1. Introduction

The planet Neptune belongs to the remote giant planets. Observed from the Earth the planet Neptune has a very small angular size and with its diameterof 2.4 arcseconds it a real challenge for amateur astronomers. Visually it is a dim, pale blue tiny disk with very little detail.

However, in the past few years the situation has changed considerably. This is mainly due to the development of a new type of camera’s based on CMOS technology. These cameras exhibit high quantum yield, low read-out noise and a fast capturing speed and perform quite well recording details on Neptune. The reflection spectra of these planets show a number of methane absorption bands in the near-infrared. The high sensitivity of the CMOS cameras in the red and near infrared makes these cameras very suitable to detect albedo differences in the atmospheres of Uranus and Neptune in this spectral range.

2. Bright features on Neptune

Since 2013 amateur astrophotographers a.o. Anthony Wesley and Marc Delcroix have detected bright features on Uranus and Neptune using CMOS cameras in combination with red or infrared long pass filters. Interestingly, after having discovered a new bright spot on Neptune in July 2015, the professional astronomer Ricardo Hueso (Spain) invited the amateur imagers to see whether this features was also detectable in amateur telescopes. This lead to  many reports of amateurs that  this was indeed the case and their data were later used to determine the longitudinal drift of this feature in time. In the course of this observing campaign it became clear that imaging  of Uranus and Neptune may easily lead to artefacts and that to obtain  results of scientific value  a number of conditions should be fulfilled.

 a. Perfect collimation of the telescope.

If the telescope is not perfectly collimated over-processing of the images might easily lead to asymmetric white spots on a planetary disk.

 b. Artefacts by over-processing of the images.

Since the images of Neptune are rather dim, you need a high number of frames to increase the signal/noise ratio (3000-5000 frames)

 c. Confirmation

When a spot has been detected it is necessary to confirm the observation. There are several options for this:

  1. Make a number of consecutive recordings of the planet and  process them identically. A real spot should be visible in each individual stack.
  2. In addition, you can make two images directly after each other the first time with the camera in the normal position and the second time with the camera rotated 90º. If a detail is genuine it will show rotation when the changing position of the camera.
  3. Another control to test the true existence of putative details is to image the planet with a time interval of a couple of hours. If a detail is real, it will rotate with the planet. Uranus and Neptune rotates in about 16-17 hours, so in a few hours time the rotation of details must be detectable.
  4. Sometimes the independent images of different individuals can be used to obtain confirmation. 

d. Measuring of coordinates

 To establish the drift of bright features on  planetary disks the coordinates of these spots  should be measured accurately. The program WinJUPOS is an essential tool for this purpose. Positional measuring on the tiny disks of Uranus and Neptune is  not so easy. When the contrast  of their images is enhanced one easily loses  a bit of the rim and the disk is reduced in size. This hampers the accurate measurement of the coordinates. For Neptune this problem is easily overcome by using the position of Triton.

To determine the orientation of the Neptune the position of Triton should be known. In version 10.2.2 of WinJUPOS  Grischa Hahn and coworkers have included the prediction of the positions of this moon. Applying this position and the position  of the center of the Neptunian disk, each image with spots could be matched with the WinJUPOS simulation. Subsequently, the position of the spots can be determined. Since the image scale of Neptune is rather small, the standard deviation of the  measured coordinates is ±5º. 

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