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GOTO vs. clockdrive

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#1 Nicole Sharp

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Posted 23 June 2018 - 01:39 AM

What are the differences in tracking between an altazimuth go-to mount, an equatorial mount with a one-axis clockdrive, and an equatorial mount with a two-axis clockdrive?

 

If you are tracking a deep-sky object at the sidereal rate, should all three be the same?  How does a one-axis clockdrive (such as for the Celestron PowerSeeker or Celestron AstroMaster) compare to a two-axis clockdrive (such as for the Celestron Omni CG-4)?  If the planet is rotating in one direction, why do you need two axes to be clockdriven?

 

For tracking an object in the Solar System, what is the difference between the three?  How much deviation would an equatorial clockdrive have for Solar-System objects (e.g. off by arcseconds per second, arcseconds per minute, or arcseconds per hour)?  Do planets in our Solar System move closer to the sidereal rate than Sol or Luna do?  What about asteroids?


Edited by Nicole Sharp, 23 June 2018 - 01:40 AM.


#2 jfrech14

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Posted 23 June 2018 - 01:54 AM

Do they make 2-axis clock drive mounts? I don't see how the dec axis could use a clock drive, only a simple motor control for minor adjustments. Tracking in alt-az is like how we look at the sky using earth-based angles from horizontal and from north. The equatorial mount is based on the axis going through the celestial pole so that the only axis that needs to track is the one that corrects for the rotation of the earth (RA). Dec only needs to be changed to change objects or to adjust for bad alignment.

 

The sun and moon have their own tracking rates in most modern go-to mounts I believe. I know the sun does. Asteroids all have different velocities and you need to put their ephemeres into the scope if the scope doesn't already have them for the objects you want to see. 

 

Deep sky objects will not track the same across all 3. The eq mounts will because I think you are meaning just a simple clock drive vs a motorized eq mount. But an alt-az mount will have field rotation as well as gimbal lock at zenith.



#3 sg6

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Posted 23 June 2018 - 01:58 AM

Alt/Ax track the object but not the rotation that occurs as the object transits the sky so for long exposure imaging they are not really useful. I would discard them as an imaging option.

 

The other 2 both driven equitorial should in effect be the same. Although a goto will apply additional compensation which may be of benefit. Additionally a goto often has better/finer stepper motors and again might be of benefit.

 

The problem of goto is that goto is basically all it is.People expect full automation these days and they don't get it. In basic form a goto mount:

You accurately polar align it, you supply location data, you supply time, you center each alignment star. If you have managed all that and done it well the scope might go to the next item you request and put it in your view. Hardly automatic.

 

Not sure of you situation but here it is easiest to get the dual axis drives, cost is similar and you may as well have a handset for movement in both options. Actually not seen a single axis option for the mounts so half think they have been dropped.



#4 ShaulaB

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Posted 23 June 2018 - 02:35 AM

"Clock drive" implies celestial motion at the sidereal rate, and these have been available for Right Ascension since the invention of electric circuits. So if you have a GEM (German equatorial mount) or a wedged fork mount, a  clock drive for just RA is fine, the scope will track objects. No need for a DEC clock drive. Perhaps this is being confused with just motors to adjust the Declination if alignment is off, or following the Sun or Moon. Note: the Sun and the Moon do not move at sideral rate.

 

With the small aperture scopes mentioned in the original post, maybe the GOTO scope is the best choice. A lot of beginners, intimidated a bit by getting into astronomy, feel more comfortable with GOTO electronics. But with an RA clock drive, a decent properly aligned finder device, and your own brain, you can find and observe anything in the sky within the magnitude limits of your optics and local light pollution.



#5 Nicole Sharp

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Posted 23 June 2018 - 03:46 AM

"Clock drive" implies celestial motion at the sidereal rate, and these have been available for Right Ascension since the invention of electric circuits. So if you have a GEM (German equatorial mount) or a wedged fork mount, a  clock drive for just RA is fine, the scope will track objects. No need for a DEC clock drive. Perhaps this is being confused with just motors to adjust the Declination if alignment is off, or following the Sun or Moon. Note: the Sun and the Moon do not move at sideral rate.

 

With the small aperture scopes mentioned in the original post, maybe the GOTO scope is the best choice. A lot of beginners, intimidated a bit by getting into astronomy, feel more comfortable with GOTO electronics. But with an RA clock drive, a decent properly aligned finder device, and your own brain, you can find and observe anything in the sky within the magnitude limits of your optics and local light pollution.

