Unguided Imaging

A lot of imagers (the majority seem to be portable) want to go unguided. This is easy enough with a short fl scope. Usually 450 to a bit less that 1000 mm in fl is where most start. There are a couple companies that make it reachable. Some imagers get it done the first try. You don’t hear from them as there is nothing wrong and they just image away but others (with the same equipment) have many issues.

The popular mounts for unguided are Software Bisque, Astro physics and 10 micron. All use modeling to accomplish unguided imaging. The mach 2 comes with absolute encoders (1100 and 1600 have them as an option). 10 micron has the same and you can order the Paramount MEII with encoders as well but the rest of the Bisque line up does not have absolute encoders (but work fine, the Taurus is an exception, the higher end models come with AE’s).

Once you get the modeling done (whole topic on its own as each manufacturers modeling setup is a bit different) you’re on to unguided but your stars are not round. They look good at 2 or 3 minutes but not at 5+ minutes, what to do. There are many things that can ruin an unguided image and you must check everything.

How long do you want to go? 

Unguided is not trivial even though some say it is.  Everything must be correct from the base to the seeing and even where you are pointing in the sky, refraction plays a part, high clouds, etc.

Make sure your PE corrects well (if an AE mount this is not applicable) and your model has few to no outliers which can be a result of some of the stuff below.

Even though every thing is tight sometimes it isn’t, you cant even feel it but any slight wobble can have a big effect at the arc second level.

Do you have thru the mount cables?  If not run correctly with a big enough loop and/or you try and jam as many as possible in that can be an issue.

Our permanent setups go 5 or so minutes unguided (Fl over 1500) when required other wise we guide (we have Paramount’s).  No thru the mount cables and the model is maxed for the horizon, 750 or so samples.  That is a high density model that lets ProTrack shine (RMS in the teens, polar alignment close enough, no reason to chase the numbers).  Some may say you don’t need that many samples (diminishing return) but it works for us where 150 or so was not what we wanted.

No CW hanging down at the end of the shaft (not a huge deal just like the weight close to the center). No riser blocks (if possible) and all cables sung but not tight to keep any tension down. 

High quality camera, FW, focuser, all ridged so nothing moves. Have found filters moving inside the FW, focuser tilting, rotator having the tiniest amount of play, etc. No visual backs with a camera nose piece or focuser held in by screw, this will usually never work, threaded, tight connections only. Your model should be able to show this and help you figure out what is going on.

The scope has been squared up (lots of messing around there (orthogonal) but its the little things that all add up.

Pour a yard of concrete all re bared up, bolt on a nice pier and add a rats nest at the top with 3 or 4 bolts and a plate to put your mount on.  It may be stable, or not, just another thing to look at but its hard to tell cause it feels tight.

Some examples

Here some set ups that may or may nor work well depending on how rigid they are, vibration suppression, repeatable flexure, etc.

Rat cage, works fine if set up correctly.
Rat cage, works fine if set up correctly.
Another example of a rat cage, many styles, not sure how these would work.
And another style, repeatable flexure?
Another cage with the pier bolted to a un isolated slab which may be connected to a house.

If any of the above setups are not stable or dont suppress vibration well or have flexure that is not repeatable (the models handle repeatable flexure fine), unguided (or guided sometimes) imaging can be a challenge. Of course there are imagers out there that have made all the above work just fine. These are just some points to get you thinking if you are having trouble getting round stars unguided.

The crazy stuff

It may sound like there is no way for any of the following to affect imaging but you must make sure if you have some of the following you can minimize them while imaging. These are just a few of the examples that were either learned by experience or someone else found and cured the problem on their setup. There are bound to be many more examples, keep an open mind.

Vibrations from roads, trucks hitting a bump in the road, a train half a mile away, dog running around, stereo in the house bumping, imaging from a deck or concrete that is not isolated (some pads are attached to the house), wind, wave on the shore, camera or attached computer fan or electronics, yours or the neighbors AC unit kicks on. Yes a lot of things make a difference and some have success even with some of these issues, each setup is different. Have seen a lot of good images from a deck but its not for everyone nor will everyone be successful from a deck.

Remember with unguided imaging, first make sure your raw PE is within spec, even with an AE mount you can still collect a raw log and see what the worm looks like. No jumps or anything unusual and you’re good. Dec is not a big deal, it never moves during unguided imaging unless you dither between images and technically your not imaging during the dither.

If you have a non AE mount make sure to train your PE. You will want under 2 arc seconds per pixel peak to peak so you’re under the (most) seeing and the PE is in the noise.

Once you are setup and imaging consistently unguided can be very nice but there is no ‘Push here’ to image button. You will more than likely have to put some time in to setup and trouble shooting to get things going well.