Jump to content


Photo

Do you want to improve GoTo accuracy?

  • Please log in to reply
31 replies to this topic

#1 Lane

Lane

    Soyuz

  • -----
  • Posts: 3586
  • Joined: 19 Nov 2007
  • Loc: Frisco, Texas

Posted 08 April 2013 - 03:49 PM

Get yourself a wide field illuminated cross hair eyepiece.

In the past I have always aligned my mounts by just estimating visually when an alignment star is in the center of the eyepiece. I would sometimes find the star with a wide field EP and then switch to a more powerful EP to try to get as close as possible. But no matter what I did I always found myself having to nudge the view just a little one way or another after every GoTo operation. It was rare to get something centered well enough that I did not feel the need to move it a little or a lot.

Then I bought the 32mm Illuminated Cross Hair EP from University Optics last month. I have now had a chance to use it with all 3 of my computerized mounts and I have been blown away by the difference in accuracy. Over the weekend I used it with the CGEM for the first time. I made sure the mount was perfectly level before starting and then did a 2 star alignment with the addition of 4 calibration stars. Normally this would have put nearly every object in the FOV of a 22mm 82 degree eyepiece in my C11, which I considered good enough. But after aligning the CGEM with the new EP, I did around 50 GoTo operations before shutting down for the night. On all but 3 of those GoTo's the objects were perfectly centered. I know the coordinates of some objects are not exact so that is what I think caused problems for those 3 objects, but they were still within 10 arc minutes of the center.

I only wish I had thought to do this a long time ago. It saves so much time not having to center objects after every GoTo.

#2 Steve Daniel

Steve Daniel

    Vostok 1

  • -----
  • Posts: 124
  • Joined: 03 Jun 2012
  • Loc: Austin, TX

Posted 08 April 2013 - 04:04 PM

Yeah - what Lane said! I've been using a K23mm with crosshairs in my CPC800 for several months. Previous mysterious goto behaviors are about 99% gone.

Steve

#3 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 43401
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 08 April 2013 - 04:17 PM

Another technique is to defocus the alignment star until it nearly fills the field of view...

Jon

#4 Jarrod

Jarrod

    Apollo

  • *****
  • Posts: 1115
  • Joined: 20 Jan 2013
  • Loc: SE USA

Posted 08 April 2013 - 05:37 PM

Yeah I was testing myself the other night to see if I could get away with skipping the crosshair EP and just eyeball it in my 1.3* widefield. After my scope was tracking accurately I lined Sirius up by eye and swapped in the 12.5mm (0.4*) reticle EP to check how I did. No bueno :grin:

#5 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10491
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 08 April 2013 - 06:26 PM

I surely hope a successful alignment of GEM type mounts does NOT hinge on having the mount level! There is no inherent reason why this must be so; a polar aligned mount cares not at all what the orientation of the tripod is. The fundamental criterion of importance is that the polar axis is parallel to Earth's axis. Then, for GoTo pointing, that the real and instrument RA are coincident. These can be met with the tripod set up on a steep slope, or nailed to the side of a barn, for that matter.

If the mount is made to good precision (RA and Dec axes mutually perpendicular), and the scope's optical axis reliably passes through the mount's polar axis (departure here is called cone error), then a polar alignment via a borescope, followed by a single star alignment, would likely be better than obtained in most actual situations.

The requirement for multiple stars is so that software/firmware can calculate and compensate for the various and sundry errors in execution.

Perhaps the biggest one is cone error, and is the one you usually can do something about. One can significantly reduce this by positioning the scope in Dec until the minimum field motion is obtained when swung in RA. Then (for GEM mountings) adjust the OTA so that either the front or aft end is 'lifted' (by shimming, if necessary) until the optical axis coincides with the polar axis.

I would not be surprised if cone error in some cases approaches 1/2 degree. That alone would be an error most worthy of reduction. The result might well be a good reduction in the number of stars required for decent pointing accuracy.

For a friend's 14" LX200, I reduced his cone error, and what formerly were typical pointing errors of 12-15 arcminutes have been reduced to 3-4. And that's not using the "high precision" mode which requires first syncing on a nearby star. For his imaging activities, which he conducts from inside his house, this has made a world of difference.