My question though is what is the actual deviation of the motion of Solar-System objects from the sidereal rate with a one-axis clockdrive?  If you have an equatorial telescope polar-aligned and pointed toward Venus, Mars, or Vesta, and are using a clockdrive to match the rotation of Terra, how quickly will Venus, Mars, or Vesta deviate from the sidereal path that the clockdrive would follow?  Same question for Luna, Sol, or any other Solar-System body?  Or do all objects follow sidereal motion except for Sol and Terran satellites?



#6 Nicole Sharp

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Posted 23 June 2018 - 03:57 AM

"Clock drive" implies celestial motion at the sidereal rate, and these have been available for Right Ascension since the invention of electric circuits. So if you have a GEM (German equatorial mount) or a wedged fork mount, a  clock drive for just RA is fine, the scope will track objects. No need for a DEC clock drive. Perhaps this is being confused with just motors to adjust the Declination if alignment is off, or following the Sun or Moon. Note: the Sun and the Moon do not move at sideral rate.

 

With the small aperture scopes mentioned in the original post, maybe the GOTO scope is the best choice. A lot of beginners, intimidated a bit by getting into astronomy, feel more comfortable with GOTO electronics. But with an RA clock drive, a decent properly aligned finder device, and your own brain, you can find and observe anything in the sky within the magnitude limits of your optics and local light pollution.

Wouldn't a two-axis clockdrive match both the rotational motion of Terra, plus the orbital motion?  The planet rotates at one revolution per day, but also moves forward in its orbit by about 1/365 of a revolution per day.  So a one-axis clockdrive would be off by about 1 degree per 24 hours compared to a two-axis clockdrive?  Or does that not need to be taken account of for sidereal tracking?


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#7 Celerondon

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Posted 23 June 2018 - 05:10 AM

My question though is what is the actual deviation of the motion of Solar-System objects from the sidereal rate with a one-axis clockdrive?  If you have an equatorial telescope polar-aligned and pointed toward Venus, Mars, or Vesta, and are using a clockdrive to match the rotation of Terra, how quickly will Venus, Mars, or Vesta deviate from the sidereal path that the clockdrive would follow?  Same question for Luna, Sol, or any other Solar-System body?  Or do all objects follow sidereal motion except for Sol and Terran satellites?

 

That is a straightforward question with some varied answers Nicole.  For earthbound observers, the starry background drifts westward at the sidereal rate.  The solar system deviation varies per object but it is easy to visualize with:

  1. Knowledge of the way that solar system objects drift (generally westward) against the backdrop of the starry sky as time passes.
  2. The aid of a planetary atlas app such as Sky Safari. 

Think about it.  Mercury and Venus almost streak across the starry background from week to week.  Mars is slower and not surprisingly, the gas giants and beyond move slower still as distance from the Sun increases.  In Sky Safari I just selected and centered a few planets and then used the time controls to compare their relative motion against the stars.  The planetary drift was visible on a weekly or monthly basis but the solar and lunar drift rates could be observed from minute to minute.  

 

Despite the differences in their apparent motion against the starry background most solar system objects do not deviate that much from the sidereal rate. The Moon and Sun are a different matter.  They move much faster relative to the stars and other solar system objects.

 

Goto is better for dealing with these differences.  Although a simple clock drive on the Right Ascension (RA) axis is adequate for observing all of these objects a decent Goto mount can find and track objects with better accuracy for the objects that don't travel at the regular sidereal rate.  However, I use a GEM with a plain RA drive and I never notice the difference while observing.  Because photographs of solar system objects require shorter exposures, they don't pose a problem either.  


Edited by Celerondon, 23 June 2018 - 05:12 AM.

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#8 Nicole Sharp

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Posted 23 June 2018 - 05:31 AM

That is a straightforward question with some varied answers Nicole.  For earthbound observers, the starry background drifts westward at the sidereal rate.  The solar system deviation varies per object but it is easy to visualize with:

  1. Knowledge of the way that solar system objects drift (generally westward) against the backdrop of the starry sky as time passes.
  2. The aid of a planetary atlas app such as Sky Safari. 

