Category Archives: Imaging train

TS Photoline 60mm FPL-53 doublet for wide field imaging

TS60ED-wI am now experimenting with a short focal length 60mm refractor for wide field imaging having given up on camera lenses. This is the Telescope Service Photoline 60mm compact apochromatic refractor in combination with the matching Photoline 0.79x reducer. The objective is a 60mm apochromatic ED doublet lens with a FPL-53 element by Ohara Japan. With a native focal length of 330mm the scope has a focal ratio of F/5.5. Using the Photoline 2 inch 0.79x 4-element reducer / corrector the focal length is reduced to 260mm and the scope is transformed into a F/4.2 astrograph.

A first light image from the scope of the relatively faint emission nebula IC1396 is shown below. This is a total integration of 2 hours (6 x 20 min sub-exposures) which indicates the potential of this scope for capturing extended nebula regions at great depth with speed.

Further details of the TS Photoline 60mm refractor can be found on the Telescope Service website. The specification of this scope is identical to the Stellavue SV60EDS and it provides an affordable alternative to the Takahashi FS-60CB.

Wide field deep sky astrophotography using camera lenses

QSI583-Pentax67-300-rig

My experiments using camera lenses for wide field deep sky astrophotography are coming to a close. I am not satisfied with the quality of images acquired. The main issue I have found is that stopping down the lenses to overcome optical aberrations and obtain useable stars reduces clear aperture which significantly affects the ability of the lens to gather enough light for extended faint objects.

The wide field rig I have been using is shown above. It is based on a QSI 583wsg mono CCD with lens adapter for Pentax 6×7 medium format lenses. The rig uses standard ADM mounting hardware (dovetails and guide scope rings) and a custom bracket manufactured by Lakeside Astro to support their ASCOM compliant motorised focussing system. The stepper motor drives a timing belt which goes around the focus ring of the lens allowing automatic focussing during an imaging session. The rig worked well using my standard imaging software stack of MaxIm DL and FocusMax under ACP control. Good V-curves for focussing were obtained for different lenses and produced accurate and repeatable results.

Two vintage SMC Pentax 6×7 lenses were tested – a 165mm F/2.8 and a 300mm F/4. Mint condition 1980’s versions of these lenses were purchased second hand from Japan for around £100 each. To produce useable stars both lenses were stopped down by two stops from wide open. This significantly reduced clear aperture for the 300mm lens and coma was still present in the corners of images. Higher quality results were obtained using the 165mm lens. My conclusion is that for deep sky work it would be worth spending a bit more and purchasing a good quality short focal length apochromatic refractor with ED glass.

See Jerry Lodriguss (Catching the Light) for a good and balanced review of camera lenses for astrophotography. High quality camera lenses are well suited for landscape astrophotography and there are numerous articles on the web on choosing and using lens for Milky Way photography.

Active Optics guiding with SXVR-H9 monochrome camera

As of January 2013 I am now using a Starlight Xpress SXVR-H9 monochrome camera for guiding in conjunction with my Starlight Xpress Active Optics (AO) unit.  The H9 camera uses the same SX Universal driver in MaxIm DL as the Lodestar which enables selection and control of an AO unit.

The H9 camera uses the sensitive Sony ICX285AL Exview HAD CCD with square 6.45um pixels and an image format of 1392 x 1040 pixels.  Operating at bin x2 the SXVR-H9 has a plate scale of 2.96 arc sec (compared to 3.85 arc sec for the Lodestar) with a field-of-view that is almost twice the area of the Lodestar.  Results so far suggest the increased resolution and larger field-of-view offer tangible benefits both to guiding precision and guide star selection.  I am now able to find guide stars more easily in the sparse star fields around galaxies.

SBIG ST-10XME with Active Optics guiding

In January 2012 I upgraded to an SBIG ST-10XME CCD camera and at the same time purchased a Starlight Xpress SXV-AO-LF active optics unit to improve guiding accuracy.

The ST-10XME has a quantum efficiency (QE) of over 85% between 560-665 nm which includes the Hα emission line at 656.3 nm. It is a no anti-blooming gate (NABG) sensor which accounts for its high QE but the negative side of this is that when a pixel saturates a bloom (or vertical streak) is produced in the resulting image. My rationale for choosing the ST-10XME was the additional sensitivity given high light pollution levels in central Bristol and my decision to operate the Megrez 120 at F/7.5 (900mm FL). I currently use the Wodaski MaxIm DL plug-in to de-bloom my images.

The SXV-AO-LF unit works very well under ACP control. ACP automates guide star selection which avoids the hot pixels in the Lodestar guider and optimises exposures to give short track times (< 0.5 seconds) and fast guide rates (≥ 4 Hz). It has transformed the performance of my EQ6 mount. I can now routinely achieve round stars (Roundness ≤ 0.05) and FWHM values ≤ 2 arc seconds under moderate to good seeing. During the galaxy season with sparse star fields I do sometimes revert to standard off-axis guiding when selecting a suitable guide star for the AO unit is sub-optimal.

Imaging | Guiding | Focusing

This was the setup for my imaging train during 2010-2011.

The imaging camera is the Atik 314L+.  The camera is based on the sensitive Sony ExView ICX285AL sensor with a 1392×1040 CCD array of 6.45µm pixels.  It has full set point temperature regulation (cooling to – 27°C below ambient) enabling the creation of matched dark frame sets for calibration.  Image acquisition is controlled by MaxIm DL.

I use an off-axis guider for guiding long exposures which are typically of 10-15 minutes duration.  The photograph shows the Telescope Service low profile off-axis guider with the Starlight Xpress Lodestar autoguider attached.  The Lodestar is based on the Sony ICX429AL sensor with 8.2µm x 8.4µm pixels.  It is incredibly sensitive and never fails to offer a choice of guide stars even though I often use 0.5 second exposures during guiding.  Guiding is performed under MaxIm DL control and I use pulse guiding to send autoguiding commands to the telescope.

The dual speed Crayford focuser on the Megrez 120 has been modified with a motorised focusing system from LakeSide Astro.  It comprises an ASCOM compliant control unit and a stepper motor assembly which is controlled using FocusMax linked to MaxIm DL.  Autofocusing with Focus Max is fast and extremely accurate and is the key to obtaining sharp and detailed high-resolution images.