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Best eyepiece for double star measurments

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#1 betaman42

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Posted 08 April 2013 - 08:06 PM

Hi all Im new and am looking forward this community..lately ive been into observing double stars with my dobsonion xt8 and Im really interested in measuring there separation and position angle but some problems arise. My telescope doesn't have any sort of drive,and I regardless I can't seem to find a good micrometer as the celestron micro guide is discontinued. Any adivce on what I can do or what my options are to best mesure double stars would be great thanksss!

#2 buddyjesus

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Posted 08 April 2013 - 09:33 PM

a good piece of kit to have then is a tracking platform. You can do drift timing using a bit of geometry without it, but it helps to have an equatorial axis for measuring position angles as this will change with time on an alt/az mount. You also want a reticle that has a fine scale.

articles in the JDSO.org site will help guide you.

#3 buddyjesus

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Posted 08 April 2013 - 09:33 PM

welcome to CN! as for eyepieces, the 12.5mm celestron reticle is considered the standard. wish i had one.

#4 Cotts

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Posted 08 April 2013 - 10:08 PM

Hi, Beta. A tracking equatorial drive is just about mandatory for measurement. In order to get good accuracy for separation measurements high magnification must be used and, with no drive you won't be able to use a reticle eyepiece.

A driven platform is no good for PA measurements because of field rotation. There will be great difficulty in determining where north is in your eyepiece.

Dave

#5 WRAK

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Posted 09 April 2013 - 03:38 AM

Here is an article describing how to measure double stars with alt/az mounted scopes which should do very well also for a dobson: http://www.jdso.org/...Frey_59_65.pdf.
Wilfried

#6 buddyjesus

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Posted 09 April 2013 - 04:45 AM

http://www.jdso.org/.../Frey_59_65.pdf

the period for the sentence screwed up the link.

I was under the impression that a dob on a tracking platform didn't have image rotation if the center of gravity for the scope was on the axis of rotation?

#7 betaman42

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Posted 09 April 2013 - 06:03 AM

Thanks for all the info and from reading the article it seems that it is possible to do double star measurements with an alt/az mount but would require extra expenses..but then again it looked likE the author was using an az/alt

#8 brianb11213

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Posted 09 April 2013 - 06:41 AM

Look up "cross bar micrometer". Sidgwick (Amateur Astronomer's Handbook) covers this very well but is incorrect in stating that it requires an equatorial mount: since the method of using the cross bar micrometer depends on the telescope drive being stopped, all you have to do to use it with an altaz mount is to orient the device so that the stars trail along the guide wire before commencing measurement. Field rotation remains an issue but so long as the star being measured is not very close to the zenith the error in the derived PA should be small compared with those which arise from errors in the timings which form the basis of the calculation. (In temperate latitudes, field rotation is less than 0.2 degrees per minute except close to the zenith)

A split action stopwatch should enable you to get all the timings in one pass - if you can't do this then an equatorial mount becomes desireable.

The cross bar micrometer is best suited to wide pairs and can be home made easily enough with any eyepiece with an accessible field stop. Ordinary orthos and plossls work well. For practical reasons (scale of parts) it is better to use a longish focus eyepiece (around 25mm) with a barlow lens to increase magnification rather than try to add the wires to the small field stop in a short focus eyepiece. High optical quality of the eyepiece is not required to use the device effectively, and field illumination is a convenience rather than a necessity.

#9 betaman42

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Posted 09 April 2013 - 02:02 PM

The article is very informative but doesn't seems to give enough info on how to actually modify an eyepiece with the micrometer....Also for an altaz mount what makes it possible to use a crossbar micrometer but not a regular filar micrometer..thanks

#10 brianb11213

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Posted 09 April 2013 - 03:12 PM

for an altaz mount what makes it possible to use a crossbar micrometer but not a regular filar micrometer

With a filar micrometer you have to bisect both stars with the PA wire and with the fixed and moveable cross wires which are used to measure the seperation. This is impossible without accurate tracking. Then with an altaz mount you'd need to apply a correction to the PA measure dependent on the star's altitude, azimuth and your geographical latitude ... that would be at best an awkward computation.

With a crossbar micrometer you don't even need a drive clock as the whole thing depends on drift due to the earth's rotation. If a star is running down the drift wire then anything close enough to be in the same field of view will track parallel to it & the timings are all you need to derive the PA & seperation as described in AAH.

#11 brianb11213

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Posted 09 April 2013 - 03:18 PM

The article is very informative but doesn't seems to give enough info on how to actually modify an eyepiece with the micrometer

There is another section in AAH on how to add threads to an eyepiece. But basically for the crossbar micrometer you do not need fine thread (spider web), thin copper wire will do & this is easily glued or soldered to the metal field stop in the eyepiece - field stops are always threaded so that they can be focused - the eyepiece types that have accessible field stops are obviously easier to modify than those with field stops embedded within them as you don't have to disturb the optics.

AAH also contains descriptions of how to build a jig to ensure that wires or threads are assembled so that they're in the right place & at the right angle (the jig allows the wires to be positioned under slight tension so that they're guaranteed to be straight too).

#12 betaman42

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Posted 09 April 2013 - 05:00 PM

THanks for all the advice brianb and everyone..looks like I will most likely attempt the crossbar rectile sounds likecool project..if any questions arise I'll post them here..thanks!






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