Think about it.  Mercury and Venus almost streak across the starry background from week to week.  Mars is slower and not surprisingly, the gas giants and beyond move slower still as distance from the Sun increases.  In Sky Safari I just selected and centered a few planets and then used the time controls to compare their relative motion against the stars.  The planetary drift was visible on a weekly or monthly basis but the solar and lunar drift rates could be observed from minute to minute.  

 

Despite the differences in their apparent motion against the starry background most solar system objects do not deviate that much from the sidereal rate. The Moon and Sun are a different matter.  They move much faster relative to the stars and other solar system objects.

 

Goto is better for dealing with these differences.  Although a simple clock drive on the Right Ascension (RA) axis is adequate for observing all of these objects a decent Goto mount can find and track objects with better accuracy for the objects that don't travel at the regular sidereal rate.  However, I use a GEM with a plain RA drive and I never notice the difference while observing.  Because photographs of solar system objects require shorter exposures, they don't pose a problem either.  

 

Yes, I am trying to figure out if I only want to make videos of Solar-System objects (such as for stacking, or filming eclipses/transits/occultations), do I really need a GOTO mount?  It seems like just any German equatorial then should be able to keep up with Solar-System objects, only requiring minor adjustments, even without a clockdrive?  What about at a really long focal length length with a narrow field of view (such as a Maksutov-Cassegrain OTA) for planetary imaging?  Would their motion still be close enough to the sidereal rate using a one-axis clockdrive?  What are the advantages of a two-axis clockdrive?  I suppose you cannot adjust for declination if there a clockdrive running on the ascension axis, unless you have a second motor?

 

Would I still need GOTO for the 2019 Mercury transit though?  Mercury and Sol probably have the most deviation I would imagine?  The transit lasts for about 6 hours.



#9 Celerondon

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Posted 23 June 2018 - 05:57 AM

Wouldn't a two-axis clockdrive match both the rotational motion of Terra, plus the orbital motion?  The planet rotates at one revolution per day, but also moves forward in its orbit by about 1/365 of a revolution per day.  So a one-axis clockdrive would be off by about 1 degree per 24 hours compared to a two-axis clockdrive?  Or does that not need to be taken account of for sidereal tracking?

I suppose that declination adjustments help somewhat for orbital tilt adjustments but most of the motion is an east/west RA distance. 

 

You are on the right track (pun intended) with that 1/365 idea Nicole!  That deviation is already built into our clocks and calendars as well as the sidereal tracking rate.  As mentioned in a prior post, adjustments in declination are required to find objects and to compensate for tracking errors that are caused by mount misalignment.  I suppose that a GoTo mount could demonstrate somewhat better tracking of objects like the Moon and Sun since their rapid relative motions include a corresponding declination drift component.  

 

Back to your original question:

  1. Altazimuth goto mounts are computerized to track in two axes because they must since they are not aligned with the Earth's axis.
  2. An equatorial mount with a one-axis clock drive will track just fine at the sidereal rate and other speeds if so equipped.
  3. An equatorial mount with a two-axis clock drive tracks just like a equatorial mount with a one-axis clock drive. 

By "track" I mean follow the stars at the sidereal rate.  My favorite design is not one of the ones that you asked about Nicole.  A goto system equipped german equatorial mount (GEM) can find and track all sorts of objects as well or better than any of the other designs that you mentioned.  As a bonus, GEM mounts have certain advantages over altazimuth mounts.  GEMs are better near the zenith and they are not cursed with field rotation.  Because of our atmosphere, the zenith is normally a great place to observe or photograph a target.  Although the field rotation problem does not affect observing it is a big deal for photography. 

 

When everything is setup and aligned properly, a goto GEM will track much like your plain one-axis clock drive equatorial.  The difference is that the goto GEM can and will make occasional tiny declination adjustments automatically.


Edited by Celerondon, 23 June 2018 - 07:21 AM.


#10 jcj380

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Posted 23 June 2018 - 06:06 AM

Hey Nicole, check out www.californiaskys.com. It’s a great site written by Curtis from the EAA forum. Much info there. (Yes, it’s skys not skies.)

#11 Celerondon

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Posted 23 June 2018 - 06:55 AM

Yes, I am trying to figure out if I only want to make videos of Solar-System objects (such as for stacking, or filming eclipses/transits/occultations), do I really need a GOTO mount?  It seems like just any German equatorial then should be able to keep up with Solar-System objects, only requiring minor adjustments, even without a clockdrive?  What about at a really long focal length length with a narrow field of view (such as a Maksutov-Cassegrain OTA) for planetary imaging?  Would their motion still be close enough to the sidereal rate using a one-axis clockdrive?  What are the advantages of a two-axis clockdrive?  I suppose you cannot adjust for declination if there a clockdrive running on the ascension axis, unless you have a second motor?

