Celestron AltAz GoTo vs Synscan

[kiloran]

I have dilemma. In the market for an AltAz GoTo mount and having difficulty deciding between Celestron (Nexstar SLT) & Skywatcher (Star Discovery). From what I've read the Celestron GoTo system is more fully-featured and easy to use, particularly for a newbie. On the other hand the Star Discovery mount has the FreedomFind gubbins which is going to be much more tolerant of my 7yo nudging the scope or the mount. Then there's also the fact the Star Discovery mount gets me more aperture - 150P vs the 130 SLT (or one of the other SLT range scopes)

I've used the Celestron system a couple of times using SkyAlign and have had a 50% success rate in getting the alignment accurate but I have no experience of Synscan. I'm reasonably proficient at spotting and naming what I guess will be Synscan alignment stars (Capella, Sirius, Betelgeuse, Rigel, Aldebaran, etc.) but I worry with cloudy skies that I don't have enough star-finding knowledge to find some of the less famous stars. Setup time is really important as I have an impatient and keen 7yo to keep occupied. His current scope is a Lidl special Bresser refractor which we'll carry on using for a while as he's quite attached to it. Ultimately either of the reflectors is going to be a big upgrade compared to this but I just can't decide about the mounts! We're only interested in visual and using GoTo merely to be able to find some of the trickier objects. He's a bit young to be teaching manual star-hopping. As such we just need to be able to reliably get an object in a 25mm eyepiece - once it is there I'll just sync the mount on it and let it track until we move on to the next target. Is Synscan going to be frustrating to use? All I've read about EQMod suggests to me that its almost not fit for purpose hence that software being written...

 

 

[chrisshillito]

EQMOD was written to purely to provide an ASCOM driver for the skywatcher EQ mounts for PC based using direct control of the mount. In order to do this we sought to replicated many of the functions that the synscan provides - this was done to provide a full featured driver not because the alternative synscan control was not fit for purpose. The synscan is perfectly fit for purpose. The accuracy of gotos are ultimately dependent on how sell you set the mount up in the first place and this applies equally to EQMOD as it does with Syncan.

Chris.

 

[Flyingfuzz]

I have the Skywatcher Star Discovery 150p and find the Synscan very easy to use. The 2 star alignment works surprisingly well. Inputting the date, time and location is easy. 

Synscan is really easy to use so do not be concerned.

The freedom find is fantastic. I have nudge the scope accidentally a few times, just reselect the object and goto, spot on.

[Geoff Lister]

I have the Synscan system on my Skymax 127 MCT and my Skyliner 250 Dobsonian. Both are AltAz, and relatively easy to use. At power-down, the software stores a lot of things that do not change often, plus, the date-last-used (USA MM/DD/YYYY format), location, time-zone, BST/GMT offset, but resets the time to 8pm at each power-up. You have to step through the display of each of these, after each power-up, until you get to the alignment stage (having re-entered the correct time).

The 2-star alignment is relatively straightforward, named star, or a suggested bright star in one of the 8 segments of the compass. I find it useful, with Venus showing well (clouds permitting) near dusk, to get first alignment on Venus, using manual slew, and then using the suggested stars for automatic slew to both of the alignment stars. The manual slew is easier if you start with the scope base (tripod or Dobsonian mount) and the scope tube level, and pointing north. The handset will tell you the azimuth and altitude angles of your chosen star. Most of the Synscan mounts have an altitude scale, and a compass will get you close-enough on azimuth, to zero in with the finder scope and eyepiece.

Depending on your location, some of the suggested stars will be behind a house, fence or tree, and, if you find that you have agreed to use an "invisible" star, then you have to re-start back at Venus again. Polaris is a good alternative, when the sky is a little darker, as it is always in the same place with respect to your garden.

