Category Archives: Informational

Informational

The Art of Exposure in Astrophotography: Short & Frequent vs. Long & Few

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Introduction
Artistic and scientific practises collide in astrophotography, which lets us capture the vast beauty of the cosmos from our own backyards. Exposure is a major factor in creating stunning photographs like these. Today, we’ll delve into one of the most contentious issues in astrophotography: the debate between taking fewer, longer exposures versus taking many shorter ones. Knowing the benefits and drawbacks of each technique will help you select the best one for your heavenly masterpiece.

Shorter But Higher Quantity Exposures

Pros:

  1. Mitigate Tracking Errors: Inaccuracies in tracking can be mitigated by using shorter exposures. Polar misalignment, periodic error, wind, and other disturbances can all cause minor deviations in the position of the stars, even with the best tracking mounts. These flaws will be less noticeable in the final product if the exposure time is shortened.
  2. Reduction of Overexposure: Overexposing bright objects or regions is avoided and detail is preserved by using shorter exposures. This is especially helpful in high-dynamic-range objects like nebulae, star clusters, and galaxies.
  3. Increased Flexibility: You can get more information out of your data if you take more pictures. Negative images caused by things like satellite trails or sudden atmospheric turbulence can be thrown out without having a major impact on the quality of the data as a whole.

Cons:

  1. Increased Noise: Noise, especially read noise from your camera, is more noticeable in short exposures. Image stacking and dark frame subtraction can help with this, but it’s still something to think about.
  2. Intensive Data Management: When taking many quick shots, you risk having too much information. The process of organising, storing, and processing such massive amounts of data can be laborious and error-prone.

Longer But Fewer Exposures

Pros:

  1. Better Signal-to-Noise Ratio (SNR): SNR is increased because longer exposures allow more light from faint deep-sky objects to be captured. Images benefit from being clearer, smoother, and more detailed, especially in low-light areas.
  2. Less Processing Required: If there are fewer frames in total, your workflow could be simpler and require fewer resources to complete. This is helpful regardless, but especially so when working with less robust computer hardware.
  3. Less Impact from Read Noise: Taking longer exposures results in less read noise from the camera sensor than taking a large number of shorter ones.

Cons:

  1. Risk of Overexposure: When using a long exposure, highlights risk being overexposed. Particularly in regions with bright stars or nebulae, this can cause a loss of fine detail.
  2. More Vulnerable to Tracking Errors: When using a long exposure, mistakes in tracking and atmospheric disturbances are more likely to occur. Stars will start to blur or trail if you move even slightly, ruining an otherwise perfect exposure.
  3. Less Room for Error: When there are fewer options, each photo is more precious. Unexpected problems (such as a plane crossing, sudden gusts of wind, etc.) that affect some frames have a greater effect on the overall image quality.

Conclusion
The equipment you’re using, the object you’re photographing, and the sky conditions all play a role in determining whether you should use shorter, more frequent exposures or longer, fewer ones. Keep in mind that neither method is better than the other. How well the strategy works to get you where you want to go is what matters.

Trying out different shutter speeds and exposures is always a good idea. After all, astrophotography is not just about getting the perfect shot; it’s also about the process of getting there. Keep in mind that the sky is always there for you to use as a blank canvas. Have fun with it!

Equipment, Informational

The CCD vs. CMOS Showdown: Why Monochrome Cameras Excel in Astrophotography Over One Shot Color

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Introduction
One of the first things photographers must decide when venturing into astrophotography is what kind of camera sensor they’ll need to capture the beauty of the cosmos. Charge-Coupled Device (CCD) and Complementary Metal Oxide Semiconductor (CMOS) image sensors are two of the most common on the market (CMOS). Each has its own set of pros and cons that make it better or worse for astrophotography in certain situations. Further complicating matters is the ongoing discussion between advocates of monochrome and one-shot colour cameras.

