Again Wrong Working Distance from Sensor - Messier M8 Lagoon Nebula

Seeing was very good on August 2nd with low Moon impact till UT24h. So ideal to make pictures of deepsky object. Based on my earlier learnings I changed the position of the flattener in order to increase the working distance between camera sensor and flattener. Unfortunately I did not measure the final distance. The result showed still aberration at the edged. Not so big as with the flattener just in front of the sensor; but still, it clearly can be seen.

So, I did some detailed measuring and found out the picture was taken with the flattener at a distance of 151mm. The pictures taken earlier had a working distance of 117.8mm; wich is below 5% of the ideal working distance according the technical data.

Focal plane of my camera is shown below :

The recommended distances from the M48x0.75 thread to the corrector via technical data is shown below. In principle this rule applies: the shorter the refractor´s focal length, the longer the working distance to the sensor has to be.

♦ focal length < 450 mm: 128 mm
♦ focal length 450-490 mm: 123 mm for my TLAPO 80/480 f/6
♦ focal length 500-550 mm: 118 mm
♦ focal length 560-590 mm: 116 mm
♦ focal length 600-690 mm: 113 mm
♦ focal length 700-800 mm: 111 mm
♦ focal length as from 800 mm: 108 mm

An underrun or an overrun of the distance of up to 5% has no visible effect on the sharpness in the field of sensors with formats up to APS-C. With larger sensors, the tolerance is reduced to 1-2%.

Mars TAL200K ASI224MC

Observation of Mars, one week after opposition. Mars was low above the south-southeast (SSE) horizon at an altitude of only 9.5°. It's still very bright with a magnitude of -2.7 and the diameter is now 24.28". As a reference, on July 9th, the planet was only 22.28", a difference of almost 10%.

Seeing was good and details on the surface were clearly seen. The surface showed clearly light and dark areas. I made a series of picutres which nicely showed clearly the rotation of the planet.

Observations are made with TAL200K and ASI224MC with Barlowx2. All pictures made with capture software SharpCap and capture are was reduced to 480x400. Stacking using DeepskyStacker. Final editing with CS4.

July Weather Summary

July 2018 was a very warm and very dry month. Two days of rain with only 3 l/m2 and an average temperature above 20°C.

Flattener correcting the view field - Importance of working distance.

Flattener at a distance of 10mm from sensor showing clearly aberration at the edges.

I was not aware about the importance of distance from the flattener to the sensor. Below pictures show the difference. The picture with Messier M56 was taken with the flattener fixed at a distance of 10mm. This pictures shows clearly aberration at all edges.  The aberration is almost gone on the picture with M39. For this picture the flattener was located at a distance of 100mm.
According to the technical data the ideal working distance to the sensor is depended of the focal length of the refractor :

♦ focal length < 450 mm: 128 mm
♦ focal length 450-490 mm: 123 mm
♦ focal length 500-550 mm: 118 mm
♦ focal length 560-590 mm: 116 mm
♦ focal length 600-690 mm: 113 mm
♦ focal length 700-800 mm: 111 mm
♦ focal length ab 800 mm: 108 mm

An underrun or an overrun of the distance of up to 5% has no visible effect on the sharpness in the field of sensors with formats up to APS-C. With larger sensors, the tolerance is reduced to 1-2%.

Flattener 100mm - Picture is sharp at the edges

M39, NGC7082, Minkowsky M1-79, Platais 1

Looking into more details around Messier M39, I found beside Platais 1, also NGC7082 and the Planetary Nebula Minkowsky M1-79 of Magnitude 13.9.

Open Cluster M39 and Platais 1

In constellation Cygnus the open cluster M39 and the faint open cluster Platais 1 can be observed. The pictures are taken with Nikon D7500 and refractor TLAPO 80/480 f/6, each picture ISO2000 and 30s exposed. Stacking using deepskystacker and limited editing with CS4.

Globular Cluster Messier M2

Globular Cluster Messier M2 is the most compact cluster in the Messier group. It's located in the constellation Aquarius at a distance of 40.000 lightyears from Earth. The cluster counts more then 1.500.000 stars.
The pictures are taken with Nikon D7500 and refractor TLAPO 80/480 f/6, each ISO800 and 30s. The moon was 83% impacting brightness of the picture. Stacking with deepskystacker and limited editing using CS4.