Particularly when dusk falls before tea-time, I have found it useful to align the scope before tea, put covers on the optics, have tea and a warm-up, and then come out again, with the tracking holding the last item viewed, before tea, still nicely central in the eyepiece. If a bright planet is not visible at dusk, wait until you can see your first star with the naked eye, (or through your finder-scope if you know roughly where it should be)

I use external power supplies, mostly the 12V, 1.5 to 2A, plug-top types, sold for powering the stick-on LED strips. I put the supply on an extension lead; with the supply and socket in a closed cardboard box, to avoid dew problems. The battery packs, supplied with the Synscan mounts will give you several hours of normal observing from a set of alkaline cells, but you need to make sure that the battery-holder contacts are clean, as any corrosion will cause reliability problems. Rechargeable NiMH cells do not give you enough useful voltage, unless you substitute a holder with 10 (instead of 8) cells.

I hope that this answers some of your questions.

Regards,

Geoff

[The Admiral]

Just a heads up that if you were interested in the Celestron StarSense in order to make setting up easier, that the version for Skywatcher mounts is a lot more expensive than the one for Celestron mounts.

Also, Polaris isn't the best star for alignment with an Alt-Az mount as it doesn't go anywhere! You ideally need stars which are changing position rapidly with time in order for the mount to give the best precision, so avoid ones which are directly North or South.

Ian

[Flyingfuzz]

I am selling my Star Discovery 150p as I have just upgraded to a 9.25 - check it out in the for sale section.

[happy-kat]

I've found using my virtuoso mount with a v3 handset easy to use and the mount also has freedom find. The android syncanint application makes set up dead easy and between use the only data I change during setup is date and time as it remembers the other location stuff.

[Geoff Lister]

13 hours ago, The Admiral said:

Also, Polaris isn't the best star for alignment with an Alt-Az mount as it doesn't go anywhere! You ideally need stars which are changing position rapidly with time in order for the mount to give the best precision, so avoid ones which are directly North or South.

I am intrigued by this statement. Intuitively, with map reading, if you want to know where you are on the ground, you use the position of the "Bus-Stop", and not that of the "Bus". Thinking back, my most accurate 2-star alignments have been with the first star low in an E to SE direction, and the second, higher up in a SW to W direction.

I made a Google search on "2-star telescope alignment algorithms" and came up with http://www.cloudynights.com/topic/535938-goto-telescope-two-star-alignment-algorithm/ which, in turn, references the maths in http://www.geocities.jp/toshimi_taki/aim/aim.htm . It is many decades since I studied matrix manipulation, (I grew up with a slide-rule and 4-figure log tables), but if using Polaris leads to an ill-conditioned matrix, then small errors in centring the star, will result in a larger error in the final results.

Again, from a gut feeling, rather than scientific measurement, I find that tracking is more accurate if I align the second star as-soon-as-possible after the first. That's why I try to do my initial homework with Stellarium, to select the best target stars, before going out into the cold. I have recently added Celestron's Sky Portal App to my tablet, and will probably use this, "on the fly", to select alignment stars.

I await some cloud, and wind, free nights to improve my alignment technique.

Regards,

Geoff

[happy-kat]

Altaz synscan v3 has very different alignment star choice for optimum tracking then eq mounts. Don't have the page number to hand but it is no more then x degrees apart and no more then x altitude but best when x degrees.

[The Admiral]

3 hours ago, Geoff Lister said:

I am intrigued by this statement. Intuitively, with map reading, if you want to know where you are on the ground, you use the position of the "Bus-Stop", and not that of the "Bus". Thinking back, my most accurate 2-star alignments have been with the first star low in an E to SE direction, and the second, higher up in a SW to W direction.

I made a Google search on "2-star telescope alignment algorithms" and came up with http://www.cloudynights.com/topic/535938-goto-telescope-two-star-alignment-algorithm/ which, in turn, references the maths in http://www.geocities.jp/toshimi_taki/aim/aim.htm . It is many decades since I studied matrix manipulation, (I grew up with a slide-rule and 4-figure log tables), but if using Polaris leads to an ill-conditioned matrix, then small errors in centring the star, will result in a larger error in the final results.

Again, from a gut feeling, rather than scientific measurement, I find that tracking is more accurate if I align the second star as-soon-as-possible after the first. That's why I try to do my initial homework with Stellarium, to select the best target stars, before going out into the cold. I have recently added Celestron's Sky Portal App to my tablet, and will probably use this, "on the fly", to select alignment stars.