#6 Phil Sherman

Phil Sherman

    Surveyor 1

  • -----
  • Posts: 1505
  • Joined: 07 Dec 2010
  • Loc: Cleveland, Ohio

Posted 08 April 2013 - 08:22 PM

A non-level mount will couple the azimuth and altitude adjustments when you try to use them to improve polar alignment. It's not really a problem because you can repeat the polar alignment routine to refine it further.

Levelling the mount makes polar alignment a lot easier. Imagine a mount whose North side is lower than the South side. As you adjust the azimuth adjustment, you'll also be changing the altitude. Remember, we're looking at wanting to make adjustments on the order of seconds of arc and on many (amateur astronomer, 8" diameter baseplate) mounts, 1/8 of a turn of the azimuth adjustment can easily move the mount about 0.7 minutes of arc. (Hopefully, I got my math right on this one.)

Adjusting the mount's position on a tripod, where there's a bit of play in the mount's rotating on the tripod just complicates polar alignment. Higher end mounts have finer threads on the alignment adjustments which facilitate accurate polar alignment. These mounts also come with a "fixed" baseplate that doesn't move on the tripod or pier when doing polar alignment. It's a lot easier to machine rotating parts to tight tolerances when you don't have to plan on them being disassembled and reassembled every time they're used.

Phil

#7 CharlesW

CharlesW

    Apollo

  • -----
  • Posts: 1221
  • Joined: 01 Nov 2012
  • Loc: Chula Vista & Indio, CA

Posted 08 April 2013 - 10:18 PM

+1 on this. I've been using the Meade 12mm reticle on all my alignments and it couldn't be easier or more accurate. I do find that if I defocus a little it's easier.

#8 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10491
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 08 April 2013 - 10:37 PM

Phil,
I can throw down my GEM mount on any ground and have Polaris dialed in within 5 minutes, 10 at most. A tripod tilt of even 10 degrees (or more, as long as stability is not compromised) imposes no difficulty.

It must be stressed that a polar alignment to an accuracy measured in arcseconds would require a lengthy drift-alignment session. And even then, no one polar altitude will apply over all parts of the sky due to refraction. In the real world, a polar alignment to 1-2 arcminutes is about all that can be expected from a borescope. But this us better than the 5 arcminutes a Celestron engineer stated is possible when relying only on the mount's controller routines using multiple stars.

If the borescope's rotating scale is used, the mount *should* be reasonably level in the E-W direction. But even if the mount were to lean to the L or R by 10 degrees, the induced error in azimuth is a pretty small 0.7 arcminutes. That's tantamount to being 'in the noise.'

My main point is to assure all that *precise*mount levelling by itself is in no way a prerequisite to good alignment and pointing accuracy. It's really extra effort for little to no gain, except for the more compulsive among us. :grin:

#9 Lane

Lane

    Soyuz

  • -----
  • Posts: 3586
  • Joined: 19 Nov 2007
  • Loc: Frisco, Texas

Posted 08 April 2013 - 11:02 PM

The manual says level the CGEM so I leveled it. I have always read that most GoTo computers don't care whether the scope is level, I don't know if that applies to the CGEM or not. But I do know that tracking will not be very good on the CGEM if you are not pretty close to level. I not only wanted to have the GoTo spot on, but I wanted the object to stay in the center for a while so I could let other people take a look.

Incidentally I did not use the cross hair eyepiece on Polaris I just roughly put it in the center of my 40mm Pentax. I only used the cross hair eyepiece on the alignment stars. This is visual viewing not AP so I really care nothing about getting a good polar alignment.

#10 George N

George N

    Mercury-Atlas

  • *****
  • Posts: 2729
  • Joined: 19 May 2006
  • Loc: Binghamton & Indian Lake NY

Posted 08 April 2013 - 11:21 PM

Get yourself a wide field illuminated cross hair eyepiece. ......

Then I bought the 32mm Illuminated Cross Hair EP from University Optics .....


I’ve found this to be true using my old 12mm guiding eyepiece to set up my CG-5 GoTo mount.

I’ve wondered if the same improvement would result from using a cross-hair eyepiece when initializing the NGC-MAX on my MI-250 GEM and my Dob??? However, that would definitely require a wide-field unit like the University Optics eyepiece.

