Woodlands

Once you have focused your camera and centred an object there are two key things left to do to get that image. The first is deciding how long an exposure to take and the second is to keep the object steady on the CCD Camera by guiding the telescope for the duration of the image.

Exposure Duration

With all exposures, the aim is not to expose the image to the point where any pixels become saturated. This can be checked by looking at the histogram of the image at the end of the exposure. If there are fully saturated pixels then reduce the exposure time. You also need to check the histogram to ensure that you have used a long enough exposure to have captured an image with enough information for image processing to yield good results. The histogram should start off at a high peak (which is the sky background noise) and then tail off in an exponential curve towards 0. If you just have the high peak and little or no exponential curve then you need to increase your exposure duration.

On my set-up I have found that I need less than 1/1000th of a second for solar and lunar images, a few tenths of a second for planetary imaging and between 10 mins and over an hour for most deep sky objects depending on their brightness.

It is possible to sum several images of the same object in order to increase the effective exposure time and increase the signal to noise ratio of the image. For example if you took four 10 minute exposures of a galaxy and then summed them together you would have the equivalent of a 40 minute exposure with a signal to noise ratio twice as good as the original 10 minute exposure. Summing short images has the advantage of lessening the affects of RA drive tracking errors. On the downside each time you read an image from the CCD camera you introduce noise into the image. So summing four 10 minute images will introduce four times the readout noise into your final image than if you had taken one 40 minute image. Have a look at my globular cluster images where I have used a single 10 minute exposure for M13 and summed 3 three minute exposure for M3.

Guiding

Errors in polar alignment or in the RA tracking of the telescope drive can cause the object to shift position on the CCD chip during a long duration (several minutes) exposure. If these errors go uncorrected the result is a blurred image and star trails.

To correct the errors as they occur you first need to be able to see them happen and then have a means of altering the position of the OTA to compensate for the tracking error without touching the telescope. Touching the telescope to operate the manual RA or DEC controls would introduce a lot of vibration in the image and ruin it. On the LX10 the handcontroller and DEC motor allow for small corrections in RA and DEC to be made without touching the telescope. On the LX200 you can use its handcontroller to make small corrections in RA and DEC without touching the telescope. So now we have a method of correcting the errors, how do we see them happen? For this we can use an off-axis guider (OAG) or a separate guidescope.

An off-axis guider is a unit placed in the optical path of the OTA. It connects to the visual back of the OTA or the focal reducer. The flip mirror and CCD camera are then connected to the rear of the OAG. The OAG has a small prism which intercepts part of the light around the edge of the field of view of the telescope. The prism is small enough so that it does not intercept any of the light destined for the CCD camera. The intercepted light is diverted into an eyepiece. This allows the user to view a small portion of the sky at the same time as the camera is taking an image. The eyepiece used with the OAG needs to have an illuminated scale and is known as a reticle. To guide the telescope with the OAG you need to locate a star in the reticle eyepiece and watch its movement against the scale. If it moves in RA (left/right) then adjust the speed of the RA drive with the handcontoller until the star returns to its original position. If it moves in DEC (up down) then adjust the DEC of the OTA using the handcontroller. Most of the corrections should be in RA as precise polar alignment should have eliminated nearly all the DEC tracking error. I use the Celestron Radial Off-Axis Guider as it has several features which make it much easier to locate a star in the reticle. I use the Celestron Illuminated Microguide Eyepiece as the reticle.

A guidescope is a separate telescope (usually a refractor) mounted on the main telescope OTA. You use the reticle as the eyepiece for the guidescope. As with the OAG, you locate a star in the reticle and correct any movement against the scale using the handcontroller. I have successfully used my Lumicon 80mm Superfinder as a guidescope on the LX10 and shortly plan to try the ETX as a guidescope on the LX200.

There are many pros and cons you should consider when deciding whether to use an OAG or guidescope for guiding. These are discussed in depth in an article by Philip Perkins. As a summary, the advantages of an OAG are that it does not suffer from flexure (movement of the guidescope against the the OTA) and it can correct for image shift (movement of the image due to movements in the OTA mirror as the telescope tracks across the sky). The main disadvantage of the OAG is the difficulty in locating a guidestar in the small field of view afforded by the OAG prism. The advantage of the guidescope is that it is relatively easy to find a bright guidestar and the disadvantages are the possibility of flexure and an inability to correct for image shift.

With the LX200 it is possible to use an autoguider to automatically make the guide corrections to the RA and DEC during an exposure. An autoguider is essentially a CCD camera that is put in the place of the reticle eyepiece on the OAG or guidescope. The autoguider selects the brightest star in its FOV as the guidestar and takes an image every few seconds. The autoguider looks for movement in the guidestar from one image to the next and sends commands to the RA and DEC motors of the telescope to compensate for the movement. Clearly the main advantage of the autoguider is that you are freed up from the work of making the guide corrections yourself. However, the other advantage is that the autoguider can 'see' stars that are often too faint to be used for visual guiding. The SBIG ST-7E CCD camera can autoguide the LX200 at the same time as taking the image.

Autoguiding an LX200 or RCX400 produces good results (round sharp stars) at lower focal lengths (e.g. 1000mm or below). However, the accuracy of the mount on these telescopes even after PEC training does not yield very good results a the full focal length of the scope (2000mm or above). This is because the mount position can only be corrected every few seconds at best and any error in the tracking of the mount during the correction periods shows up in the images taken at longer focal lengths as large of elongated stars. It is not just the stars that are affected though. The fine detail in the object being imaged is also lost as it is smeared out over the CCD chip during the exposure. A very good solution to this problem is to use the AO-7 adaptive optics system from SBIG. This unit attaches between the CCD camera and the telescope. It has a mirror which reflects the light from the telescope onto the CCD chip. Very short exposures of a guide star (up to 40 images per second) are taken using the guide chip in the CCD camera and the AO-7 tilts its mirror between each exposure to compensate for any movement caused by poor tracking or atmospheric disturbance. If the guide star drifts too far over time for the mirror tilt to compensate, then the AO-7 sends pulses to the mount to adjust the guide star position back towards the centre of the mirror. The frequent and very accurate correction of the tracking that is possible with the AO-7 mirror means that inaccuracies in the tracking of mount are corrected as well as some effects atmospheric disturbance. You can see the difference between guiding with the AO-7 and the RCX400 in these images of M1.