I await some cloud, and wind, free nights to improve my alignment technique.

Regards,

Geoff

Phew Geoff, I thought I was going to be challenged on the statement, and I'm not sure I could defend it . Still, I'll have a go! I like your analogy with buses, but of course in terrestrial terms, things do stay put, but we have the more complicated situation of needing to align with the moving buses!

This is a little off topic, so please bear with this diversion. Like you, it's many decades since I did any matrix mathematics, and my brain is now so addled with age that they don't really mean much! It's also very much a gut feeling too. I prefer to think in terms of pictures if I can, and my take on it, for what it is worth, is as follows.

If you think of the celestial hemisphere thus:

The AZ mount has in its memory the celestial map, but what it doesn't know is the relationship of this map to the observable heavens. In other words, it doesn't know which way it's pointing at any particular time and how out of alignment the diametric plane is compared with that of the heavens. This isn't quite the same for an EQ mount of course because it does at least know where the north celestial pole is. To characterise its alignment, the mount needs to know the 'tilt' of the diametric plane in two orthogonal directions, or at least be able to calculate what it is. And it needs to be able to locate stars with time. So, to my way of thinking, it's best to choose alignment stars which are reasonably close to 90° apart in azimuth. Clearly, if they were 180° apart the mount wouldn't be able to deduce anything about the tilt in the orthogonal direction. As to locating stars with time, it seems to me that the quicker the stars are moving in azimuth and altitude with time, the easier the mount is able to calibrate itself precisely, given that one is required to centre the alignment star in the FoV at a particular time (known by the mount). That is not saying that aligning on a star which isn't moving much with time cannot give a sufficiently high precision, it's just that the centring of the star would need to be ultra-precise in order to be able to achieve it. That is why I also try to choose alignment stars which are not close to north or south, because in those directions the altitude varies only slowly with time. I think Polaris is a particular case where as it moves so little with time it won't really provide sufficient information for the mount to adequately align. Strangely, I can't see anything in my Nexstar SE Alt-Az mount instructions that mention Polaris as being an unsatisfactory alignment star.

Like you, I've also wondered what effect the delay between aligning on the two stars makes. I've assumed that the mount is clever enough to be able to account for this, but I can't be sure. Then again, one can improve tracking by doing a third alignment subsequently, so I'm guessing that this is no different to adding the second star alignment to the first.

Anyway, make what you will of the above, bearing in mind that it certainly isn't a definitive guide, just my own conclusions.

Ian

[Filroden]

35 minutes ago, The Admiral said:

I've also wondered what effect the delay between aligning on the two stars makes

I think it has no effect at all. You could align the first star at 19:00 and the second at 22:00 and it would work so long as you don't move the mount in between. Of course, it couldn't track or goto between those two times! Think of it this way, you are telling the mount where it is pointing (a known star with known RA and Dec, at a specific time and date, from a specific location). You repeat this with another star at another time. The mount knows how far in alt and az it moved between these two frames of reference. It now has two complete frames of reference from which to calculate how it is aligned to the sky. Although the maths is harder because of the time difference, this is trivial to the mount. I think any further additional alignments help it improve its model by taking into account that the mount doesn't rotate perfectly and each alignment helps reduce the effects of these imperfections.

[Geoff Lister]

Thank you Ian & Ken for your feedback. Both explanations make good sense, and I hope that they reflect the philosophy adopted in the Synscan & Nexstar az-alt alignment software. I still feel that it should be possible to reduce some of the, small, errors if the mount's azimuth axis is truly horizontal. In any case, once I have selected my first alignment star, the handset tells me to slew to az and alt coordinates. By having the mount level, and starting from roughly north, I can set the alt angle using the handy scale on the mount top, and use a compass for the approximate az angle. This usually gets me the star in the finder. The second star is easier because the mount slews automatically to (roughly) the correct position.

The Synscan software has a PAE option to improve alignment, and thus tracking, in a small area of sky around a chosen object.