#11 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10491
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 09 April 2013 - 03:15 AM

Lane,
Centering Polaris is perhaps as important as any other star if used like other stars in the computational routines.

Actually, I would prefer if all mounts could just not use Polaris at all, or at most only as a way to first get a rough alignment. It's so near the pole that mount and (especially!!!) cone error can result in the mount 'thinking' its polar axis is closer to the pole than it is. Of course, the numerous subsequent star alignments (nominally) compensates for this by modelling the very error. This is OK for visual, where pointing accuracy is paramount, but the potentially large polar misalignment could result in not insignificant Dec drift, limiting the duration of unguided astro-images (including piggybacking with telephoto lenses, not just through the scope.)

My take is that if something can be done to assure reliably better polar alignment *and* improved pointing accuracy, why not do it? Cone error is one area in which it pays to at least assess. If it's found to be no worse than about 10 arcminutes (5 is a better limit), you may 'safely' overlook it.

#12 Jon Isaacs

Jon Isaacs

    ISS

  • *****
  • Posts: 43401
  • Joined: 16 Jun 2004
  • Loc: San Diego and Boulevard, CA

Posted 09 April 2013 - 03:21 AM

But I do know that tracking will not be very good on the CGEM if you are not pretty close to leve



Is this something you have tested? Polar alignment does not require that the scope be level, it just requires that the RA axis of the scope be aligned with the earth's polar axis, the axis of rotation.

Jon

#13 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10491
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 09 April 2013 - 08:33 AM

To reiterate. Polar alignment for good tracking is not necessarily a requirement for pointing accuracy. The former can be well in error--with concomitant poor tracking--and not impair pointing accuracy if the mount's firmware has fully modelled this error (and others, such as non-perpendicularity and cone.)

I've read somewhere here a couple of months back a relaying from a Celestron engineer who stated that a mount-assisted polar alignment (presumably relying on what I consider to be a dubious use of Polaris) could be expected to not be better than 5 arcminutes. After using the all-sky (or some such name) alignment procedure. A polar borescope can handily best this. Levelling not required.

Does this mount permit the option of selecting alignment stars *after* you've boresighted on Polaris, and thus already have a pretty decent polar alignment? I ask because if the alignment procedure requires you to UNDO YOUR GOOD POLAR ALIGNMENT obtained via the borescope by now adjusting the mount in order to bring Polaris into the field center, that's not so nice.

(The just-outlined condition is what the LX200 mount imposes when you want to align it. After setting up a very accurate drift alignment, when it slews to where it 'thinks' Polaris is, it's best to hit ENTER without moving *a thing*; certainly not the alt-az adjustments, for then the hard won polar alignment is buggered.)

And so the best procedure, if available, is to polar align with the borescope, then run through the star alignment routine, *perhaps* leaving Polaris out of the equation, all while without adjusting the mount head whatsoever.

If the mount's axes are nicely perpendicular and the cone error is small, it may require to use no more than two strategically selected stars to realize good pointing. The very requirement to employ numerous stars in order to finally enjoy decent pointing is a clear sign of one or more of the three principal errors rearing their heads; polar axis alignment, axis perpendicularity and cone error. Not to say other gremlins aren't present, such as flexure/sagging/looseness and awful mirror flop on a Cat. But they can be easily checked for and minimized.

In order to use a polar borescope with confidence, it must be itself boresighted to the mount's polar axis. Instructions cannot be far away.

I've described the essence if the procedure to assess and correct cone error.

Mount axis perpendicularity can be assessed via creative use of a digital level (clinometer/inclinometer) resting on the OTA, while flipping and rotating the mount about. Or...

With a good polar alignment, find a pair of stars sharing the same RA (for the current epoch!) and which are very nearly equidistant from the celestial equator (e.g., one at +22.5 and the other at -22.5 Dec.) While the mount tracks, sight one star under the crosshairs, preferably oriented N-S/E-W. then slew to the other star, noting how far E or W of the crosshairs it lies. Go back and forth a few times to ensure consistency.

