Tips

Skysensor 2000 three star alignment procedure

For best tracking performance of the Skysensor driven mount you should try to get a good three star alignment. In the beginning I often could not achieve a three star alignment. When pressing 'Align' for the third star the SS2K to my frustation simply 'confirmed' a TWO star alignment, and the tracking performance was unacceptable for CCD work. Now I have established a method that will always get a three star alignment.

First select your alignment stars. They should obviously be visible and they should form the corners of a roughly equilateral triangle. Avoid three stars on a line! They should also be as far as possible from each other, at least 30 degrees. I typically select two stars in the western hemiphere and then one in the eastern hemisphere. If you use an unaligned equatorial mode do a rough polar alignment. For the best result use a reticle eyepiece in your telescope to center the star precisely. Slew to the first chosen star. You will most likely miss it. Loosen the clutches of both the RA and declination axis and manually center the telescope on the star. Lock both clutches and do the final centering of the star with the SS2K. Make sure the last correction in RA is towards the west to eliminate any backlash. Press 'Align' until you hear the beep. Slew to the second star and center it using the SS2K controls (remember the last RA correction should be towards west). Press Align. You may want to slew back to the first star to check if it is still well centered. If not realign and then slew to the second star to check it again. Now slew to the third star and repeat. It should tell you that you have a '3 star' alignment.

If you have troubles getting a 3 star alignment despite following the above procedure you may be suffering from excessive backlash in your drive. The SS2K manual has instructions for adjusting the worm gears of the GP (DX) mount to minimize backlash.

If you use a 'Polar aligned mode' you can still do a 3 star alignment to correct for the residual error in alignment. It is however a requirement that your polar alignment is pretty good

Skysensor 2000 daytime alignment using the Sun

Warning:
The procedure described below involves pointing the telescope at the Sun. DO NOT try this if you don't follow proper precautions for observing the Sun. That means covering the telescope with an aperture solar filter made from a material intended for this purpose (e.g. Baader AstroSolar film). Also don't forget to cover the finder scope either with a similar filter or an opaque dust cap

There are quite a few objects, especially planets, that can be observed during daytime. In particular Mercury is often better observed in daylight when it is high in the sky than in the twilights when it sits in the poor seeing near the horizon. I have successfully observed Mercury on several occasions in bright sunshine where its looks like a miniature Moon with phases. Even the relatively faint Saturn is visible during daytime, although at low contrast. On one occasion in May 2002 I managed to observe all 5 bright planets, the Moon and several bright stars during the same observing session in the afternoon. ut how do you find a planet in that bright sky? There are a few options:
1) Do a good polar alignment the night before, leave the telescope (mount) outside and rely on setting circles or GOTO computer to seek the planet the next day, for example by offsetting from the Sun.
2) If the Moon is visible you could make a two point alignment using the Sun and the Moon (ALWAYS remember to mount the solar filter before slewing to the Sun!).
3) If you have a Vixen Skysensor 2000 it is also possible to do an alignment using only the Sun (this is a trick I discovered by accident). Here is how it works:

Do a rough polar alignment, e.g. with the aid of a compass. Mount the solar filter on the telescope and finder, put the telescope in the initial position and switch on the Skysensor. Select "Sun", press ENTER then GOTO. The telescope will probably miss the Sun. Center the telescope on the Sun using either the filter covered finder or by looking at the shadow of the telescope on the ground. When the telescope is centered in the eyepiece press ALIGN until it beeps. You get a one point alignment. Now wait at least 45 minutes and recenter the Sun using only the direction controls. Press ALIGN again and you should get a two point alignment. This should be good enough to find planets not too far from the Sun. Practice on easy targets like Venus before you try the more challenging objects like Mercury or Saturn.

If you can't find a "Sun" object in the SS2K main menu you have to enable the Sun as a selectable object (it is disabled by default for safety reasons). Select SETUP -> OBJECT SELECTION -> SUN OBSERVATION MENU and enable the Sun.

Another WARNING: Be careful when observing objects less than 15 degrees from the Sun (such as Venus near inferior conjunction). This applies especially to Schmidt-Cassegrain telescopes as the short tube allows sunlight at a small angle to hit the mirror and possible focus it just in front of the telescope (danger!). Use a dew shield to increase the effective tube length (it will also improve contrast due to less stray light). Never remove a solar filter before the telescope has been slewed to a safe position away from the Sun. Also think about where the telescope is pointed if you leave it for some time. I have once seen a big dobson telescope catching fire (or at least smoking badly) because the Sun had moved into "view"

Using MX5 high resolution mode

The Starlight Xpress MX5 cameras support a high resolution mode of 500x580 pixels. It is more difficult to use since the CCD chip has to be read out in an interlaced mode. However you will be rewarded with an increase in the vertical resolution of the image. The method described here assumes you are using MSB AstroArt with the Deinterlace plugin. For deep sky exposures select 'high res progressive' in camera settings. Then do your exposures as usual. Make a number of dark frames in the same mode (you cannot use 1x1 binned dark frames!). If necessary also make some flat field exposures, but for flats it is possible to rescale a 1x1 binned flat to 500x580 with good results. Do the preprosessing of all images as usual applying dark frames and flat fielding. You will probably notice an interlace pattern ('Venetian blind effect') of bright and dark lines in your stacked image. These are caused by the fact that the two fields are read out of the CCD chip at different times (the 2nd field continues to be exposed during the readout of the first field). It is here you will need the 'deinterlace' plugin (can be downloaded from the MSB web site). This plugin adjusts the relative brightness of the two fields by moving a horizontal slider. When the interlace patterns has been completely cancelled out close the plugin window and save your image. The final step is to resample the image. If your image is really sharp you can resample it to 777x580 pixels but more typically you will want to use the standard 500x373 format. Any other formats with a 4:3 ratio are also possible. If you compare this image to a similar image made in 1x1 binned mode you will notice a better vertical resolution.