Understanding CCD and CMOS Sensors
Light is converted into electronic signals by the CCD or CMOS sensor at the centre of a digital camera. The image quality, cost, and power consumption are all impacted, but in fundamentally different ways.

CCD Sensors
CCDs have been the go-to sensors for astronomy photography for quite some time. They are well-known for the exceptional clarity and sensitivity to light of their photographs. These sensors generate low-noise, high-quality images by transferring charge across the chip and converting it into voltage in a single spot: the array’s corner. In turn, this improves light collection by allowing for a greater pixel fill-factor. CCDs, on the other hand, are typically more costly and power-hungry than their CMOS counterparts. In addition, they experience ‘blooming,’ an effect in which overcharged pixels leak their energy into neighbouring ones.

CMOS Sensors
In contrast, CMOS sensors have shorter processing times and use less power because light is converted to voltage right at each pixel’s location. They have lower manufacturing costs, making them common in smartphones and consumer-grade cameras. Their read noise and sensitivity are typically higher than that of CCDs, though. Recently developed technologies have allowed CMOS sensors to catch up to and even surpass CCDs in terms of performance, closing the gap between the two.

Monochrome vs. One Shot Colour Cameras
After settling on a CCD or CMOS camera, the next big decision in astrophotography is whether to use a monochrome or one-shot colour camera.

One Shot Color Cameras
As the name implies, a One Shot Color camera takes a complete colour picture with just one click of the shutter. The Bayer mosaic used in these cameras covers each pixel with red, green, and blue filters. The greatest benefit of these cameras lies in their ease of use. Even amateur astronomers can easily take stunning, colourful pictures of the night sky with these instruments.

Monochrome Cameras
Images taken with a monochrome camera are grayscale. These cameras capture red, green, and blue light through individual filters and combine them into full colour in post-production. Even though using a monochrome camera is more difficult and time-consuming, there are some benefits.

Why Monochrome Cameras Excel in Astrophotography
In general, monochrome cameras have higher sensitivities than single-shot colour ones. They are up to three times as sensitive as cameras that use a Bayer filter because all of the light that reaches the sensor is used to create the image. This heightened sensitivities is especially helpful in low-light astrophotography.

Additionally, more options and control can be had during the imaging process when separate filters are used with a monochrome camera. Using a hydrogen-alpha filter, astronomical photographers can isolate and emphasise specific wavelengths of light, such as the ionised hydrogen regions in nebulae. Imaging in light-polluted skies or capturing narrowband images greatly benefits from this ability.

Because the information for each colour channel is captured by the entire sensor rather than just a subset of pixels, as in one-shot colour cameras, the resulting images have greater resolution and detail.

Conclusion
In conclusion, CCD and CMOS sensors each have their uses in astrophotography, and the one you settle on will depend on your particular goals, financial constraints, and level of experience. Comparing monochrome and one-shot colour cameras, the former has better sensitivity, flexibility, and resolution while the latter is more user-friendly and saves time. Therefore, the investment in a monochrome camera and separate filters can be well worth it for serious astrophotographers seeking to capture the highest quality images.

Informational

Creating a Hubble Palette Image from OSC Dual Band Data

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Many people like myself have transitioned from a MONO camera to a One Shot Colour (OSC) for whatever reason, for me it was all about not being able to get the required amount of time due to weather conditions here in the UK. When I first considered moving to an OSC camera, it dawned on me that I would not be able to produce the vibrant Hubble Palette images that I could produce by imaging with specific filters on my MONO camera, specifically Hydrogen Alpha (Ha), Oxygen 3 (OIII) and Sulphur Dioxide 2 (SII) which would then be mapped to the appropriate colour channels when creating the final image stack.

Now along came Dual and Tri band narrowband filters for OSC cameras which peaked my attention, the Dual Band filters allow Ha and OIII data to pass, the Tri Band filters allow Ha, Hb (Hydrogen Beta) and OIII to pass but at a high Nm value. I reached out to my friends at Optolong who had two filters, the L-eNhance and the L-eXtreme, the L-eNhance is a Tri Band filter, but after speaking with Optolong it would not work well for me at F2.8, so I went with the L-eXtreme Dual Band filter which has both the Ha and OIII at 7nm.

After receinving my ASI6200MC Pro, I decided to start acquiring data on a 1/2 to 2/3 moonlit nights on the North America Nebula, and so far when writing this post I had acquired a total of 60 frames of 300 seconds each at a gain value of 100, I processed the image my normal way in PixInsight and below is the result of the image:

North America Nebula, 60x300S at Gain100, Darks, Flats and BIAS frames applied with the ASI6200MC Pro using the Optolong L-eXtreme Dual Band 2″ Filter

I thought that my data looks good enough to work with and experiment with trying to build an SHO (Hubble Palette) image with, and I have spoken with Shawn Nielsen on this exact subject a few times so he gave me some hints and tips especially with the blending of the channels. So off I went to try and produce an SHO image.

Before we start, there are some requirements:

  • This tutorial uses PixInsight, I am not sure how you would acomplish this with Photoshop since I have not used PhotoShop for Astro Image Processing for a number of years
  • Data captured with a One Shot Color (OSC) camera using a Dual or Tri Band Narrowband filter
  • Image is non-linear…so fully processed

Step 1 – Split the Channels

In order to re-assign the channels, you have to split the normal image into Red, Green and Blue channels, I found this to work better on a fully processed “Non-Linear” image as above, once this was done, I renamed the images in PixInsight to “Ha” – Red Channel, “OIII” – Blue Channel and “SII” – Green Channel, this makes it easier for Pixelmath in PixInsight to work with the image names. Once this was done, I used PixelMath to create a new image stack with the channels assigned, and this is how PixelMath was configured

Red Channel = SII
Green Channel = 0.8*Ha + 0.2*OIII
Blue Channel = OIII

Once applied this produced the following image stack (do not close the Ha, OIII or SII images, you will need these later on):

SHO Combined image from PixelMath

Step 2 – Reduce Magenta saturation

As you can see from the above image, some of the brighter stars have a magenta hue around them, so to reduce this, I use the ColorMask plugin in PixInsight (You will need to download this), and selected Magenta

ColorMask tool with Magenta selected

When you click on OK, it will create the Magenta Mask which would look something like this:

Once the mask has been applied to the image, I then use Curves Transformation to reduce the saturation which will reduce the Magenta in the image


The result in reducing the magenta can be seen in this image, you will notice there is now no longer a hue around the brighter stars

Result after Magenta Saturation reduced using Magenta ColorMask and Curves Transformation

Step 4 – ColorMask – Green


Again using the Color Mask tool, I want to select the green channel, as we will want to manipulate most of the green here to red, so again ColorMask:

This then produced a mask that looks like the following:

Step 5 – Manipulate the Green Data

Once the Green Mask has been applied to the image, since most of the data in the image is green, we are looking to manipulate that data to turn it golden yellow, so for this we use the Curves Transformation again

The above Curves transformation was applied to the image three times whilst the the green mask was still im place, and this resulted in the following image changes:

Resulting image after green data manipulated in the red channel using Curves Transformation

So as you can see we are starting to see the vibrant colours associated with Hubble Palette images

Step 6 – Create a Starless version of the OIII Data

Now remember I said not to close out the separated channel images, this is because we are going to want ot bring out the blue in the image without affecting the stars, so for this we will turn the OIII image into a starless version by using the StarNet tool in PixInsight

Here’s the OIII Image before we apply StarNet star removal:

Default settings used in the StarNet process

This resulted in the following OIII image with no stars:

Step 7 – Range Selection on OIII Data

Because we do not want to affect the whole image, we will use the range selection tool on the starless OIII image to select areas we wish to manipulate, now we have to be careful that the changes we make are not too “Sharp” that they cause blotchy areas, so within the range selection tool, not only do we change the upper limit to suit the range we want to create the mask for, but we also need to change the fuzziness and smoothness settings to make it more blended, these are the setings I used:


Which resulted in the following range mask

Step 8 – Bring out the Blue with Curves Transformation

We apply the Range Mask to the SHO Image so that we can bring out the Blue in the section of the nebula where the OIII resides, with the range mask applied we will use the Curves Transformation Process again as follows:

Curves transformation process to increase blue, reduce red and increase saturation of image with rangemask applied

The result of which is:

Result after first curves transformation with RangeMask applied

As you can see we have started to bring out the blue data, but we are not quite there yet, with the range mask still applied, we will go again with the curves transformation only this time, just reducing the red element:


The result of the 2nd curves transformation with the Range Mask is as follows:

Resulting image after 2nd pass with Curves Transformation to remove the red elemtn in the range mask

Step 9 – Apply Saturation against a luminance mask

On the above image, we extract out the luminance and apply as a mask to the image, and we then use the Curves Transformation for the final time to boost the saturation to the luminance

Luminance Mask to be applied to image
Curves Transformation with Luminance Mask applied

Final Image

I repeated the same process on my Elephant’s Trunk Nebula that I acquired the data when testing out the ASI2400MC Pro and this was the resulting image:

I hope this tutorial helps in producing your SHO images from your OSC Narrowband images, I know many of my followers have been waiting for me to write this up, so enjoy and share.

Equipment, Gear Reviews, Informational

Sharpstar 20032PNT F3.2 Paraboloid Astrograph Review

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Having owned the Sharpstar 15028HNT, I decided I wanted a larger light bucket without really sacrificing on speed, so I opted for the big brother of the 15028HNT which is the SharpStar 20032PNT.

I picked up my 20032PNT from Zoltan at 365Astronomy, and could not wait to get it home and unbox it, so after removing it from not just one carboard box, but two, I was presented with a very large flight case, which evidently is a larger version than the one that came with the 15028HNT.

Once I had the scope unpacked and inspected everything, the first thing I noticed was the focuser, the 20032PNT has a large focuser, which is big enough to accomodate the reducer/corrector that has an M68 connector thread as well as an M54 and an M48 connector thread.

The scope is well built, as I would expect from the build quality of the 15028HNT, the red annodised alluminium tube rings just give that final touch of finese. The 3 inch focuser is very smooth, and will no doubt be able to handle quite a load of equipment.

The first thing I planned to do was ensure that the primary mirror was secure and did not rock back and forward as well as replace the stock fan. I have fed back to SharpStar that they should mount the fan externally and also mount it with shock absorbing rubber mounters, and have the airflow into the tube from the back, rather than drawing air down the tube from the secondary. Here are my images of the fan replacement:

Primary Mirror assembly removed from OTA
Fan assembly with mirror removed
Stock Fan

Stock fan removed and added in a PWM fan connector should the fan ever need to be replaced, it can be replaced without removing the mirror assembly
Anti vibration fan mounting points
External fan connected to PWM connector
How the fan looks from the outside, the image is missing the fan filter which I added afterwards

Once everything was back together, I mounted my Eagle4Pro onto the top bar, as well as added an extra long losmandy plate because I wanted the OTA as far forward as I could get it in order to have the camera in the right location without it hitting the mount at all.

And here is the scope on the mighty EQ8 Pro mount

My first set of image testing did not go so well. My previous 15028HNT did not protrude above the walls of the observatory, so despite the fact that the secondary mirrors on both scopes are right up at the top of the tube, the 20032PNT was picking up stray light from my neighbour, so I had to adopt a dew shield that would extend the OTA by around 5 inches:

Scope with dew shield attached

Flats
I first started to have issues with my flats that was taken with a flat panel, the flat frames would “Overcorrect” the images, but one thing I noticed that there was a lot of vignetting happening. Sky flats seemed to work better, but I was not happy with the vignetting. Now since I am using a full frame sensor on the ASI6200MM Pro, and the scope supports full frame, I was a little intrigued as to why I was getting so much vignetting, you can see from the master flat below that there was indeed a significant amount of vignetting.

Red Master Flat in PixInsight

I did some calculations and found what my problem was. Since my camera is full frame, it has a diameter of 44mm. The M54 connector on the telescope is 55mm away from the sensor, so a simple equation tells me that my light cone is larger than the M54 connector:

Sensor diameter + (distance from sensor / focal ratio)
44 + (55/3.2) =61.1875mm

The internal diameter of the M54 connector is around 51mm, so the light cone was being restricted by around 10mm. So I had a custom M68 to M54 adapter made which is 28.5mm in length, the reason for this is because the backfocus from the M68 connector is 61mm, so if we apply our formula:

44 + (61/3.2) = 63.06mm, this is way below the internal diameter of the M68 connector male thread, so vignetting should be minimised. Now because I do have some M54 in my image train, I know I would not completely elliminate vignetting and this is why, using the above formula, we can work out the light cone at varying part of the imaging train:

12.5mm (EFW mating to camera) = 47.9mm
18mm (50.4mm Filters distance from sensor) = 49.62mm
32.5mm (Light entrance to EFW distance from sensor) = 54.15mm

As you can see, I should expect some vignetting to occur because the light cone at the EFW M54 connector (with around 51mm internal) is 54.15mm, so I would be clipping the light cone slightly, but the result is as follows, again red filter, you can see that the vignetting is significantly reduced:

Collimation was done using the exact same process I used on the 15028HNT, you can read the guide here.

Conclusion:
SharpStar have again produced an outstanding quality astrograph, with a massive focuser to take on the largest of imaging trains, as well as finishing off the product with high quality annodised OTA rings. I am extremely happy with the performance of the telescope, below is my first image which happens to be a 2 panel mosaic:

Iris Nebula, 2 Panel Mosaic, Each Panel consists of 151x60S frames at Gain100, for L, R, G and B, for the full resolution image please use this link

Backfocus information:
M42 connector: 53mm
M54 connector: 55mm
M68 connector: 61mm

Focal Length (With Reducer/Corrector): 640mm
Focal Ratio: F3.2
Newtonian Type: Paraboloid
Focuser Size: 3″

The only complaint I have is with regards to the fan, which I have made a suggestion to SharpStar on that. Good job again SharpStar!

Informational

A step by step guide to Collimation

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If like me you own some sort of reflector telescope, whether this be a Newtonian, Dobsonian, Ritchey Chretien or as I have a Hyperboloid Astrograph then you’ll know that there is a very strong importance on collimation, the faster the optics the more critical collimation becomes, especially for imaging. After recently removing the rear mirror assembly for cleaning, as well as changing from the QHY183M to the QHY268C-PH amongst onther stuff in the imaging train, I wanted to share my experience and knowledge around collimation. Let’s start off with the details on what I use

Part 1 – Aligning the Secondary Mirror with the Focuser

Now on my SharpStar 15028HNT, they recommend you unscrew and remove the corrector from the focuser, however I have found no difference in collimation with or without the corrector in place and because it is part of the optical train I’d rather include it in the collimation, so the first step for me since my primary mirror was currently removed was to check the secondary alignment with the focuser, as well as the rotation of the secondary in relation to the focuser, in order to do this, I use the Teleskop-Service Concenter eyepiece, the eyepiece itself has a set of rings engraved into the plastic apperture like so

Teleskop-Express Concenter Eyepiece markings on lower end of barrel

I ensure that my focuser is at the most inward position and since my SharpStar has an M48 thread on the focuser, I used a 2″ extension tube that has an M48 thread on it, and placed the concenter eyepiece in there:

M48 threaded 2″ Extension tube with Teleskop-Express Concenter Eyepiece

This serves well to get the rotation and alignment of the secondary with the focuser by ensuring that the mirror appears as a perfect circle between the rings, now you can adjust your focuser position in order to get the edge of the mirror to appear on the lines, this is what the view looks like through the concenter eyepiece:

Here you can see the secondary mirror appears circular and in line with the concenter eyepiece markings showing a successful alignment with the focuser

The blue at the top right of the image is a piece of card I stuck behind the secondary in order to show the edge of the mirror better.

As you can see my secondary mirror is pretty much perfectly aligned with the focuser and square with the focuser also, if your mirror shows up as more eliptical, this means the mirror needs to be rotated, if the mirror does not fit in within the circle itself, for example if it is over to the left or right, you will need to move the mirror forward or backwards by means of loosening or tightening the central screw that holds the secondary.

You can see from the following image, I have a central screw which is used for moving the mirror up or down the tube away from or closer to the primary, as well as rotation of the mirror, but then there is also the three collimation screws that are used to adjust the mirror direction itself which we will talk about in the next section

Here you can see the central adjustment screw for adjusting the mirror rotation and centering the mirror with the focuser, the three outer scres are used for adjusting the tilt of the mirror to align with the primary

Part 2 – Aligning the Secondary Mirror with Primary Mirror

Now that we have our secondary mirror lined up and square with the focuser, the next step is to align the secondary with the primary, now for this I will use my FarPoint Astro Laser collimator, which itself has recently been collimated by FarPoint Astro, now you can re-use use the 2″ extension tube and place the laser into the tube, but for the SharpStar I will use the M48 to 1.25″ lockable adapter like so:

FarPoint Astro laser collmator in the SharpStar M48 to 1.25″ Adapter

Now the point of this part is to ensure that the laser hits the centre spot of the primary mirror, if it does not, then this is where you would adjust one or more of the three screws on the secondary, as you undo one, you should tighten the other two, as you can see from this image, I need not make any adjustments as the laser hits the centre of the primary perfectly:

Here you can see that the laser hits the primary mirror centre spot

Part 3 – Aligning the Primary Mirror

Now since I do not have to make any further adjustments to the secondary mirror, it is time to focus on the primary mirror, the trick here is to get the laser beam to return to the point of origin, here’s an example of the primary not being correctly aligned:

You can see two dots here, one is the laser aperture, the other is the reflection of the laser from the primary mirror, this reflection needs to meet the aperture

You can clearly see the red dot to the top left of the laser apperture, this means that the primary needs some adjustment by means of the three collimation screws which are situated on the rear of the primary mirror assembly:

Here you can see the primary mirror collimation screws, the larger push/pull the mirror, the smaller are locking screws to secure the mirror in place after successfully collimating.

Most telescopes have a push – pull method here, turning anti-clockwise will push the mirror further up the tube, whereas turning clockwise will pull the mirror towards the bottom of the tube, it is very important not to keep turning anti-clockwise because this could result in the screws becoming disconnected from the primary mirror. After an adjustment on a couple of the collimation screws, my primary is now aligned properly as the laser beam returns into the laser apperture:

Here you can see that there is no additional dot, the dot is centered right on the laser aperture indicating primary alignment is complete

Once the laser collimation has been completed, it is easy to verify this with the FarPoint Auto-Collimator, the eyepiece has a mirror inside which allows you to see where the centre spot of the mirror is and will form a slightly pale dot in the middle, if the dot appears in the middle then you have your collimation pretty much spot on after following the above, maybe a very slight adjustment on the primary collmation screws is all that is required, you can see here what the view looks like:

It is also normal on faster telescopes to see the mirror appearing offset as opposed to central to the OTA itself. Once completed, I would typically then perform a star field test and I prefer to use the Multi Star Collimation in CCD Inspector for this, you can of course use the de-focused star method.

I hope you found this useful, I just thought I would share my process in performing collmation to help others who may be on that journey also.