You MUST slew in Dec only! Therefore press just the Dec buttons, or perhaps better yet, unlock thecDec axis and move manually (if this can be done without inducing any unwanted shifting in RA and such.)

You can estimate the offset of the star from the crosshair, or if significant, drift time it (in seconds). The actual angle of this offset in degrees equals

Drift time / 239.3 * Cos(Dec)

For example, suppose the stars are at +/-22.5 degrees Dec, and the offset was drift timed as 4 seconds. The angular offset is

4 / 239.3 * Cos(22.5) = 0.0154 degrees, or 0.93 arcminutes.

Because we are sampling over a limited arc on the celestial sphere, this angle does not reflect the actual error in axis perpendicularity. We need the *true* angle at which the scope crosses a line of RA while passing through the equator. A rigorous treatment requires spherical trigonometry. But fear not! A simple approximation suffices for our purpose. The formula is

ATN (drift angle / Dec range), where ATN is the inverse tangent

We calculated our drift angle (for 4 seconds) as 0.0154 degrees, the stars' Dec range is +/-22.5, or 45 degrees. And so the approximate departure from axis perpendicularity is

ATN ( 0.0154 / 45 ) = 0.0196 degrees, or 1.18 arcminutes.

This approximation becomes more accurate the smaller is the Dec range for the chosen stars. But of course you want a reasonably large Dec range for increased sensitivity. I find stars at Dec +/- 10 degrees are fine, as you can still detect non-perpendicularity down to the hyper-fine level of better than 10 arcseconds (which requires estimating the star's offset from the crosshair, it being too small to drift time.)

If the axes are perpendicular to within 1-2 arcminutes, you're doing fine. You could predetermine the conditions in a pass/fail scenario, where for the selected stars (from planetarium software, and while reasonably near the meridian, preferably) you have precalculated a maximum drift time for your permissible departure from perpendicularity.

I know this is all details that might seem to be far beyond anything necessary. But you never know, do you? You might discover you have a gem of a mount, or one that warrants replacing...

#14 EFT

EFT

    Vendor - Deep Space Products

  • *****
  • Vendors
  • Posts: 2544
  • Joined: 07 May 2007
  • Loc: Phoenix, AZ

Posted 09 April 2013 - 09:38 AM

I have one of the big Stellarvue 32mm cross hair eyepieces and love it. I never liked the 12mm varieties very much because of the small field of view. I would never consider aligning without the cross hair eyepiece. But if you were stuck without one, then using a low power eyepiece to locate your star and then switching to a high power eyepiece defocused to center the star will certainly work. Just guessing at the center of the field is not likely to get you very good gotos.

#15 mountain monk

mountain monk

    Surveyor 1

  • *****
  • Posts: 1851
  • Joined: 06 Nov 2009
  • Loc: Grand Teton National Park

Posted 09 April 2013 - 10:10 AM

I too got the University Optics 32mm crosshair ocular last month and I've been using it in combination with a Nexus, SkySafari, and an iPad 3. A marvelous setup, stunningly accurate. It brings the graphic interface to observing; it leaves the Sky Commander and other pushbutton systems feeling like DOS. Just IMHO.

Dark skies for all beings.

Jack

#16 Lane

Lane

    Soyuz

  • -----
  • Posts: 3586
  • Joined: 19 Nov 2007
  • Loc: Frisco, Texas

Posted 09 April 2013 - 03:40 PM

I too own a 12mm cross hair eyepiece, but it is to narrow to use in the alignment routine. I don't want to have to use a wide angle to find the star and then change to the cross hair eyepiece to align the star. With the 32mm the FOV is wide enough so one eyepiece does the whole process.

#17 mich_al

mich_al

    Vanguard

  • *****
  • Posts: 2328
  • Joined: 10 May 2009
  • Loc: Rural central lower Michigan Yellow Skies

Posted 09 April 2013 - 03:44 PM

I've been using a Meade 9mm crosshair (about 250x) with wireless illumination. I get the star centered in the finder first then in the hi-mag EP. It works really good except the Meade wireless illuminator is a real POS. Poor quality and self destructed after less then 2 years use.

