out Alt adjustments of the mount. Please use similar procedure and tricks as in the step 2. Once the two stars are in the cross hair positions of the telescope, the polar alignment is completed. Step 4 ( only if you could not carry out Step 3)
      Site the pair of stars of the same RA from the table 1 below. Now, while choosing the pair, please select another pair which is off zenith. Please try to select such pair which has both stars approx same elevation from horizon, so that their atmospheric refraction is almost same. Effectively, we cancel the atmospheric refraction influence. Please note that in step 2, NCP/SCP is located to be on one of the RA lines. Now, we use another RA line with this newly selected pair. Again, for these stars to be centred, please keep Eq axis fixed and only move Dec axis ( similar to step 2). However this time, the mount corrections to be done are using Alt adjustments. Once the two stars are in the cross hair positions of the telescope, the polar alignment is completed. Mathematics
       
      The starting point was the star catalog where the Epoch 2000 is taken as baseline. Then I selected the stars brighter than mag 4.5. I applied the corrections due to Earth Precession and also the individual star’s proper motion. With the base data was ready for today's’ star positions. Then I programmatically picked up all pairs for same RA (within 0.001 difference) and later all pairs with same Dec (within 0.001 difference).
      I found mag 4.5 to be heuristically optimal. This magnitude is sufficient for visual locating these stars. Also, the number stars shortlisted from the main catalog is good enough to give sufficient number of required pairs.
      The pairs located today may not be valid after say couple of years due to Earth Precession and stellar proper motion. The below two tables will need fresh computation then.
      Disclaimer: I have tried few of the above mathematically found pairs from my location 19 Lat 73 Log. I use Bresser ExOS 2 mount. After the polar alignment, the tracking was tested for 10 min which was adequate for my current level of astrophotography.
      At different altitudes, different latitudes, this is not tested. I believe, the method will definitely work for small exposures. It is to be validated if this method works for very long exposures.
      Star Pairs
      Table 1 : Star pairs with same RA ( useful for Step 2 and 4)
       
      Sr No
      First star  (name)
      First star HD Id
      Second star ( name)
      Second star HD Id
      1
      Gam Cas
      5394
      37 And
      5448
      2
      Nu Per
      23230
      19 Tau
      23338
      3
      Ups Tau
      28024
      71 Tau
      28052
      4
      90 Tau
      29388
      53 Eri
      29503
      5
      Kap Lep
      33949
      Rho Ori
      33856
      6
      The Aur
      40312
      Del Aur
      40035
      7
      Gam Mon
      43232
      Eta Gem
      42995
      8
      Eps Gem
      48329
      30 Gem
      48433
      9
      13 CMa
      50013
      V0415 Car
      50337
      10
      Omi CMa
      50877
      The CMa
      50778
      11
      P Pup
      63922
      Xi Pup
      63700
      12
      Chi Car
      65575
      11 Pup
      65228
      13
      Del Hyd
      73262
      E Vel
      73634
      14
      B Vel
      74180
      V343 Car
      74375
      15
      Iot Cnc
      74739
      Eps Hyd
      74874
      16
      31 Leo
      87837
      Alp Sex
      87887
      17
      Pi Cen
      98718
      Sig Leo
      98664
      18
      Lam Mus
      102249
      Nu Vir
      102212
      19
      Alp Crv
      105452
      Del Cen
      105435
      20
      Gam Cen
      110304
      Gam Vir
      110380
      21
      5 Boo
      120477
      2 Cen
      120323
      22
      SHT 56
      129116
      Alp Lup
      129056
      23
      Del Her
      156164
      Pi Her
      156283
      24
      102 Her
      166182
      Pi Pav
      165040
      25
      110 Her
      173667
      Phi Sgr
      173300
      26
      Zet Cap
      204075
      Gam Pav
      203608
      27
      Del Gru
      213009
      Del Cep
      213306
      28
      Iot Cep
      216228
      Mu Peg
      216131
      29
      Bet Peg
      217906
      Bet Psc
      217891
       
      Table 2 : Star pairs with same Dec ( useful for Step 3)
       
      Sr No
      First star  (name)
      First star HD
      Second star ( name)
      Second star HD
      1
      7 Cam
      31278
      Gam UMaj
      103287
      2
      Iot Cyg
      184006
      The Boo
      126660
      3
      H Persi
      26630
      Dmi?? Cass
      4180
      4
      Pi Aur
      40239
      Iot Her
      160762
      5
      39 Cyg
      194317
      Omi Persi
      23180
      6
      Eta Peg
      215182
      Zet Cyg
      202109
      7
      Iot Cnc
      74739
      Bet Tau
      35497
      8
      Bet Peg
      217906
      Vet Cyg
      183912
      9
      54 Leo
      94601
      Alp Vul
      183439
      10
      Alp Tau
      29139
      Gam Gem
      47105
      11
      Mu Ceti
      17094
      Lam Ori
      36861
      12
      Omi Psc
      10761
      Bet Cnc
      69267
      13
      Pi Ori
      30836
      Del Hyd
      73262
      14
      3 Agr
      198026
      Lam Agr
      177756
      15
      Iot Ori
      37043
      Iot Vir
      124850
      16
      Lam Eri
      33328
      Eta Eri
      18322
      17
      The Lib
      142198
      Del Crv
      108767
      18
      Bet Cet
      4128
      Bet CMaj
      44743
      19
      88 Aqr
      218594
      Pi Sgr
      178524
      20
      88 Aqr
      218594
      Xi Oph
      156897
      21
      4 Xi CMaj
      46328
      3 Eri???
      18978
      22
      Rho Pup
      67523
      Omi CMaj
      50877
      23
      Omc Cau???
      56139
       
      61555
      24
      Bet Hyd
      103192
      43 Eri
      28028
      25
      2 Cen
      120323
      Eps Sgr
      169022
      26
      Alp CrA
      178253
      Mu Seo
      151890
      27
      Bet CrA
      178345
      Lam Gru
      209688
      28
      Phi Cen
      121743
      Eta Cen
      127972
      29
      Eta Col
      40808
       
      73634
      30
      Vel
      78647
      Gam Phe
      9053
       
      Good luck
      Ashirwad Tillu ( ashirwadtillu@gmail.com), user name ( antariksha)
    • By Demonperformer
      The other night, I got a rather weird thing happen using Sharpcap. In live-stacking, the first image arrived and it was full of stars. So, without changing any settings, I did a "platesolve" (which also realigns the scope) to centralise my target. It did its single frame capture and I got a practically blank screen and a message saying it could only detect 3 stars. I tried reducing the "sigma" setting, as advised by the on-screen message, with little effect. What gets me is that the same settings produced a whole mass of stars in each of the subs that were being live-stacked. Never had this happen to me before, and no, of course I didn't do anything intelligent like saving the log file!
      I can always try to replicate this next time (although I always find trying to replicate something that isn't working to be a weird activity in itself!), but on the off-chance that anyone can see what was happening without the log file, I thought I would ask.
      Thanks.
    • By MarsG76
      This exposure of the Orion Nebula region is really just a quick and lazy session since I didn't want to waste a clear night by doing nothing and the scope was already setup and focused so I wouldn't be spending much time on setup. I also didn't have a plan for imaging another object it seemed like a good idea being a bright and easy object to image.
      I already imaged this object in the past, but by comparing the setup, procedure and improved tracking accuracy of the past together with the now cooled 40D, I know that the result would have been an improvement if I would have dedicated the necessary exposure time, through the necessary NB filters.
      This image all consists of RGB/OSC, IRCut filtered, 31x15s, 32x30s, 16x60s, 10x90s, 11x120s ISO1600 subs. 
×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.