Category: Astronomy

Mirror grinding

To keep the mirror edge from chipping and breaking we are putting a bevel on it. Most guides tell you to do that with a sharpening stone which is made from carborundum bonded into a stone-like material. I thought doing the bevel by hand with the stone was much too slow, so I tried it with a diamond-bit on a dremel:

This works much faster and a 1-2 mm bevel can be made in a few minutes. After the bevel grinding you see our 'grind-o-matic' machine. It's driven by a 90 W DC motor with a 30:1 gear-head connected to a 12 mm steel axle which supports an aluminium disk on which the mirror or tool sits. There's a sheet of plastic to keep the wood and floor dry.

After about 2 hours of grinding with nr. 60 carborundum we achieved 1 mm of sagitta. A drop of glycerol in the grinding slurry helps to avoid stiction between the mirror and the tool. For a 240 mm diameter F/6 mirror the target sagitta is 2.5 mm so there is still some work to do.

Starting to build a 240mm Newtonian

More aperture is better. I've started to build a 240 mm Newtonian with some mirror blanks from Tammilasi and knowledgeable help and grinding materials from Teknofokus.

The first task was to grind the 240 mm diameter (40 mm thick) borosilicate mirror blanks flat on each side. This took around 1.5 hours per side using 60-grit silicon carbide and grinding against a flat steel plate.

See also mirror grinding video by The Sky at Night/BBC.

Sun 22.4.2008

I got an OD=5 solar filter for my 80 mm refractor and took a first look at the sun this afternoon. Two sunspots are clearly visible. The large spot at 3 o'clock is dirt on the camera sensor. The apparent rotation period is supposed to be around 28 days, so by taking a picture each day it should be possible to make a time-lapse movie.

To really see structure in the sun requires a very narrow (~0.7 Ångström) bandpass filter centered around the H-alpha emission line at 656.28 nm. Unfortunately these range from expensive to very expensive...

M95, M96, and M105

A little left of Regulus and where Saturn is right now in Leo there's a group of three Messier objects: Two spiral galaxies M95 (magnitude 11.4), and M96 (magnitude 10.1), and an elliptical galaxy M105 (magnitude 10.2).

This is a stack of around ten 8 s exposures through an EF 70-200/4L set to 200mm and full aperture. Resolving details from the galaxies will require a longer focal length and much longer total exposure time. Focus is pretty good, and stars appear as points even in the corners - but the image shows severe vignetting. I need to learn how to use flat-frames to correct for that.

For those of you that don't spot the galaxies immediately from the pic above :), below I've indicated their positions:

Clear skies - Finally!

It's been cloudy almost every night for about three weeks now, but finally today a half-decent sky-watching night. The moon is full, so my moon pictures were predictably quite bland. I tried to search for some messier objects, but with a poor finder-scope and 25mm being the widest eyepiece that didn't work out too well (even with Stellarium on the laptop right beside me...). Here's the best of around 10 shots of Saturn.

102mm refractor, f=1000mm (~F/10), 2x Barlow, Canon 20D, 1/3 s exposure at ISO100

Barlow test

The big picture shows a normal photo through an f=1000mm F/9.8 telescope with a Canon 20D. The field of view should be around 1° 17‘ 20" according to this FOV calculator. The small frame is taken with an additional 2x Barlow lens between prime focus and the camera sensor. It ends up magnifying the image around 2.7- to 2.9-fold. I guess this could be tuned a little each way by inserting the Barlow differently in the focuser or with a T-ring standoff between the camera and the Barlow.

Something to try on the moon or bright planets once the skies are clear again...

Saturn

Had hoped to shoot the Pleiades with some longer exposures today, but clouds rolling in prevented that. So some snapshots at ISO100 and 1/4s of Saturn instead. These are 100% crops, so maybe I need to get an adapter for eyepiece-photography for shooting planets at higher magnification?

The clouds caused this round halo-effect around the moon. By 23:00 it was impossible to shoot stars.

Moon photo

Pooling our hardware resources together in the lab, we now have a 102 mm F/9.8 (f=1000 mm) refractor on an EQ6 equatorial mount and either a Canon 20D or a Canon 400D to shoot with. When one camera is coupled to the scope the other one can simultaneously take a wide-field photo. Did not bother with polar-aligning the mount today, so just looked visually at the moon, mars (it happened to be close to the moon), and M42. The moon is so bright no tracking is really needed.

Here is the moon through a 102mm F/9.8 (f=1000mm) refractor with a Canon 400D at prime focus, set to ISO400 and 1/160s. Around 21:40 local time on Friday 15 Feb 2008.

Astrophotography in Finland is a cold hobby, I was somewhat unprepared for the weather so around 60 min in -7 C was enough for me...