A very small, wide-field, guided imaging rig Part 1. The equipment

The imaging scope is the latest edition to the Touptek family of guide scopes; the Touptek GS PAPO Guide scope. It is a 4 element self-flattening Planar Apochromatic ED refractor. It is truly tiny: F = 100mm, D = 25mm, f/4. it has a 1“ imaging circle and gives good stars to the corners of a sensor such as the Sony IMX 533.

There is very little back focus so it is not possible to use a camera rotator to facilitate the framing of subjects. One slight concession to this is that I have fitted two 1mm spacer shims, as each one is added the camera can be rotated to a different position to tighten it up. It remains to see if they will have to be discarded when we attempt to focus on a star using a Bahtinov mask.

We have mounted a 2“ Altair filter drawer on the front of the scope and have added a 30mm M48 extension tube after that to act as a dew/ light shield.

This is an unconventional setup and it will be interesting to see how it performs.

View of the equipment

A view from the front

The guide-scope to be used is the  SVBONY SV165 Guide-scope with F = 120mm paired with a QHY-5II-M guide camera. This harks back to the days when manual guiding used a guide-scope with a longer focal length than the imaging scope, creating forgiving guiding for the image being captured with the imaging scope. A small movement in the guide-scope is an even smaller movement in the imaging scope.

The camera we are using initially is the uncooled ZWO ASI585MC. The ZWO ASI585MC uses an IMX585 sensor which is BSI, and built on Sony's STARVIS 2 technology. It has a full well depth of 40k to 47k electrons and a Read noise of 0.7 to 5.5 electrons depending on gain. It has a high QE of 91% at 500nm and 80.9% at 656nm.

Simulating the field of view of the GS-100 and the IMX585 in Stellarium, if one centres the image on the star HD198626, the whole of the Cygnus Loop can be framed:

Framing of the Cygnus Loop

Even though it is called the GS-100 Guide scope, it is heavily marketed by Touptk as a mini, very wide field imaging scope because of its high quality optics. They show that is is a good match for the new innovative cooled/uncooled camera the Touptek Astro AE676C as well as other cooled or uncooled cameras. They also show that filter drawers, electronic filter wheels and filters can be attached to the front of the scope and that it is directly compatible with the ToupTek Astro AAF electronic focuser.

Whilst as I said earlier, we are using an unconventional configuration, we will be, by no means using the scope out of spec.

Cable clips placed at strategic positions to facilitate cable management

A Bahtinov mask was contructed by using epoxy resin to attach a 3D printed Bahtinov mask intended for a Seestar S50 onto a 10mm M48 extension tube so that is can be screwed onto the 30mm M48 extension tube dewshield.

The attached Bahtinov mask

This will be needed to get the stars well focused.

It can be stored conveniently in a small plastic box of the type we use to store 2“ filters.

An opportunity arose to test the focus of the rig with a Pegasus Astro UV/IR cut filter in the filter drawer. A pre-release version 3 of AstroDMx Capture was used. The field of view of the GS-100 was plate-solved and the scope was sent to the star Vega. The Bahtinov mask was then used to bring the star to focus.

Screenshot of the focused star on the preview screen of AstroDMx Capture

Clouds prevented further testing but this test was a success!

In Part 2 this wide-field imaging rig will be tested with the latest pre-release of AstroDMx Capture version 3 

Getting HOO narrowband images from a Dwarf Mini

The Dwarf mini has a dual-band filter that effectively separates the O3 into the green and blue channels and the H-alpha into the red channel of the colour image it produces. 

We are going to take the 16 bit PNG produced by Stellar Studio and re-process it into a real HOO image. The image we shall use is a mosaic image of NGC7000, the North America Nebula.

We shall convert:

RGB

and convert it into:

HOO

Let us look at one method of doing this:

Click on any image to see it much closer

We shall use GIMP in the main.

There are various ways of doing this. The first thing to do is to save the 16 bit PNG as a 16 bit TIFF. The reason for doing this is that some software only treats PNG files as 8 bit files and converts them after reading them. We want to retain the 16 bits until the very end of the processing.

We shall start with a starless version of the original image plus the stars image. These could have been obtained from the original image in a number of programs so for that part we shall not be prescriptive.

Starless RGB image

We have to split the RGB image into it's three monochrome channels; R,G and B.

Colours

    Components

        Decompose     (NOT to Layers; Colour model RGB)

Three monochrome images will appear:

Red channel: H-alpha, H

Green channel: O3

Blue channel: O3

We next have to combine the Green and Blue channels as they contain the O3 data.

Select the Green channel (the middle one at the top).

Edit

    Copy Visible

Select the Blue Channel (the right hand channe at the top

Edit

    Paste as

        Single layer             (depending on the version of the Gimp you are using, it might say New layer)

At the right hand side, select Mode Addition.

Then:

Image

    Flatten image

This has created the O3 monochrome image.

Then delete the Green channel as it is no longer needed.

You might then need to use levels to bring up the brightness of the new O3 image to a level similar to that of the H image.

The adjustments can be seen in the Levels dialogue in the screenshot below.

Note that is has not just been made brighter, but the dark areas have been preserved by bringing in the left hand slider a little. 

This process of making the H and O3 images of similar brightness distributions is called normalisation.

(This and the next step can be repeated until you have the required amount of blue in the HOO image)

The H and O3 monochrome images now have to be combined into an HOO false colour image.

The H is placed in the Red channel and the O3 is placed in both the Green and Blue channels of the colour image that will be created.

Colours

    Components

        Compose

A dialogue like the one in the screenshot below will appear.

Take your time and make sure that you assign red to red

blue to green and blue to blue

When you click on OK the HOO image will be created.

Save the image with a sensible name like: NANEB_HOO.tif

At this stage it is still 16 bit.

Then load The HOO image into GraXpert

DON'T extract the background because this will already have been done by Stellar Studio right at the start.

Denoise the image.

When Denoising is complete:

Select Saving

Set to 16 bit Tiff

Click on Save Processed:

GraXpert will give a new filename by adding Graxpert to the end of it

The name we can see that has been generated is NANEB_HOO_GraXpert.tiff

Click on Save



You can then load this image into GIMP

If you need to, you can select:

Colours

    Hue-Saturation

        and increase the Saturation a LITTLE as shown in the dialogue in the screenshot below

Then load the Stars image into GIMP

Select the Stars image

Edit

    Copy Visible

Select the HOO image

Edit

    Paste as

        Single layer                                  (or New layer in a different version of GIMP)

On the right hand side

Select Mode Screen

This will screen the stars back into the image.

Export the image as NANEB_HOO_starsback.tif

If you then Export the image as NANEB_HOO_starsback.jpg

It's bit depth will be reduced to 8 bits as a JPG image suitable for sharing.

The original image and the HOO image animation