Newtonian astrograph - f=585 mm, f/2.9

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astrofotografický speciál
astrofotografický speciál astrofotografický speciál astrofotografický speciál astrofotografický speciál astrofotografický speciál astrofotografický speciál
Telescope: astrograph (own design)
Optics: newton 200/800 mm
Corrector/Reducer: 2" ASA 2KORRR 0.73x
Focal lenght, speed: 585 mm, f/2.9
FOV with APS-C: 130' x 87'
FOV with KAF-8300: 106' x 79'

Currently, this is the main telescope which I use for deep space photography. The reason for using of this astrograph is a relatively poor value of focal ratio of Pentax SDHF (75/500 mm, see below), which I used till 2011. According to the fact, that high speed astrographs on the market are relatively expensive and lower priced class of scopes are mechanically very doubtful, I decided to make custom design and construction of the telescope. Presented scope is a classical Newtonian design, with a primary mirror D = 200 mm, f-ratio = 800 mm (Telescope Service), the secondary mirror with a small semi-axis 70 mm and 2" motor focuser JMI. As a corrector/reducer is used a product of the Austrian company ASA - 2CORRR with 0.73x focal lenght reduction.

During the design of the scope, emphasis was mainly on the sufficient rigidity of the whole structure. All parts except tube are made of high-quality EN AW-2030 T4 [AlCu4PbMg] alloy. Tube is made of 2 mm thick aluminium alloy. Inside of the tube is covered with black Velur sticker which is sprayed with matt black paint to minimize internal reflections from the tube walls. Primary mirror is held in the position with three point carrier. Scope is equipped with in-built heating againts dew problems during humid cold nights.

Pentax SDHF - f=500 mm, f/6.67

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Pentax SDHF
Pentax SDHF Pentax SDHF Pentax SDHF
Telescope: Pentax SDHF
Optics: APO refractor
Focal lenght, speed: f=500 mm, f/6.67
FOV with APS-C: 153' x 102'
FOV with KAF-8300: 123' x 93'

Pentax SDHF is apochromatic refractor with in-built field flattener, which draws a perfectly flat field and pin point stars acros 60 mm diameter plane. It is approximately three times the size of the APS-C sensor in DSLR. When used with a DSLR (chip size 22.2x14.8 mm) field of view is 2.54 ° x 1.70 °. Focuser is made very precisely, equipped with easy lock (see image above). My own personal experience is very good, lift moves very smoothly and easily. The stiffness of movement is well adjusted with the locking lever. Focusing is very precise without jumping or backslash. After the focus, position can be locked, so during the night does not travel anywhere. The advantage of the telescope is an excellent optical performance acros the field, at least in terms of APS-C size sensor (I've not tested it yet on larger sensor)and perfect mechanical design.

Some con's of the telescope. At first, very expensive accesories (2" adapter, focal reducer or tripod fixture). Poor chromatic aberation correction in extreme blue part of the spectrum. This is the issue only if CCD is used, where is high quantum eficiency in blue below 400 nm. With DSLR Canon this is not so bad. It could be clearly seen on the images below. Both images depict same area, but one is taken with CCD (GII-8300) and second with Canon EOS 300D.

Pentax SDHF

This scope did a lot of work for me when I started with astrophoto and now I use it as pointer for my astrograph (585 mm f/2.9) or for terestrial photography.