M81 (through clouds)

[JohnH]

Getting my excuses in first - I lost a lot of detail in my Ha and R frames because of high clouds, thick enough to interfere with guiding on occasion. The Moon was 78%.

The image shows M81 bottom left. There appear to be a lot of star forming Ha rich regions in the spiral arms. Oddly, I have read that it used to be thought that this was an elliptical galaxy but the arms appear clear even in my image.

M82 is on the bottom right - you can see the hydrogen gas disrupted by the gravity of M81.

With enough imagination you can make out NGC3077 (which is another spiral galaxy) top left.

Imaging was on 12th January from my back garden (Bortle 7).

Scope: Sharpstar 15028 HNT on iOptron CEM25P.
Camera: ZWO ASI 1600 Pro MM (Mono) with Asiair Pro.
Processing was PI with Topaz DeNoise and Topaz Sharpen.

Ha: 150 minutes of 120 second subframes (only 15 frames were really clear but I used most of the rest.)
R: 45 minutes of 45 second subframes.
G and B were 30 minutes each of 30 second subframes.

I used the Ha as the Luminance layer. I combined Ha and R at a ratio of 3:1 for the red layer.

Total time 3 hours 45 minutes.

Still have some odd shaped stars. I am checking literature, the scope should have a fl of 425mm, Plate Solve says it is 419mm. There may be issues with the real back focus of the flattener/reducer.

I keep telling myself I will improve!

Regards,

John

[Carole]

I can't believe you managed to get that from Bromley.  I always thought Bromley was Bortle 8, I am just up the road from you.  I certianly think we are bordering 7/8. 

You still have those weird looking stars and there must be something in the imaging train to cause that extra spike.  I am guessing at the dew heater?  Can you experiment and see what happens to the stars if you remove it? Then at least we may find out the cause.  I never used a dew heater with my Newtonian, or if I did I wrapped it around the ube near the secondary and relied on a long dew shield made of foam. 

Really well done. 

[JohnH]

Hi Carole,

Thanks. In fact, I had taken the dew heater out for these. I had imaged the same subject at the beginning of the month with the dew heater in, most of those images were spoiled by clouds and were rubbish. I had even tried curving the cable to prevent a spike but I think that that cause other problems. I hope that I can do without a heater, there is a school playing field behind me with a culverted river. If the temperature is low (ie a clear sky!) the humidity is high and the equipment is covered in moisture and/or ice by the time I take it in. Also, there are things sticking slightly into the tube as I look down, eg fixings for attachments to the tube.

I am rather groping around in the dark (metaphorically and literally) to get perfect back focus for the flattener/reducer in the system.

Another possible reason could be my filters which may not suite such a fast (f2. system.

I think that you are right and we are right on the border of Bortle 7 and 8. I got the figure from my weather App (Xasteria+). I keep meaning to see what the "Dark Sky Meter App" reads but never get around to it.

At least I have advanced in my use of the AsiAir Pro control computer. I have now learned how to frame the image and store the coordinates so that I can return to that precise location on subsequent occasions. I think that this is a major step in helping me acquire more data for a subject.

Regards,

John

John

[Carole]

there are things sticking slightly into the tube as I look down, eg fixings for attachments to the tube.

I think this might well be your star problem, there shouldn't be things protruding into the tube.

Carole

[JohnH]

If I were to stick felt patches over the intrusions into the tube (to smooth them out) would I be doing more harm than good?

John

[Carole]

Can you post up some photos (in another thread), showing us what you are referring to.  Looking down the tube including one strught down the tube, and also of the outside where these protrusions are fixed on.

Carole

[Carole]

Any chance of seeing either a single sub of this, or an unprocessed stack, I am wondering why the stars (in additon to the weird spikes) are elongated and also look like they have had something odd done to them in the processing.

Carole

[JohnH]

Hi Carole,

I am not sure if this his what you meant:



This is the Ha stack after calibration and cosmetic correction in WBP with the Screen Transfer Function applied via HT, and



Without.

I think that part of the weirdness occurred during noise reduction which didn't look quite right but I couldn't get rid of (the spikes made it difficult to create masks).

John

[Roberto]

Excellent image - and framing - regardless of the star issues.    You have a mixture of tilt and central obstruction.  Doesn't deter from the beauty of the galaxies.

Roberto

[JohnH]

Thanks Roberto,

I have been worried about the possibility of tilt. I have no idea how I can remedy it. The focuser has adjustable screws but I do not know how I can, realistically, remove it. II tried to create a grid of artificial stars with very fine optical fibre but it was not practical.

Regards,

John

[Carole]

Glad Roberto agrees with me about some sort of central obstruction. 

The single sub Looks OK (apart from the spike and elongation problem), so something odd is happening to the core of the stars during processing.  I know nothing about Pixinsight, so can't really advise on that unfortunately.

Hope some-one can help.

Carole

[Mac]

I tried to create a grid of artificial stars with very fine optical fibre but it was not practical.

Try just using a single fibre star and double diffraction grating at 90 degrees to get your grid.
If you make a home made start, use glass blowers glass (boro silicate) with a weight on and heat it with a blow torch to draw it out.
once drawn out, paint it matt black to keep the external breakout reflections to zero (adds a small bit of strength), then break the glass where its thinnest.
You can get some Very very small stars, which can be very bright if used with a laser.

I now use single mode fibre from work, 10um