 

Would I still need GOTO for the 2019 Mercury transit though?  Mercury and Sol probably have the most deviation I would imagine?  The transit lasts for about 6 hours.

You can make those videos without a goto mount.  Although any GEM can "keep up with solar system objects with minor adjustments" without a clock drive, you get two big advantages by using one.  Keeping up or tracking will be easier and more accurate with a clock drive.  Easy is a good thing and more accurate tracking means that your images will be better.

 

Be careful with really long focal lengths.  Longer focal lengths increase the requirements for mount stability and tracking accuracy.  

 

Transits and eclipses are FUN!  I have a suggestion.  Practice!

 

A short tube refractor would be an inexpensive way to start and learn how everything works together.  If you use an aperture under 100mm your mounting requirements will decrease while portability increases.  Believe it or not, you don't need much focal length to get good pictures of the Sun.  

 

How much do plan to spend?  Will you fly to your Mercury transit observation site?  Next year I hope to go to Chile for the total solar eclipse. My telescopes, mounts, and camera equipment will need to travel with me.  Prior eclipse and transit trips included a 70mm refractor and one-axis driven GEM.  When I flew to observe and photograph the 2012 Transit of Venus the telescope was carry-on luggage and the mount traveled down below.  Last summer when we drove to Oregon for the total solar eclipse, I took the 70mm refractor, a 100mm refractor and the driven GEM as well as another manual altazimuth mount for visual observations.

 

Goto is not required to observe or photograph an eclipse or transit.  In fact, goto is probably not much of an advantage for those activities.  Even though the transit of Mercury lasts several hours your exposures will be quite short.  Clock driven tracking is a different matter.  Although I enjoy observing an eclipse or transit with a manual altazimuth or GEM mount sharing the experience with others is more challenging.  Challenges and complications during an eclipse or transit are distracting and detract from the experience.

 

That said, goto can enhance your experience while observing.  My next mount will be a goto mount.  I have been looking at the Celestron AVX and the new Meade LX85.  The combination of a decent weight payload and high portability interest me.  Although I will eventually end up with something like a Losmandy GM811G those two smaller mounts seem better for my current plans.  

 

Here is my nagging question.  Should I take a goto GEM and 100mm refractor to Chile or trim my traveling astronomy kit for this trip?  My current driven GEM is just as bulky as a goto mount but makes taking pictures easy compared to my manual altazimuth mount.  I could take the 70mm refractor but it was really nice to have both scopes in Oregon.  (observing/photography)

 

Practice Nicole.  If you purchase a decent mount and a simple telescope you can determine what works for you.  Add a solar filter and camera to your kit and soon you will be advising others on solar system photography.  


Edited by Celerondon, 23 June 2018 - 07:09 AM.

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#12 Nicole Sharp

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Posted 23 June 2018 - 07:20 AM

So it sounds like I might be able to save a lot of money by not getting GOTO, if a German equatorial can keep track of Solar-System bodies?  I was hoping to get a Maksutov-Cassegrain though, probably with the longest focal length I could afford or the mount could support.  I can use it to get nice photos of the Mars opposition next month, and a really close-up view of Mercury passing sunspots.  But the field of view would be very narrow, and much less at principal focus with a planetary webcam, and even less with eyepiece projection.  Any small deviations in declination from a manual or one-axis clockdriven German equatorial mount and I would lose the planet.  Would something like a 12X50 finderscope (with removable Solar filter) solve this in helping me to keep track of where the telescope is pointing?  Or would I still need GOTO for principal-focus or eyepiece-projection planetary imaging?

 

I am lucky enough to have the Mercury transit and two Solar eclipses (2017-2024) here in my hometown, so I don't need to travel.  Though I do plan to drive a couple of hours east of here to see totality in 2024.


Edited by Nicole Sharp, 23 June 2018 - 07:24 AM.


