18 replies to this topic

[Jeremy Perez]

Hi folks,

I've been trying to break in a Meade Astrometric eyepiece I picked up last year. I haven't put it into practice on many doubles yet, but the process has been very educational so far. For stars in the 1.5 to 4 arc second range, my success with PA has been within ± 5 degrees and separation within ± .8 arc seconds. I put together a tentative tutorial on the materials and the process I'm using here:

Measuring Double Stars

Since I'm a novice with this tool, I'm interested in critiques on the process I'm using. If you have a chance to take a look, and find I'm missing some important steps--or if I'm making it unnecessarily complex in places--I'd appreciate your thoughts.

Thanks for any input!

[Rich (RLTYS)]

Jeremy

I scanned thru your tutorial with great interest. (will read it more throughly later) From what I read I was quite impressed. I had just, this Christmas, received a Celestron MicroGuide Eyepiece (12.5mm) and would eventually like to try it on doubles. Your article will be a big help.

PS: Does your scope have a motor drive or are you using a Dob?

Thanks and Clear Skies.
Rich (RLTYS)

[Jeremy Perez]

Ah! Great point Rich. I've been making the double star measurements with my 6" f/8 SkyView Pro which does have a drive. I know it would be possible to measure doubles without a drive, but I can see that it would be more difficult to fine tune the measurements. I did make diameter measurements on Comet Holmes with my undriven dob, but that wasn't as demanding as the high-power double observations.

[lunator]

Jeremy

Out of interest why are you using an external PA dial?

I used one with a cross hair eyepiece whilst making measures using the chronometric method.

Tom Teague sets out a very simple process that avoids the need for an external dial and is suitable for the meade & celestron (& Baader) astrometric eyepiece.

I have used to measure doubles down to 2"

It's set out in chapter 12 of Bob Argyles' (ed.) book

Cheers
Ian

[Jeremy Perez]

Out of interest why are you using an external PA dial?

Hi Ian,

I have not read Argyle's book. I did recently pick up a digital stopwatch (with huge numbers) so I could try the chronometric method for separation measurements. I'll be interested to see what method Teague uses for PA that avoids the external dial.

[Rich (RLTYS)]

"I did recently pick up a digital stopwatch"

Jeremy

Curious, where did you get your digital stopwatch I've been trying to find one.

Clear Skies.
Rich (RLTYS)

[Jeremy Perez]

With regrets, I must confess: Walmart

[Rich (RLTYS)]

"With regrets, I must confess: Walmart"

Jeremy

Works for me. Thanks for the info.

Rich (RLTYS)

[lunator]

Out of interest why are you using an external PA dial?

Hi Ian,

I have not read Argyle's book. I did recently pick up a digital stopwatch (with huge numbers) so I could try the chronometric method for separation measurements. I'll be interested to see what method Teague uses for PA that avoids the external dial.

Hi Jeremy

Tom's method is very simple. once you have aligned the central 'linear scale' to the position angle of the secondary from the primary then you put the star into the centre of the eyepiece. In the case of the Meade that would be at the 25th division mark on the linear scale.

Turn off the drive and the star will drift west. When the star reaches the outer protractor scale you note the point it crosses the dial and that is the PA.

Cheers

Ian

[Jeremy Perez]

Thanks Ian. I'll give that a try and compare how it works for me.

[GregB]

Jeremy,

It so happens there is a preview of portions of Argyle's book that you can find on books.google.com, just search for Observing and Measuring Visual Double Stars and you will find the full text of the chapter on using the Celestron and Meade reticle eyepieces to measure doubles.

By the way, it is a great book and valuable addition to your reference library.

Greg

[Karl Fabian]

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Tom's method is very simple. once you have aligned the central 'linear scale' to the position angle of the secondary from the primary then you put the star into the centre of the eyepiece. In the case of the Meade that would be at the 25th division mark on the linear scale.