The Skymax tripod has a small bubble level. This is OK for the tripod, but I wondered if this translated up through the mount base, and thus into the az axis. I set the tripod's bubble central, and then fitted the mount, with the OTA reasonably level and then sat a spirit level on its top. I made some small alt adjustments to set the edge of the spirit level's bubble between two alignment marks. With a perfectly aligned az axis, this bubble offset (alt angle) should be maintained at all az angles. I then rotated the az axis 360 degrees. The bubble moved, indicating that the az axis was slightly off vertical. I made some trial and error adjustments to the tripod's legs until I had corrected the error. I then used my Swiss Army knife to adjust the bubble level, and then re-fixed it with superglue. I now have better vertical alignment.

Now I just need some clear skies, and a reduction in the gusty wind, to improve my alignment skills.

Geoff

[The Admiral]

That's going to some lengths Geoff! My Nexstar has a separate flat-topped tripod and I use a traditional spirit level in two perpendicular directions to adjust it to level. The small bubble level that came with the mount is hopelessly inaccurate and I don't think it gives the precision required. With the ordinary level it only takes a few mm adjustment of a tripod leg to significantly alter the bubble's position. But you are correct in that I have no idea how true the az axis is. But that is as far as I'm prepared to go, and I'm assuming that the alignment routine will account for any slight error.

I think it is also essential to be fairly accurate with the time setting too. I use a suitable website on my tablet giving me the time (www.time.is), which is probably about the best I can do, as I don't have a GPS input.

With my Nexstar I use a manual 2-star align, selecting my chosen stars from a list of stars, aligning to each in turn. I use a RDF initially and then home in using the magnified live-view feature of the camera.

"Now I just need some clear skies, and a reduction in the gusty wind, to improve my alignment skills." Hmm, that's the most frustrating part.

Ian

[kiloran]

Thanks all - I've decided to go with Synscan - in the form of @Flyingfuzz 's Star Discovery 150P

I think the two-star alignment process will be find - and acutally quicker than using Celestron's SkyAlign. I hadn't realised that in fact you only need to know one star with that method and I'm blessed with dark skies and relatively few obstructions so that should be fine. Plus I can always use mount in manual mode.

[The Admiral]

You can use one star align with Celestron, but as with all I think, you get better accuracy with more stars.

Ian

[kiloran]

Finally got to use the mount this evening and pleased to report that this relative novice had no issues with the Synscan approach. Surprisingly quick and with the added benefit of FreedomFind I'd have no hesitation recommending it to a beginner - particularly if you've got young children. My LO grabbed the telescope and FreedomFind re-aligned bang on despite him slewing it a good 30 degrees. Impressive stuff. It also happily tracked well enough to keep an object more or less centred in a 20mm eyepiece when I left it for 45 minutes.

[happy-kat]

That's a great report on your new mount.

To follow on from my previous comment synscan v3 manual pages 16 and 17, for Alt-azimuth mount:

• It is recommended that the altitude of the two alignment stars are between 15 and 60 degrees and the deviation of altitude is between 10 and 30 degrees.

• The azimuth deviation of the two alignment stars can be between 45 and 135 degrees, it is best to be close to 90 degrees.

 

Edit: opps that's for a different thread.
 

[Flyingfuzz]

Hi Kiloran, glad you got on well with the scope. Freedom Find is a huge advantage if the scope gets knocked at all and should be on every scope.

Don't forget you can manually move the scope to a different part of the sky and then let the goto do the final alignment. 

Good luck.

[kiloran]

3 hours ago, Flyingfuzz said:

Hi Kiloran, glad you got on well with the scope. Freedom Find is a huge advantage if the scope gets knocked at all and should be on every scope.

Don't forget you can manually move the scope to a different part of the sky and then let the goto do the final alignment. 

Good luck.

I really like this feature as it cuts down slewing time dramatically. Another tick in the box for Synscan over Celestron.

[happy-kat]

Got to use this freedom find tonight on the Virtuoso, wanted to check my camera balance on the elevation using so undid the clutches to test balance, did them back up and requested my target and the mount adjusted the camera to point at it again. It's cool.