Drift Method of Polar Alignment

(With Assist From a Micro-Guide Eyepiece)

James J. Janusz

1. Preparation. Set up your scope as normal and turn on the drive. Use any of the many methods to roughly align using Polaris. This can be as easy as just getting Polaris in the center of your finder scope. The closer you get, the shorter time you will spend drifting. (Hint: Be sure to level your tripod - if not level, adjustments in one axis will affect the other.) Those with goto mounts can use the built in North Polar Calibrate mode to get closer.
2. Put a diagonal and illuminated reticule, guiding eyepiece, in your scope. If you have an eyepiece like the Celestron Micro-Guide use it, as I’ll give additional instructions for it later. (Omit the diagonal for a Newtonian.) About 200 power is the minimum required for adequate sensitivity during drifting, so use a Barlow if necessary. Rotate the eyepiece so that a star moves parallel to the crosshairs in Dec and RA when using the slow motion controls. Align it so that Dec is up and down (North & South) and RA is right and left (East & West). This is very important!
3. Azimuth Adjustment. Find a star slightly east of the meridian, about two hours, and at about +10 degrees declination and center it in the guiding eyepiece. Let your scope track and watch for Dec drift (up or down). Unless your alignment is very close, you will see drift in 5 to 30 seconds. You may guide in RA only if you wish but it is not necessary.
4. If the star drifts up, turn the mounts azimuth knob that makes the star move to the right in the field. (Hint, remember, upright) If the star drifts down, turn the azimuth knob that makes the star move left in the field (These adjustments are reversed for a Newtonian). After adjustment, use the slow motion controls to re-center the star. For long exposure film photography, repeat this until there is no drift for at least 15 minutes, for shorter exposure CCD imaging 5 to 10 minutes may be sufficient. (Hint, although you will want 15 minutes with no movement, I only go for 5 minutes at this point. I then move to the altitude adjustment for the full 15-minute drift and then return here to finish out the 15 minutes. This assures that altitude adjustments have not affected azimuth and the remaining small adjustments will not affect altitude.) (Caution, don’t allow the mount to drift past the meridian, as that will reverse the direction.) If you see drift in less than 5 seconds at 200X, you are probably 10 or more eyepiece fields off in azimuth. Give the knob a good crank. This may have to be repeated 3 or 4 times to notice the drift slowing. If you don't see any drift for 30 seconds or so, you’re getting close. Make your azimuth adjustment accordingly. If after adjustment the star drifts in the opposite direction, you went too far. Note: You may use a star west of the meridian instead of east, but the adjustments must be reversed, that is, if the star drifts up, adjust the azimuth to move the star left and if the star drifts down, adjust the azimuth to move the star right.
5. Micro-Guide users can use the small markings or divisions to assist in the process. As you get close, no drift for a minute or so, use this formula. If after 15 minutes the star drifts up or down one division (marking) the correction required is four divisions. At 5 minutes and one division, it’s 12 divisions and at 10 minutes and one division, it’s 6 divisions of correction. Any number of divisions and time can be extrapolated from this. See the tables on page 3. You will need to rotate the Micro-Guide to horizontal to make the adjustment right or left. Your particular system may require a slightly different number but this is a good for 200x.
6. Altitude Adjustment. Find a star 15 to 30 degrees above the Eastern horizon and about 0 degrees declination. I use the altitude of Polaris, +32 as a Guide. The closer to 30 degrees the less seeing will affect the drift process. As in the azimuth process, if the star drifts up, adjust the mounts elevation control to move the star down. If the star drifts down, adjust the elevation to move the star up. Repeat until there is no drift for at least 15 minutes. (Hint: Remember to move the star in the direction opposite the drift.) (Note: You may use a star in the west instead of the east but the adjustments must be reversed, that is, if the star drifts up, adjust the elevation to move the star up and if the star drifts down, adjust the elevation to move the star down.) Repeat this until there is no drift for at least 15 minutes.
7. Micro-Guide users can use the small markings or divisions to assist in the process. As you get close, no drift for a minute or so, use this formula. If after 15 minutes the star drifts up or down one division (marking) the correction required is ten divisions. At 5 minutes and one division, it’s 30 divisions and at 10 minutes and one division, it’s 15 divisions of correction. Any number of divisions and time can be extrapolated from this. See the tables on page 3. Your particular system may require a slightly different number but this is a good for 200x.
8. If you made a large correction in elevation (several degrees or more), go back and check the azimuth, otherwise you are done. I always re-check the azimuth. With a little practice, it is possible to complete the procedure before the end of twilight. Try it in your back yard until you are confident. This method is accurate enough for astrophotography of up to 2 hours for small fields (less than 1 degree) and up to 1.5 hours for larger fields (up to 5 degrees) for declinations between +70 and -70 degrees. For exposures longer than these and closer to the poles, a drift of 30 minutes or more may be required.
9. It must be emphasized that the star cannot drift at all for 15 minutes to achieve the exposure times stated above. If you judge the drift by bisecting a star with a line in an illuminated eyepiece the star must stay bisected for the full 5 minutes. I prefer to sharply focus the star and put it right on the line. This shows the drift more accurately. If the star has drifted as little as half its diameter then field rotation will begin to creep into long exposures. One last disclaimer, this method is designed for portable set-ups. Permanently mounted telescopes will benefit from more accurate alignment and error modeling.

Tables of Micro-Guide Adjustments

Azimuth Adjustments

Minutes >>>	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15
Reticule Marks
1	60	30	20	15	12	10	8.6	7.5	6.7	6	5.5	5	4.6	4.3	4
2	120	60	40	30	24	20	17.1	15	13.3	12	10.9	10	9.2	8.6	8
3	180	90	60	45	36	30	25.7	22.5	20	18	16.4	15	13.8	12.9	12
4	240	120	80	60	48	40	34.3	30	26.7	24	21.8	20	18.5	17.1	16
5	300	150	100	75	60	50	42.9	37.5	33.3	30	27.3	25	23.1	21.4	20
6	360	180	120	90	72	60	51.4	45	40	36	32.7	30	27.7	25.7	24
7	420	210	140	105	84	70	60	52.5	46.7	42	38.2	35	32.3	30	28
8	480	240	160	120	96	80	68.6	60	53.3	48	43.6	40	36.9	34.3	32
9	540	270	180	135	108	90	77.1	67.5	60	54	49.1	45	41.5	38.6	36
10	600	300	200	150	120	100	85.7	75	66.7	60	54.5	50	46.2	42.9	40

Altitude Adjustments

Minutes >>>	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15
Marks
1	150	75	50	37.5	30	25	21.4	18.8	16.7	15	13.6	12.5	11.5	10.7	10
2	300	150	100	75	60	50	42.9	37.5	33.3	30	27.3	25	23.1	21.4	20
3	450	225	150	112.5	90	75	64.3	56.3	50	45	40.9	37.5	34.6	32.1	30
4	600	300	200	150	120	100	85.7	75	66.7	60	54.5	50	46.2	42.9	40
5	750	375	250	187.5	150	125	107.1	93.8	83.3	75	68.2	62.5	57.7	53.6	50
6	900	450	300	225	180	150	128.6	112.5	100	90	81.8	75	69.2	64.3	60
7	1050	525	350	262.5	210	175	150	131.3	116.7	105	95.5	87.5	80.8	75.0	70
8	1200	600	400	300	240	200	171.4	150	133.3	120	109.1	100	92.3	85.7	80
9	1350	675	450	337.5	270	225	192.9	168.8	150