Turn off the drive and the star will drift west. When the star reaches the outer protractor scale you note the point it crosses the dial and that is the PA.
************************************************************

Hi all,
I realize this is an old thread but had to add some information regarding acquiring PA readings with the Meade Astrometric eyepiece. The drift reading acquired on the scale is NOT the actual PA. The reading must be corrected to arrive at the PA. The method used for the Meade Astrometric with mirror reversed image is thus:

For drift readings NORTH of (W)90 or (W)270 position on protractor: If drift reading is N of 90, subtract 90 from the uncorrected degree reading and multiply the answer X2. Then subtract that number from the uncorrected degree reading to obtain the actual PA degrees. Example: Uncorrected reading 100 degrees. 100-90=10 10x2=20 100-20=80 degrees PA. For uncorrected degree readings where 270 instead of 90 point is on preceding half or field use the same method just substitute 270 for 90. Example: Uncorrected reading 300 degrees. 300-270=30 30x2=60 300-60=240 degrees PA.
For drift readings that are SOUTH of W270 or W90 point on protractor scale: SUBTRACT the uncorrected degree reading from 270 or 90 degrees (depending on which protractor point is on preceding half of field). Then take that result and multiply it x2. Finally ADD that result to the uncorrected reading to obtain the actual PA degrees.
Example: Uncorrected protractor drift reading 79 degrees: 90-79=11 11x2=22 79+22=101 degrees PA. For south readings where 270 is on preceding half of field merely substitute 270 for 90. Hope this helps make things more clear without giving anyone a migraine.

Clear & steady skies,
Karl

[7331Peg]

Thanks for posting that info, Karl.

I tried using the drift method in the Argyle book this past summer and fall, and got absolutely nowhere with it. I finally discovered I could get a correct reading by using the hand controller on my G8 to move the star in question to the outer circle -- worked like a charm every single time.

I just checked the figures I had generated using the drift method, and using the formulas you posted, I can come up with the correct PA. I have to wonder why the Argyle book and other sources, such as articles in the JDSO, don't mention this correction formula.

Minus the corrections you've posted, I certainly don't see any way to arrive at the correct results via the drift method.


John

[Karl Fabian]

Thanks John
Simply put, as soon as the eyepiece linear scale is rotated from the original E-W drift axis to bisect the secondary, the number of degrees N or S of that axis obviously must be accounted for. I searched all over the internet and found nothing at all on doing the correction. Got frustrated so just sat down at the desk playing with the eyepiece , drawing circles, rotating, etc, and then it became clear. Actually with the Meade Astrometric it is a simple math exercize but took some time to figure out.

Clear and steady skies,
Karl

[7331Peg]

I searched high and low and kept playing with various combinations of addition and subtraction to find that magic formula, but it kept eluding me. I knew there had to be one, though.

My Star Splitter hat is off to you for figuring it out -- many, many thanks!




John

[Karl Fabian]

Thanks John for the great double star observations & descriptions on Star Splitters. I enjoy all of them and they are often on my "check it out list" of things to observe.

Karl

[Ain Soph Aur]

Karl, what would the correction formula be with the Celestron astrometric eyepiece?

[Karl Fabian]

Hi, Don't have a Celestron Microguide to play with but I'll try to figure it out by looking at an online image of the reticle. The same basic math would be involved, of adding or subtracting degrees from the drift postion angle, but must be modified because of the different outer scale degree arrangement of the Celestron unit. If I happen to figure it out I'll post it here. An easy way to get a start on trying to figure it out is to aim a level altazimuth scope at a paper target with dots arranged like a rendition of a double star with the position angles shown and NSEW labeled just like you would see in an astronomy textbook showing double star position angles. Target would have 0 (N)and 180 (S) vertical and 90 (E) and 270 (W) horizontal. Then bisect the pair then slew the scope left or right in azimuth toward the W (270 deg) on the target chart and note where primary crosses outer degree scale. You already know the correct PA from the labeled target so must add or subtract the degrees on the scale to agree with the actual PA of the drawn target chart "double star". After doing this on a number of different PA angle targets the formula should hopefully reveal itself as it did for me with the Meade unit. It can give you some aggravation but eventually it becomes obvious by doing this method. It is just a chore in simple math of adding and subtracting or multiplying degrees depending on where the primary crosses the scale using the simulated drift method.

Clear skies,
Karl

[Ain Soph Aur]

Excellent advice. Thanks Karl!