#18 Lane

Lane

    Soyuz

  • -----
  • Posts: 3586
  • Joined: 19 Nov 2007
  • Loc: Frisco, Texas

Posted 09 April 2013 - 03:46 PM

Polaris is not used in the computations on the CGEM unless you actually select it as one of the alignment stars.

So just having a rough alignment on polaris is good enough. And by rough I mean that it was probably off by less than 30 arc-minutes. Combined with a level mount, that was good enough to keep the object in the center for way longer than I needed, over 10 minutes at least.

This isn't AP I am doing here just visual viewing. I only need the GoTo to center the object and then have the mount hold it there for at least 5 minutes. By then I am off to the next object anyway.

#19 Lane

Lane

    Soyuz

  • -----
  • Posts: 3586
  • Joined: 19 Nov 2007
  • Loc: Frisco, Texas

Posted 09 April 2013 - 03:54 PM

Defocusing a star only works well in a smaller field of view and I need a wide field eyepiece to find the alignment star. I have tried defocusing before and did not see that much of an improvement. My ability to center the defocused star in the center of a wide field wasn't much better than my poor ability to center a sharp star in the center of a field.


The really big improvement was not actually on the CGEM but instead using this new cross hair eyepiece with my Sky Commander and push-to mount. You only get 2 stars to align with on Sky Commander and getting them both dead on is very important. It still is not nearly as accurate as the CGEM but it gets much closer now and allows me to keep a higher power eyepiece in the diagonal as I push the mount around from object to object.

#20 cn register 5

cn register 5

    Viking 1

  • -----
  • Posts: 760
  • Joined: 26 Dec 2012

Posted 09 April 2013 - 05:16 PM

My reticle EP is home made, a 25mm Plossl that cost me $12 from Orion in Cupertino with hairs (mine) super-glued across the field stop.

It works really well, especially with obstructed scopes because if you are a bit defocused you get a doughnut with a tiny bright spot in the centre - it's a diffraction effect. The doughnut shows the cross hairs and the bright spot gives an accurate reference. The trick is not to move the EP or refocus during the alignment process, this way errors in the positioning of the cross hairs don't matter.

Chris

#21 Stargaz18

Stargaz18

    Mariner 2

  • *****
  • Posts: 297
  • Joined: 04 Jan 2011
  • Loc: Wichita Falls, TX

Posted 09 April 2013 - 10:01 PM

I bought my illuminated EP at the Astronomics Grand Opening last year and like you have been blown away with the difference!

#22 gfeulner

gfeulner

    Mariner 2

  • *****
  • Posts: 259
  • Joined: 23 Nov 2007
  • Loc: Bergen county, New Jersey

Posted 09 April 2013 - 11:29 PM

Excellent advice! I recently bought an IOPTRON eq45. The GOTO'S were very good with only a one star alignment. Generally put the object in the field of my 32mm eyepiece. I did eyeball centering originally and then read your post. Tonight after careful polar alignment I did the one star alignment but this time I centered the star on my 12mm crosshair. The objects that I slewed to were either on the crosshair or very near it. This is at 160x on my c8. Thank you very much for the info. This will make GOTO'S much more enjoyable! Gerry

#23 GlennLeDrew

GlennLeDrew

    Voyager 1

  • *****
  • Posts: 10491
  • Joined: 17 Jun 2008
  • Loc: Ottawa, Ontario, Canada

Posted 10 April 2013 - 02:08 AM

That the IOPTRON mount delivers good pointing accuracy after an alignment procedure using but one star is a testament to its admirable precision of manufacture.

This is what I'm talking about!

#24 James Cunningham

James Cunningham

    Gemini

  • *****
  • Posts: 3224
  • Joined: 07 Aug 2010
  • Loc: Maryland

Posted 11 April 2013 - 02:33 PM

Glenn, can you put that description of how to get rid of cone error in laymans terminology. Thanks.
Jim

#25 astro_baby

astro_baby

    Viking 1

  • -----
  • Posts: 998
  • Joined: 17 Jun 2008
  • Loc: United Kingdom

Posted 11 April 2013 - 03:01 PM

As an alternative make sure your finderscope and main optics are well aligned and use the finderscope for the alignment routine......the finder has crosshairs in it for you.

Thats what I use and it works a charm. Improves GoTo no end.






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics