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 of the target do not trail too much
• 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?

Information from the NEOCP is used to generate alternative and useful predictions at the following sites:

NEOfixer (independent predictions for NEOCP objects and prioritises observational urgency for NEOCP objects and all known NEOs)

Scout (independent predictions for NEOCP objects)

NEOScan (independent predictions for NEOCP objects)

Other lists of NEOs are available that are in need of further astrometry, e.g.:

Dates of Last Observation of NEOs (MPC)
Bright Recovery Opportunities (MPC)
New Priority List (NEODys)

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 UTC over the internet or using GPS systems.

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.

I generally use my CCD binned 2x2 which results in a pixel size of 2.1 arcsec square. When measuring with Astrometrica I use an aperture radius of 3 pixels, giving a diameter to fit the target into of about 13 arcsec. To get good centring exposures are kept short enough so that trailing is less than 13 arcsec and ideally somewhat shorter, say 6 arcsec, so that Astrometrica can achieve an accurate centroid. For an object moving at 30 arcsec per minute then 6 arcsec would be covered in 6/30*60 seconds = 12 seconds exposure. 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 and whether even shorter exposures may be able to be used to achieve less trailing.

For faster moving objects exposures can be very short, sometimes only a few seconds or less, so getting as many images as possible as the object crosses the frame can be very important and having a short 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 20 seconds duration. Exposing longer than this results in a percentage of images to be slightly trailed due to poor guiding.

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 EUR 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 February 2024:

• Windows 10 64-bit operating system running on a 4-core i5 laptop with 8 Gb RAM
• 400 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.