Near infrared deep sky observation by using a small refractor

 

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The present observation is conducted with a small astrograph FSQ-85ED (Takahashi) combined with a QSI-583 CCD camera. The telescope is equipped for the occasion with the focal reducer Takahashi X0.73 (final F/D = 3.9). The quality of the sky corresponds to urban conditions (Castanet-Tolosan observatory, in the close vicinity of the city of Toulouse - south of France).



The equipment used for this observation. The FSQ-85ED telescope is mounted on a G11.
The guiding camera is a model Atik314L+ oddapted of the focus QSI WSG system.

First an image of the star CW Leo - probably one of the first made by an amateur. This star is a very discrete object in visible part of the spectrum, but becomes the brightness object of the sky when it is observed with wavelength upper to 5 microns. It is located in the Leo constellation at coordinates (2000.0): RA = 9h47m57s, DEC = 13 16'44 ".

This is an ancient carbon star embedded in a very opaque dust cloud. It was discovered in 1969, and now well observed in professional observatories having infrared imaging capabilities in particular.

A ultra faint envelope in the visible can be detected by asking a very long time with best telescopes (a challenge for amateurs!). See for instance this picture made at CFHT:

http://www.cfht.hawaii.edu/Science/Astros/Imageofweek/ciw021000.html

(the light that illuminates the nebula comes from stars in our own galaxy, not from the central object itself. Dust cloud of CW Leo act as a mirrors for the other stars of the Galaxy).

Comparison between an image taken with the FSQ-85ED using only the visible spectrum (conventional RGB image) and a color image by adding a near infrared channel instead of the conventional filter R (a Schott RG850 filter):

The exposure time is 5 x 300 seconds in each filter. The infrared band is obtained with a filter RG850 (in place of the filter Astrodon R). As its name implies, this filter passes only the wavelengths above 850 nm (it is totally opaque to the eye). While the CW Leo is virtually invisible in the RGB image, it becomes obvious using near infrared light.

The document below shows the four negative images acquired in each filter:

A similar infrared observation was made on Messier 8 nebula (FSQ-85ED-583 + QSI camera + infrared spectral channel constructed unn RG850 filter):

IR sources in the heart of M8 are clearly visible in the IR image + V + B (bottom) - like red dots. These stars were almost invisible in conventional RGB images (top). Infrared sources are obscured by the presence of localized dust clouds between us and them. Exposures: B = 2 x 300 s, V s = 2 x 300, R = 4 x 160 s, IR = 5 x 300 s.

Here is another example of comparison aspect visible / infrared galaxy for galaxy Messier 51:

... here a color version by combined some colered filters:

The infrared image reveals areas of dust and has a diffuse appearance. Conversely, the image taken through a narrow filter isolating the red line of hydrogen,s well localized target, corresponding to regions of creation of stars (HII regions). Exposure times: Band B = 3 x 300 s, V band = 6 x 300 s, band R = 6 x 300 s, infrared band = 4 x 300 s, Halpha filter = 4 x 600 s.

This page shows the possibility to extract information of interest (detection of highly reddened stars by dust) by using a small instrument (85 mm diameter) and in difficult viewing conditions (high light pollution, mag. 2 are the faintest stars visible to the eye in the Messier 8). The use of a well corrected apochromatic refractors in the near infrared is a condition of operation success. The FSQ-85ED has this feature.
 

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