#13 Celerondon

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Posted 23 June 2018 - 07:27 AM

Yes, I am trying to figure out if I only want to make videos of Solar-System objects (such as for stacking, or filming eclipses/transits/occultations), do I really need a GOTO mount?  It seems like just any German equatorial then should be able to keep up with Solar-System objects, only requiring minor adjustments, even without a clockdrive?  What about at a really long focal length length with a narrow field of view (such as a Maksutov-Cassegrain OTA) for planetary imaging?  Would their motion still be close enough to the sidereal rate using a one-axis clockdrive?  What are the advantages of a two-axis clockdrive?  I suppose you cannot adjust for declination if there a clockdrive running on the ascension axis, unless you have a second motor?

 

Would I still need GOTO for the 2019 Mercury transit though?  Mercury and Sol probably have the most deviation I would imagine?  The transit lasts for about 6 hours.

Yes, the Sun, Mercury and Venus are the hot rods of this solar system.  All three zoom together but I think that fleet-footed Mercury is the fastest of them all.



#14 Nicole Sharp

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Posted 23 June 2018 - 07:27 AM

Oh.  But Mercury will cross the meridian for the 2019 transit (at least here in Maryland, USA).  I forgot about that.  So then I will either have to use a GOTO altazimuth or wedge mount or a Celestron Advanced VX (which doesn't require meridian flips)?  I assume all manual or clockdriven German equatorial mounts would require a meridian flip when Mercury reaches the meridian?


Edited by Nicole Sharp, 23 June 2018 - 07:39 AM.


#15 Rusted

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Posted 23 June 2018 - 07:36 AM

Don't most equatorial mountings continue tracking well beyond the meridian before a flip becomes essential?

It would be easy to confirm this for yourself if you own a driven, equatorial mounting.



#16 Celerondon

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Posted 23 June 2018 - 01:37 PM

Oh.  But Mercury will cross the meridian for the 2019 transit (at least here in Maryland, USA).  I forgot about that.  So then I will either have to use a GOTO altazimuth or wedge mount or a Celestron Advanced VX (which doesn't require meridian flips)?  I assume all manual or clockdriven German equatorial mounts would require a meridian flip when Mercury reaches the meridian?

You probably have nothing to worry about Nicole.  A GEM doesn't immediately turn into a pumpkin when it crosses the meridian.  wacko.png

 

Is this your transit schedule?

Partial begins:   Nov 11, 2019 at 7:36:05 am
Full begins:       Nov 11, 2019 at 7:37:46 am
Maximum:         Nov 11, 2019 at 10:20:15 am
Full ends:           Nov 11, 2019 at 1:02:42 pm
Partial ends:      Nov 11, 2019 at 1:04:23 pm

 

If this is your schedule some practice with any decent GEM will show you how to handle mount setup for the transit.  For this schedule I would plan for the morning and check a few times for scope and camera clearance for an hour or two past the meridian.  That is all there is to it!  According to these numbers, most of the transit will be during your morning hours.  

 

If you plan to take images (such as a video stream) across the meridian, you can take measures* to increase your clearances.  Otherwise, a meridian flip is no big deal.  It takes just a few moments to flip a telescope to the other side of the mount.  Camera gear and other attachments can complicate the procedure so like I said before...

 

You should practice.  Practice solar safety!  Practice with your mount.  Practice pointing your telescope at the Sun.  Practice focusing and solar observing.  Practice mounting and focusing your camera.  Practice pointing the telescope at the Sun when your camera is mounted on it.  Practice adjusting your camera and taking pictures. 

 

Nicole, will you have any family or other guests present for your transit of Mercury session?  Because Mercury is rather small and distant, eclipse glasses don't work as well for transit viewing.  Six hours is a long time.  You may want to set up a second telescope for direct viewing of the transit while your main rig is busy taking pictures.  

 

If guests are present, they will distract you.  Once you are comfortable with solar observing and photography you might even want to practice taking pictures and observing with guests present.  

 

*Pier extension, https://www.cloudyni...flip/?p=5971743


Edited by Celerondon, 23 June 2018 - 04:51 PM.


#17 Nicole Sharp

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Posted 23 June 2018 - 02:39 PM

You probably have nothing to worry about Nicole.  A GEM doesn't immediately turn into a pumpkin when it crosses the meridian.  wacko.png

 

Is this your transit schedule?