Gradient removal in AstroArt 2

If you make CCD images in a severely light polluted sky you will often be suffering from a gradient in your image due to differential sky brightness (the horizon is much brighter than zenit). At the same time it is difficult to make a perfect flat field that will compensate for all vignetting and CCD sensitivity variations. Luckily you can most of the time clean up your image using the Gradient Removal plugin in AstroArt. I have used this technique a lot and would like to pass on some of the small tricks I have discovered.

First of all you need to install the plugin. It is available from the Astroart web site. Download and unzip it and drop the DLL file into the directory in which you installed Astroart.

Here is my gradient correction procedure:
First make a copy of your image (F2), then apply a strong median filter to this image (median N x N, radius 15 or so). This part is important as it smooths the background and gets rid of most stars (this is what I consider my invention here). Now open the gradient tool ('Tools' -> 'Plug In Commands' -> 'Gradiant Removal') and add a number of points to the plugin window. These points should be evenly distributed over the smoothed image carefully avoiding any area having nebulosity etc. I typically select 8 points around the periphery plus a couple of points in the interior unless there is too much nebulosity there. Do not select points too close to the edge, however, partly because the median filter may be less well behaved near the edge, partly because some CCDs are "hot" near the edge (my MX516 is notorious for this) although it is mostly cured by flat fielding. There is no need to select more than about 10 points. To see the artificial flat field that will be applied click 'Create flat field'. To actually correct the original image make that window active and click 'Correct Image'.

The plugin will only correct for 'global' gradients. It is no replacement for flat field correction, as it cannot handle sharp local gradients. It can to some degree correct mild vignetting but the typical vignetting is normally too strong to be completely eliminated. Dust donuts cannot be removed this way!

Making lunar mosaics

Large high resolution lunar mosaics can be fascinating as you can pan over the surface of the Moon like looking through a virtual eyepiece. However making such a mosaic is a tedious process and a few things can go wrong spoiling an attempt to make a mosaic from your raw material. Typical show-stoppers are: Changing exposure, changing sky transparency and the real killer: Missing parts! It is not difficult to get it right but you have to do the imaging in a systematical manner. The following method assumes you use a webcam but most of it also applies to other camera types like CCD cameras and video. 'West' and 'east' in this text refer to celestial coordinates, not to directions on the lunar surface where "west" is at the eastern side as seen from Earth.

1. Choose your weather. Only attempt making a mosaic if the sky is transparent. If there are any signs of cirrus or halo around the Moon wait until another night. As mosaics are usually made at moderate focal lengths you don't need a superb seeing for making a mosaic but abort if images look blurred

2. Polar align your mount such that you can track features on the Moon for some time. A computer controlled mount able to track the Moon is strongly recommended.

3. Rotate the camera such that the horizontal axis is precisely aligned to either right ascension (RA) or declination. Check the alignment by slewing the scope slowly in RA while monitoring features along the edge of the image. They should move parallel to the image edge

4. Use manual exposure control. Adjust exposure on the brightest part of the lunar surface such that bright parts look very bright but not overexposed (it is ok to overexpose Aristarchus due to its extreme brightness). Also check that there is a little bit of signal when the camera points towards the dark sky (to avoid clipping dark details at the terminator). When the exposure is correct don't change it any more. If the camera has white balance controls let it adjust the white balance on the lunar surface and then freeze the white balance.

5. Scan the lunar surface in a systematical order. I usually scan the surface in "row order", that is starting from the north scanning the surface from east to west row by row, see figure. Moving westward avoids back lash problems. It is very important to make some overlap between neighbouring images, first of all to prevent any accidental gaps but also because many CCD sensors have bright or dark zones at the edges that you want to crop away (you could also flat field correct your images but that is beyond the scope of this article).

6. Before moving to a new field fix your eye on a feature near the west end of the image, then slowly slew the scope in RA only until the same feature is near the east end of the image. When starting a new row first slew back in RA to the eastern edge, choose a detail near the south edge and then slew in declination while keeping an eye on that feature. If you don't have computer control of your mount the Moon will probably have drifted since you started that row (the Moon moves one diameter across the sky in about one hour). In that case check the position with the previously recorded image

7. With a webcam shoot about 10 second AVI at 5 frames per second for each field. This should allow enough frames to select good frames for stacking or a single best frame.

8. Process all your images identically and stitch them together in Photoshop, Gimp or whatever image processing program you prefer. When building the mosaic start with an image near the center (image 9 or 10 in the figure) and work your way towards the limb and terminator.

Don't be too ambitious in your first try at making a mosaic. Keep the focal length moderate, say 1000 mm or make a mosaic of only a part of the Moon. I usually stick to about 1200 mm partly due to the typical seeing and partly to keep the amount of work managable.

Home

Ole's Astronomy Site