Jump to content

  •  

- - - - -

New approach for Polar Alignment


Discuss this article in our forums

New approach for Polar Alignment

(no dependency on siting Polaris)

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.

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 centre 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 centre 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 todays’ 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 catalogue 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

Second star ( name)

Second star HD

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 ****

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), Cloudy nights user name ( antariksha)


  • bsavoie likes this


0 Comments



Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics