Sunday, October 15, 2017

Double Cluster, Heart Nebula and Soul Nebula

This is the second image with my Pentax-based imaging system:

(click on image for full resolution image)
The three main objects in this image are the Double Cluster, the Soul Nebula (top) and Heart Nebula (below). They are all in the Perseus arm of our galaxy (earth is in the Orion arm) and in similar distance. The Double cluster is 7500 light years away, both nebulae 6500 light years.
The Double Cluster is relatively young (12.8 million years) and appears to be slightly blueshifted. This is a result of its movement - it races towards earth with 38/39 km/sec!
The cluster can be seen with the naked eye in really dark areas and easily with a binocular. It was discovered 130 B.C. by the greek astronomer Hipparcus.
Both nebulae are actually one gigantic complex that is 300 light years wide! They are connected by a bridge of gas. Both are birthplaces of stars in their center (which is why their centers are less red: a lot of gas has already been consumed by new stars). The stars in their centers are just a few million years old - and they are younger the further they are away from the center.

Processing this image was made difficult by the bloated stars in the LRGB images:
LuminanceHa

I compensated for this already in the linear state by shrinking the stars using the MorphologicalTransformation process in Pixinsight (again, using one of he awesome tutorials on lightvortexastronomy.com):

First, I created a "contour star mask" from the stretched image:

This star mask should cover exactly the stars. Here is how the inverted mask looks::

Now, we apply the MorphologicalTransformation process:

And here is the result before and after:
Before:After:

It's a subtle difference (which is good as we don't want to completely change the image) - but makes a huge difference further downstream.

Sunday, October 8, 2017

The North America Nebula

This is the first successful image of my Pentax-based imaging rig:
(click on image for full resolution)
Processing this image was a challenge - mostly because the imaging scale is so different then my images from my Takahashi TOA-130 scope. It has A LOT of stars and very little true background.

This nebula is four times the size of the full moon (which demonstrates the HUGE field of view of the 55mm lens!!!) The nebula and the Pelican Nebula (the lower, smaller nebula) are part of the same interstellar cloud of ionized hydrogen - which is forming stars. Between the nebulae and us are dust lanes that create the shape. We don't know for sure how far the nebula is away from us and what its dimensions are. Some speculate that Deneb (the very bright star in the lower part of the image and one of the brightest stars in our skies) ionizes this nebula. which would put it at a distance of 1800 light years and a diameter of 100 light years!

Comet Tracking

I read about C/2017 O1 ASAS-SN. As this comet is currently nice and high in the sky (at least in the second half of the night) I decided to give it a shot.

First, I just pointed the mount to the point in the sky where the comet is and started imaging:

(OK, ignore the dust motes for a second :-) But you can clearly see the comet in the middle right. Zoomed in:

Yep! There it is. But of course elongated as the scope is tracking the stars - not the comet.

So, I used Horizons (part of APCC Pro) to track the comet. I think Ray Gralak did a great job with Horizons. It's a little complicated at first to setup, but once you went through it once or twice, it's actually very easy!

With that, now, my images looked like this:

The comet is in the middle - and you can already see that the stars are elongated. Zoomed in:

:-(

It looks as if Horizons adjusts the mount only every few minutes (these are 10 minute exposures) ...

... on second thought, this could also be my PEC gone wrong. Tonight, I'll try 10 minute unguided exposures with and without PEC to see what happens ...

Sunday, October 1, 2017

A Pentax lens based imaging rig

1+ year ago I started to work on a Pentax lens based imaging system. I saw some wide angle shots that were made with 30 - 60mm lenses that looked amazing. And because of the resolution, these images can be taken without guiding and just a good enough pointing model. Almost "point and shoot" style.

Why Pentax lenses?
Pentax 67 lenses have three properties that make them suitable for astro-imaging:
  1. Backfocus
    These lenses have a backfocus of 84.95mm(!!!) - enough to put focuser and filter wheel between it and the camera
  2. Imaging circle
    These lenses have an imaging circle of almost 8cm(!!!) - more then what most CCD cameras have.
  3. Quality
    Being made by Pentax, the optics of these lenses is remarkable. Definitively good enough for astro-imaging.
I purchased a Pentax SMC 67 45mm F4 lens - it's very short and will create a LARGE image circle of more then 700 arc minutes!!!

Adapter

First, I had to get an adapter. I was planning to use a PDF focuser from FLI and the CFW2-7 filter wheel from FLI plus my MLx694 camera:

MLx694: 21.0 mm
CFW2-7: 20.9 mm
PDF: 29.84 mm
___________________
50.1 mm

That means that the adapter has to be 84.95mm - 71.74mm = 13.21mm thick - from the Pentax bayonet to the PDF focuser thread. Unfortunately, preciseparts can't make the adapter shorter than 24.00mm. I actually ordered the 24mm adapter, but couldn't achieve focus with it.

I checked some local companies who make adapters like these. But most didn't even get back to me - they probably only make adapters in MUCH larger numbers.

Next, I ordered a Pentax bayonet adapter with a different other side and tried to make one myself - but my skills on the lathe weren't nearly good enough to make one that is rigid and exact enough ...

A different approach would be to use a Robofocus focuser to actually turn the focuser of the lens itself. This would remove almost 30mm from the system and be plenty enough (see here for an example). But that sounded tricky as I would have to mount the focuser next to the lens stable enough that it can turn the lens. This didn't pass my test for a simple, robust system...

Finally, Richard borrowed me his adapter that he made for himself for the same purpose a few years back.


Problem #1 solved.

Mounting

Now, assembled, this is the whole imaging train:

My first attempt was to 3D-print an adapter to put this on top of a losmandy dovetail:


This worked, but it moved the whole thing pretty high up which made it less stable and also required A LOT of counterweight to balance it.

Luckily I realized that the holes in the FLI camera are exactly the same distance as the holes on the dovetail!!! That made it much easier to mount it!


First Light - weird halos

Finally, I could take out my camera and try it. This was the result:


Not too bad! But when you zoom in:

There are some weird halos around the stars. This turned out to be a major pain in the ...

First, I tried to use a UV cut filter. These lenses are very sensitive to UV light and it could be that this is the cause (though unlikely as this is a narrowband image, i.e. UV should already be filtered out).

Next, I thought that this might be stray light and put a lens cap on - same result.

I tried different f-settings - no improvement.

Finally, I took my setup to Richards ranch and we worked on it. He also couldn't find anything wrong. But he checked the setup of the lens. And in a diagram, he noticed that the last element of the lens is almost extruding from the lens and is curved. He suspected that what I see are reflections from the lens.

First Light 55mm lens

To check this, I purchased a PENTAX 67 SMC P 55mm F4 F/4 lens. I'm glad that I didn't go the route of using a Robofocus focuser, otherwise, I'd have to fuzz around with the setup as the focuser wheel of this lens is in a different position as the 45mm lens ...

First light:

Looking good! Zooming in:


Yei! No halos. Some fuzziness around the bright stars left - which is probably a result of the UV-sensitivity (I should get a UV cut filter for this lens).

Plate Solving

I want to use this rig with my MyT mount, so that I can build a pointing model and use that to track without a guidescope.

Sounds easy, but plate solving at this scale is a challenge!

Using the entire image usually doesn't work as there are WAY too many triangulations (it's kind of fun to watch this in TheSkyX which shows which frames are considered - it's jumping around like crazy!!!)

Pinpoint doesn't work at this scale at all to begin with.

Through a lot of trial and error, I determined that these are the best settings:

Filter: Luminance
Exposure: 8 sec
Binning: 2x2
Image Crop: 50%

One challenge remains if I image from out backyard: light pollution! Because of the relative small aperture, the stars don't stand out from the light-polluted background and are often missed. Nothing I can do about that :-(

Another challenge is that often earthly objects (trees, our house...) cover some of the image and create "artificial stars". Though the resulting images are sometimes really cool:

Focusing

This was (and still is) really tricky.

Because of the short focal length, the critical focus zone is 78 microns(!!!)

This means three challenges:
  1. Focusing has to be very exact
  2. The focuser can only be a little bit away from the focal point - otherwise the stars are so out of focus that they aren't recognized anymore.
  3. I have to refocus a lot during the night.
And of course, focusing routines also suffer from the same problem as plate solving: our light polluted backyard - which makes stars often not stand out enough ...

I am using the following settings (in SGPro):
Number of data points: 9
Step size: 70 (these two settings are the most sensitive ones: too small and I have to move the focuser too close to the focus point to start with plus it doesn't move out far enough to create a good v-curve, too large and the focuser moves too much out and SGPro doesn't recognize any stars)
Minimum Star Diameter: 6
Binning: 4x4
Exposure time: 20 sec for LRGB, 45 sec for narrowband filters

Flats

Taking flats is something I have not figured out at all:

  • Sky flats always have gradients (because of the large image circle). Even if I put a white t-shirt on it to diffuse the light - there are still gradients.
  • Flats with a flat panel create weird patterns (probably because of the proximity of the panel to the lens).
:-(

First Images

With all this figured out just in time for OSP, I took my first two images: the North America Nebula and the Double Cluster/Heart Nebula/Soul Nebula together. I took the LRGB data for both images at OSP and augmented it with narrowband data taken from our backyard.

Result

I have a lightweight, wide-field imaging rig!!!

But because of all the remaining instabilities in focusing, plate-solving, flats... it's pretty far from the point-and-shoot vision that I had :-( I'll keep trying, but maybe instead I should reconsider getting that awesome RH 200 Veloce from Officina Stellare :-)

Sunday, September 17, 2017

NGC 7822

NGC 7822 is a young, star-forming region in Cepheus - some of it's regions are no more then a few million years old. It is 40 light years across and lies 3000 light years away above our galaxy. Inside the region is a supernova remnant - which indicates that a massive star in it has already exploded. Also, it contains one of the hottest stars discovered near our sun - it has a surface temperature of 45000 Kelvin (the surface temperature of our sun is 5778 Kelvin). It's luminosity is about 100,000(!!!) times that of the sun.

(click for full resolution)
I took the LRGB data (4.5 hours) at OSP and Ha and OIII (13 hours) in our backyard in San Jose. I used the Ha to enhance Red and Luminance and OIII to enhance Green and Blue.

When processing this image, I ran into something that I haven't experienced before. After stacking, color correction, background extraction and enhancing the images with my narrowband data) I ended up with the following RGB and Luminance image (stretched):
But when I combined them (using LRGBCombination in Pixinsight), I got this:

Zooming in reveals that applying the Luminance image indeed increased the detail, but it took out almost all color!

I tried this several times, finally asked around. And it turns out that LRGBCombination only works on stretched images! I have no idea why I didn't encounter this before - I am sure I had done this before. But once I did this, everything else went smoothly!!!

Thursday, August 31, 2017

LBN 468

This nebula is 1,600 light years away from earth. One of the most interesting parts of the nebula is the bright part in the middle right. This nebula is called Gyulbudaghian's Nebula. It is a bipolar reflection nebula that is illuminated by a proto star.

(click on image for full resolution)
This is unfortunately only one half of the whole nebula. I wanted to take a mosaic of this but the last night at OSP didn't have good skies and I couldn't take enough data of the other half.

Eclipse Corona Image

Here is my main image from the solar eclipse:
(click on image for full resolution)

This was my biggest challeng: to process the 3 bracketed image sets that captured the corona of the sun. The corona has a very high dynamic range. My first tries to use any HRD algorithms (in Lightroom, Nik HDR Efex...) did not go very far.

I found a couple of useful tutorials on the web:
They all employ a similar workflow:
  1. Align all images precisely
  2. Create a composite image of the sum of all individual images
  3. Run a radial blur filter or a Larsen-Sekanina filter of the composite. This will create an image with all the detail but low contrast (mostly grey or black)
  4. Multiply the sum image with the detailed, low-contrast image
  5. Make final adjustments (stretch, curves...)
Fitswork has an interesting approach: pick two images, overlay by subtracting one from the other, this will make it easy to align them. It does work. ... but I found it too cumbersome. I was looking around for another solution and found the FFTRegistration (Fast-Fourier-Transformation) script in Pixinsight. This is often used to align comet images.


Enter a reference image, add all the images, I wanted to store the registered images, so I checked this and click "OK". ... takes a while and this was the result:

I took this image into FitsWorks and selected the Larsen-Sekanina Filter:

There are only two settings:

Rotation and Radius. The tutorial recommended to start with Radius=2.0 and Rotation=1.31. I chose those settings and got this result:

It has a lot of detail - and almost not other information (low contrast, wrong colors...)

But multiplying both images gives this result:

Doing a simple stretch:

And some curves, saturation and color adjustments:

And then some final tweeks in Lightroom (devignetting, cropping...)