Partial begins:   Nov 11, 2019 at 7:36:05 am
Full begins:       Nov 11, 2019 at 7:37:46 am
Maximum:         Nov 11, 2019 at 10:20:15 am
Full ends:           Nov 11, 2019 at 1:02:42 pm
Partial ends:      Nov 11, 2019 at 1:04:23 pm

 

If this is your schedule some practice with any decent GEM will show you how to handle mount setup for the transit.  For this schedule I would plan for the morning and check a few times for scope and camera clearance for an hour or two past the meridian.  That is all there is to it!  According to these numbers, most of the transit will be during your morning hours.  

 

If you plan to take images (such as a video stream) across the meridian, you can take measures* to increase your clearances.  Otherwise, a meridian flip is no big deal.  It takes just a few moments to flip a telescope to the other side of the mount.  Camera gear and other attachments can complicate the procedure so like I said before...

 

You should practice.  Practice solar safety!  Practice with your mount.  Practices pointing your telescope at the Sun.  Practice focusing and solar observing.  Practice mounting and focusing your camera.  Practice pointing the telescope at the Sun when your camera is mounted on it.  Practice adjusting your camera and taking pictures. 

 

Nicole, will you have any family or other guests present for your transit of Mercury session?  Because Mercury is rather small and distant, eclipse glasses don't work as well for transit viewing.  Six hours is a long time.  You may want to set up a second telescope for direct viewing of the transit while your main rig is busy taking pictures.  

 

*Pier extension

  

If guests are present, they will distract you.  Once you are comfortable with solar observing and photography you might even want to practice taking pictures and observing with guests present.  

 

I still think I might be better off with a Celestron Nexstar for an easy-to-use "grab-and-go" telescope.  But I think I might also want a manual mount, especially in cold or hot weather that might be bad for the expensive electronic mount (such as a January Lunar eclipse or a November Mercury transit).  I don't think a German equatorial would be difficult to operate, but I think it could be difficult to set up.  Hopefully that is just something that gets easier with experience though?  The Explore Scientific German equatorial mounts you can get for as little as $30 with an OTA, but I don't think they have a clockdrive available.

 

And yeah, a few minutes of lost footage from a six-hour video is probably only a minor inconvenience.  I think the camera (and the YouTube community) might appreciate it if the video is cut into one-hour segments.  I thought about using two telescopes and two cameras for the Mercury transit, one with full disc, and one as close-up, but I think it wouldn't be worth the effort since Mercury is so small.  It would be much more interesting to use a Mercury-tracking mount (as opposed to a Solar-tracking or sidereal-tracking one), and get a really close-up view I think at high magnification if that was possible with a small telescope on a lightweight altazimuth mount.

 

BTW, a good way to practice for the Mercury transit is to look for Solar transits of the ISS.  I think the ISS might actually be bigger than Mercury, but it moves a lot faster.  They don't happen very often though, unless you travel for it.

 

I learned some hard lessons in 2017.  I had wanted to film the whole thing on video then, but I had camera problems.  So I was able to share the view with random strangers due to the eclipse-glasses shortage, which wasn't so bad.  I want to be prepared for the Mercury transit though (they are more rare than Solar eclipses!) with a good tracking mount, a better view, and a better camera.  My eventual goal for a new telescope though is to try to develop a video telescope.  I want to use electronically assisted astronomy and a laptop or video monitor to view the telescope with.  That way lookey-loos can just watch the video monitor instead of pawing at the expensive optics, lol.


Edited by Nicole Sharp, 23 June 2018 - 02:45 PM.


#18 Celerondon

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Posted 23 June 2018 - 05:44 PM

I still think I might be better off with a Celestron Nexstar for an easy-to-use "grab-and-go" telescope.  But I think I might also want a manual mount, especially in cold or hot weather that might be bad for the expensive electronic mount (such as a January Lunar eclipse or a November Mercury transit).  I don't think a German equatorial would be difficult to operate, but I think it could be difficult to set up.  Hopefully that is just something that gets easier with experience though?  The Explore Scientific German equatorial mounts you can get for as little as $30 with an OTA, but I don't think they have a clockdrive available.

 

And yeah, a few minutes of lost footage from a six-hour video is probably only a minor inconvenience.  I think the camera (and the YouTube community) might appreciate it if the video is cut into one-hour segments.  I thought about using two telescopes and two cameras for the Mercury transit, one with full disc, and one as close-up, but I think it wouldn't be worth the effort since Mercury is so small.  It would be much more interesting to use a Mercury-tracking mount (as opposed to a Solar-tracking or sidereal-tracking one), and get a really close-up view I think at high magnification if that was possible with a small telescope on a lightweight altazimuth mount.

