Methods for small telescopes

Introduction

Where to look?

Overview of methods at the telescope

Short is good

Stacking is the cheapest way to buy yourself better hardware...

(stacking techniques)

It's all in the timing...

Direct NEO Distance Determination by Parallax (added 30 Dec 2004)

Introduction

Performing NEO follow-up work often involves imaging fast moving (sometimes very fast moving) faint objects, so therefore to provide good astrometric positions:

• Exposures need to be short so that images are as near round as possible
• The total exposure needs to be as as long as possible to give a good S/N ratio (the stronger the better for measurement accuracy)
• The timing of the exposure needs to be good in absolute terms so that any error in the position introduced from the timing is negligible. For fast moving objects this means timing to better than one second.

These factors have led me to develop the following techniques to suit my equipment. Other observers with different equipment will no doubt have their own methods - these work for me.

Where to look?

Overview of methods at the telescope

A number of short exposures are taken and stacked together taking into account the object's motion. The pc clock is kept aligned with UT over the internet and software is used to ensure that exposures start as close as possible to an integer second.

Short is good

(Lots of) short exposures taken at sidereal rate have several advantages over longer exposures taken tracking the object:

• Often the original ephemeris prediction for a NEO is somewhat off, based on possibly a very short arc at discovery. Stacking short exposures means that if the original rate of motion gets revised significantly the images can be re-stacked with the correct motion allowing a more precise position to be measured. Taking a long exposure and tracking at the originally published rate does not have this advantage.
• When a NEO passes close to a field star the exposures where the object is at its closest can be left out when stacking, allowing a measure to be made. If a long, single exposure had been taken the field star would have ruined the image permanently.
• If there are other objects in the field of view (e.g. main belt asteroids, moving at different rates) these can be stacked independently to obtain better S/N ratio and reduce trailing, allowing all objects to be measured with good sharp images.

As a general rule I try and keep exposures short enough to stop the object trailing more than about the size of 1 pixel (or 3 arcsec when I'm using 2x2 binning) during the exposure. e.g. if the object is travelling at 30 arcsec per minute then I would expose for 1/10th of a minute or 6 seconds, to keep the trailing to 3 arcsec. The brightness of the object will dictate how many exposures need to be taken, so that the total integration when stacked is enough to record the object well.

For faster moving objects I normally use 2x2 binning to speed up image download. Exposures in these cases can be very short, sometimes only 1-2 seconds, so getting as many images as possible as the object crosses the frame can be very important and the shorter download speed can be critical.

Accuracy limitations of the LX200 mount and the advantages of re-stacking mean I normally limit all my exposures to a maximum of 30 seconds duration. Exposing longer than 30 seconds cause a small percentage of images to be slightly trailed.

Stacking is the cheapest way to buy yourself better hardware...

As the accuracy of astrometry is so dependent on the strength of the image being measured, I use Astrometrica to stack multiple exposures together to increase the signal strength. For $25 you can get results as if you had a telescope or CCD several times larger or more sensitive than you actually have.

Astrometrica provides functionality to:

• Stack multiple images together
• reduce astrometric positions
• provide magnitude information
• create a report file in the correct format for sending to the MPC
• Provide an overlay for any moving objects in the field of view (as long as the orbital elements have been downloaded)
• detect moving objects in a set of three  or more images 
• preview images prior to stacking to discard any that may have defects 

I strongly recommend Astrometrica to get the best from your equipment.

Check out the Stacking Techniques pages for more information on how to get the most out of Track & Stack.

It's all in the timing...

Good timing is vital when observing fast moving near-Earth objects and exposures need to be timed with an accuracy significantly better than ± 1 second for many objects during close approaches to Earth.

At Great Shefford timing accuracy of ± 0.04 seconds is achieved with the following combination of hardware and software, as of August 2021:

• Windows 10 64-bit operating system running on a 4-core i5 laptop with 8 Gb RAM
• 300 Mbps fibre broadband internet connection with Gb ethernet cable to the observatory
• Meinberg NTP Service and Monitor software
• Maxim DL image capture with shutter latency configured

See the Check system timing accuracy page for an overview of how the timing accuracy was measured.