One caveat of all GOTO commands is their reliance on an accurately leveled & polar-aligned setup. This article doesn't advocate sloppy setup and I haven't tested it with a wildly unaligned, tilted mount but it is of great use, if you use a mobile setup and do a good job in getting it right but don't want to labor another hour to get from good to excellent (run drift-align).

As said in the introduction, this technique uses your existing imaging camera plus laptop, basically the same setup you already have in place, if you either use auto-guiding or control the camera from a PC to perform a sequence of exposures and review results on a "big" screen..

System requirements & costs -- the (additional) cost is $0 and if your laptop already can talk to your camera & telescope mount for auto-guiding, you are good to go. If not, adding two USB cables, one USB-to-RS232 adapter and maybe another RS232-to-mount cable is all you need. The second USB cable plugs into your camera, but you knew that
Disc-space requirements is a bit on the high side -- you need space to store all the stars in the sky. My setup uses 4.5GB, since I've downloaded very high-resolution maps. The narrower your telescope's FOV, the more detailed the maps need to be and thus the larger the installation. A SLR on a 700mm telescope has a wider FOV and requires a smaller set of maps than a 1/4" CCD on a 3500mm scope. For most laptops, 4.5GB of HD space isn't an issue.

How to obtain -- the tool goes by the odd name of "AstroTorilla" and the author explains the name because a tortilla wraps around the food you want to eat. Don't look to your nearby mexican restaurant to deliver this, instead head to SOURCEFORGE :
Choose the matching libraries -- I may have downloaded more than needed but with a 2350mm reflector + 1/2" CCD, the FOV is narrow. If in doubt, download more than needed. Later, when it comes to solving, narrowing down the FOV before starting to solve is way more important.
Follow the instructions to download & install -- that is pretty straight forward. If you want to peek under the hood, the Plate-Solver is a Linux-based astrometry tool and for it to work AstroTortilla installs a small Linux-to-Windows interface (Cygwin) in addition to the "tortilla" itself. All is automated. Download will take time -- especially if you choose a large set of maps.

How to use -- Don't get scared by the comment about the Linux-based astrometry tool. You don't have to deal with that -- that's where the tortilla wrapper comes in. The interface is a regular PC window with a number of entries you can adjust.

First things first -- top to bottom

FIRST you need to specify how to communicate with the mount.-- you can choose ASCOM and directly specify the mount's driver or choose ASCOM and use the generic hub. Using a hub has the advantage of enabling multiple programs to share access to the mount and that is what I want -- and with the GENERIC HUB that works very well. On the other hand, the POTH HUB isn't always reliable and tools like autoguiding use features, the POTH doesn't support. For solving & slew commands, I've used both options.
You can leave this blank / disconnected. The "current position" gives the solver a hint where to search but it isn't required. A wrong hint might even prolong the search.

SECOND you have to tell AstroTortilla how to acquire the images it should solve. It isn't picky, even allows you to choose files on a HD & screen capture. But for a more efficient use, a direct connection to a real camera and mount is recommended. AstroTortilla supports direct ASCOM camera drivers as well as using intermediaries like APT, Nebulosity & Maxim DL.
  • Currently I only use a Canon-SLR and APT is the only suitable option -- there are no ASCOM-drivers for any of the dSLRs
    update : APT + ASCOM CCD also works
  • It is unfortunate but Nikon SLR bodies don't find a lot of support in the astronomy area. You are not completely shut out but you will have to resort to specifying a JPG-file and repeat that step over & over again.
  • Nebulosity is a tool which combines image capture and post-processing and has built-in support for a number of popular astro-cameras (incl Canon SLR)
  • direct ASCOM camera control, skipping all the intermediate levels is possible-- but not for dSLR users

At the moment, I use a Canon SLR and have used APT successfully. Nebulosity + Canon SLR may be another option for me but APT has a couple of additional features I like, especially controlling the telescope mount and run Canon-exposures with different ISO and add mirror lock-up delays. Nebulosity has the edge over APT when it comes to image post-processing but its controls lack some of the SLR-specific options. The trial version of Nebulosity inserts diagonal strips into the image to encourage you to buy the full version That however, makes it impossible to test with AstroTortilla.
[update] APT plus a (one shot color) CCD-camera via ASCOM interface also is very successful in sending images to AstroTortilla and getting them solved.

THIRD -- setting the parameters for the solver. That's the part that can make or break your success with AstroTortilla and the settings shown above finally work with my camera and a variety of lenses. Solving now takes usually less than 30s. Occasionally ~120s. If you mess up focus, exposure or solver parameters, you can wait 30 minutes or more for the solver to finish crunching the numbers and still come up empty handed.

  • Be careful in setting the three "scale" parameters (min, max & unit) -- if you choose e.g. 0...360 degree you may cover all possible combinations but waste a lot of time. My setting is 10...360 arc-min. That's 1/6°...6° and this FOV covers a wide range of camera + scope combinations. Adjust according to your needs and keep it realistic.
    6° is approx 300mm on a DX sensor, 1/6° is narrower than the FOV of my CCD + reflector. CCDcalc, Stellarium or AstroPlanner, all can assist you with the FOV calculation.
  • JNow vs J2000 -- Currently, I use JNow but if I remember correctly, the important issue was to make sure all tools use THE SAME standard. CGEM & iEQ also have options to select which standard to follow. Differences are small.
  • In the custom options you may want to experiment with the "-sigma 50" setting. While it works for me, gradually increasing or decreasing that number can benefit your results.
    Keep an eye on the (brief) display of "number of stars found" that appears after the image has been acquired. Less than 100 isn't good, more than 500 may be too many, if you see over a thousand, your image is too noisy. After a couple of succeful attempts, you will see, which settings (incl exposure time & ISO) work best for you & your combinaton of lens & camera.
  • exposure time (and in turn ISO if you have a SLR, BIN2x2 with a CCD) -- if you use this tool to help you setup & align a mount, chances are good, your setup isn't 100% accurate (no offense, I have the same issue) ==> instead of using long exposures, find a good balance between higher ISO (noise) and elongated stars (misaligned, too long exposure). For this, I have no magic one-fits-all solution but near the NCP / polar alignment, stars move very little.
    For other plate solving tasks, exposure time can be critical as too elongated stars may not be solved.
  • very accurate or at least good focus -- a sure way to drive the solver crazy and waste lots of time waiting for useless results, is to capture images with stars not in focus. The solver can be picky and can keep you waiting 30+ minutes before coming up with .... nothing.

The combination of APT & AstroTortilla did a lot to improve my satisfaction with the astronomy setup I have and also helped me to accelerate good, mechanical polar alignment. avoiding long drift-align procedures or crawl down underneath the telescope to look through the polar-scope : /stargazer95050/28462461
Instead of using the polar-scope, I use APT + AstroTortilla. But this time, I set the mount into "zero" position and STOP THE MOUNT from tracking. Now, the telescope is supposed to point at the Celestial North Pole and the solver will calculate the offset. You can use the mechanical adjustments to improve the initial alignment -- with practice, you will even recognize the East-West tilt in your tripod legs and might adjust them : /stargazer95050/28534127

Here is an example of using APT + AstroTortilla -- video is in realtime therefore you need a little patience. But watch for the movement at the end. That's where AstroTortilla makes the necessary correction : /stargazer95050/28668501


[updated] 2014-05-25
1st release -- Dec 2013

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