 

BTW, a good way to practice for the Mercury transit is to look for Solar transits of the ISS.  I think the ISS might actually be bigger than Mercury, but it moves a lot faster.  They don't happen very often though, unless you travel for it.

 

I learned some hard lessons in 2017.  I had wanted to film the whole thing on video then, but I had camera problems.  So I was able to share the view with random strangers due to the eclipse-glasses shortage, which wasn't so bad.  I want to be prepared for the Mercury transit though (they are more rare than Solar eclipses!) with a good tracking mount, a better view, and a better camera.  My eventual goal for a new telescope though is to try to develop a video telescope.  I want to use electronically assisted astronomy and a laptop or video monitor to view the telescope with.  That way lookey-loos can just watch the video monitor instead of pawing at the expensive optics, lol.

Great Nicole!  It seems like your previous experiences will give you a head start with the your Mercury transit preparations.  You are correct about learning how to use a GEM.  Fraunhofer knew what he was doing!  A GEM will become quite intuitive after you use it for a while.  Here is one my favorite telescope reviews (The GEM part begins on page 4). http://www.astrosurf...t_refractor.pdf  Mr. Struve was fortunate enough to obtain the first GEM!  

 

Ignore inexpensive mounts.  Really cheap GEMs are never suitable for what you want to do.  A lighter altazimuth mount would be fine for cold weather observing or grab-and-go astronomy but they are not suitable for serious photography.  

 

What is your price range?  Under $500, under $1000, and under $1500 yield increasingly capable systems which can approach or exceed your targets.  If you have more dollars dedicated to astronomy I am sure that your options will multiply accordingly.   Recent advertisements for the new LX85 series from Meade include this instrument.  https://www.meade.com/lx85-mak-6.html

 

If your ideal photo setup is a longer focal length telescope like a Mak-Cass mounted on a quality goto GEM, you might want to consider that Meade package or an AVX based alternative.



#19 Celerondon

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Posted 23 June 2018 - 06:04 PM

You may have something with that long focal length goal of yours, Nicole.  

 

Here is an example of the type of work that a beginner with a 6" Mak-Cass can do on a mid-sized goto GEM.  Dark Matters is a Cloudy Nights forum user.  https://www.cloudyni...x/#entry8645649

 

Click Dark Matter's Flickr link to see his images.



#20 Rusted

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Posted 24 June 2018 - 12:55 AM

You must expect Mercury to be really tiny and to move across the Sun's surface very slowly.

 

IMG_1826 mercury rsz 500.JPG

 

In an earlier Mercury transit, with high cloud, I had flat batteries in my camera, no time to recharge them and had to get back to work after lunch.

The batteries were brand new but I subsequently discovered they would not hold any charge at all. Taking batteries back for a refund 15 miles away will not provide much satisfaction if you miss capturing an astronomical event of the century!

 

Never take anything for granted if you haven't rehearsed as if the event was truly  live. Needless to say my "record" of that transit was indistinguishable from specks of dust. Preparation and practice are essential for such an event. Just pointing a telescope at the Sun is really difficult unless you are fully prepared. 

 

I once tried to look through the unprotected 9x50 finder telescope to find the Sun!!!! What an idiot!!! It was such a familiar habit after 50 years of looking at every other object up there. I didn't observe the Sun very much back then. All it needed was a scrap of Baader Solar Foil filter taped over a bit of cardboard tube to safely cover the lens but I hadn't even thought about that beforehand!

 

I now use a piece of card to make the telescope's shadow as round and small as it will go. I promise myself to make a proper "sun finder" eventually. It will save hours of messing about just trying to find the Sun. When fitted with a PROPER solar filter you can't see the sun [at all] until it is in the field of view. The rest of the sky is just plain black everywhere else when you look through the eyepiece.


Edited by Rusted, 24 June 2018 - 12:58 AM.


#21 555aaa

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Posted 25 June 2018 - 09:03 AM

Clock drives used to be purely mechanical, like a clock. As far as what the rates are, solar tracking is about 2.5 arc seconds per minute different from sidereal. The sun has an apparent diameter of about half a degree or 1800 arc seconds so that's about twelve minutes for the sun to move one diameter if you are tracking at sidereal rate. For Mars or the outer planets the rate is typically less than one arc second per minute.
  • Nicole Sharp likes this

#22 Nicole Sharp

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Posted 17 July 2018 - 11:15 AM

Just thinking here, but if I only wanted to do planetary, Lunar, and Solar imaging, then it sounds like I shouldn't need GOTO at all?  Just a decent German equatorial mount, preferably with a clock drive, so I can keep the planet in the field of view at a high magnification, during which I can film video for stacking?  Is there a particular effective focal length though at which I would begin to need GOTO tracking to keep a planet in the field of view at a very high magnification for planetary imaging?  E.g. a 3X Barlow on a 2700/180 Maksutov-Cassegrain OTA is over eight meters of effective focal length.  Would that be the kind of OTA where a noncomputerized equatorial mount would fail for?  My preference would probably be something like a 5" or 6" Mak though for planetary imaging.

 

Could a Mercury transit still be an exception to that rule for tracking with a GEM clockdrive at a high magnification?  The actual transit only occurs over about half a degree of sky though, so maybe it is not that bad?  But the polar alignment would not be very accurate in the daytime, using just a compass to estimate north.

 

The price difference between a clockdriven GEM and a go-to GEM might be enough to get a hydrogen-alpha Solar telescope for 2024.  :-o


Edited by Nicole Sharp, 17 July 2018 - 11:28 AM.


#23 photoracer18

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Posted 17 July 2018 - 02:36 PM

There is sidereal rate, king rate, lunar rate, and solar rate. Technically all the planets deviate from sidereal because they are not celestial objects but solar system objects. Some that are in the outer solar system still stay pretty close to sidereal because their apparent deviation from the rate is small. Basic clock drive does sidereal, or as close to it as the motor and gear speed can come.

What go-to does is allow much better tracking accuracy if the mount is not perfectly polar aligned (or is alt-azimuth, which is the definition of a mount that is as far from polar aligned as it can get). But once you have enough alignment objects added, the mount can figure out how to drive both axis motors to emulate a perfectly aligned GEM (but with field rotation as a side effect).

If you want perfect solar alignment and tracking you want to polar align it before dawn while you can see Polaris. Also the Solar rate is not the same as sidereal, like the lunar rate. With short focal length scopes this is not an issue but it can be with long focus scopes. Magnification increases the apparent speed of the object in the field of view. For that reason many who did imaging in the film era of only clock drives, used what is known as a drive corrector. This allowed the scope to track the other rates in addition to sidereal by producing a square/sine wave AC output to vary the speed of the AC synchronous drive.

 

If you have a dual axis clock drive with an auto-guider port you can make it keep the object in the center by guiding. Not possible with a single axis drive. In the old days we used guidescopes with our eyes and a dual axis controller to guide with. That was before digital guide cameras were developed.

Personally while I have go-to mounts I tend to prefer mounts just prior to go-to with precision quartz driven motors and auto-guider ports (also with encoders for push-to for finding things I don't already know). Unless I am doing public star-parties or outreach when I will gladly use one of my go-to mounts. Most GEM go-to mounts can also do clock-driven, or at least they used to be able to do it, in case you want to skip the alignment stage and DIY.


Edited by photoracer18, 17 July 2018 - 02:37 PM.


#24 Nippon

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Posted 21 July 2018 - 02:46 PM

The only goto EQ mount I know of that tracks or compensates in declination for poor polar alignment is the Vixen Sphinx and it was not well received here in the US. My AVX did not track in declination at all so with poor polar alignment it was no better than my dual axis non goto GM8 which also has king, lunar, solar and sidereal.


Edited by Nippon, 21 July 2018 - 02:46 PM.


#25 Nicole Sharp

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Posted 21 July 2018 - 02:51 PM

Was just looking at the Celestron Omni CG-4.  If you are using a 1900/152 Maksutov-Cassegrain OTA at prime focus with a 2X Barlow (so 3.8 meters in effective focal length), would that mount be able to track Mercury during a Solar transit for 6 hours with a clockdrive?  Presumably I would have to keep a close eye on it and make frequent adjustments.  But it should be doable?  If it does lose Mercury from the sidereal tracking, I should still be able to find it again?  Presumably I can also mount a small refractor with a Solar filter to use as a manual guidescope to find Mercury again if it is lost from the field of view of the Maksutov?


Edited by Nicole Sharp, 21 July 2018 - 02:51 PM.



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