The basic idea of EAA astrophotography is for a telescope to take many very short exposures which are ‘Live Stacked’. As an increasing number of frames are aligned and stacked, the image will show faint objects gradually appearing out of the noise. This is quite magical and great for use at outreach events. There are now quite a number of dedicated EAA scopes ranging in aperture from 50 mm up to 114 mm. These are controlled by a smartphone or tablet on which the captured image appears. (The Unistellar EVScope 2 does, however, have an ‘electronic eyepiece’ through which one can view the gradually improving result.)
These are very easy to set up and use but only have small imaging sensors so that the field of view is somewhat limited. There are many YouTube videos about them.
It has to be said that they can take quite impressive images as this one of the Swan or Omega Nebula taken by the Unistellar EVScope 2.
This can be achieved by those who are not particularly into astronomy so, for some, they can be a worthwhile purchase. The Unistellar EVScope 2 has perhaps the highest specification of EAA specific telescopes, so this is the telescope I wished to emulate.
This article describes a mount/scope/camera configuration that will give even better results than the best of these at lower cost – although it does take a little effort. Many astronomers will already have some of the required items to put one together. One great advantage is that eyepieces rather than a camera could be used with the telescope to visually observe the Moon and planets at higher magnifications.
A low cost mount/combination to use for EAA
Looking through the lists of affordable ‘goto’ telescope/mount combinations, one stood out to me. This the Celestron NexStar 130 SLT. This employs a 130 mm parabolic mirror in a Newtonian configuration having a focal length of 650 mm (so with a focal ratio of f/5) mounted on a single arm computerised Alt/Az mount. As it happens, I own one of the very first to come into the UK when I reviewed it for Astronomy Now many years ago.
It employs a wonderful 3 star alignment system called SkyAlign where one simply finds and centres any 3 bright stars spread across the sky . It current cost is ~£500 and I thus thought that this would make an excellent basis to compete with the Unistellar EVScope 2 which employs a 114 mm Newtonian telescope having a focal length of 450 mm giving a focal ratio of f/4. In terms of light capture the 14% increase in aperture of the 130 SLT should partly compensate for the lesser focal ratio of the EVScope 2. So I suspect that these would be very comparable in imaging performance. Where the 130 SLT system could outperform the EVscope2 is the fact that a larger sensor camera could be employed giving a correspondingly larger field of view.
The EAA specific scopes have rather small sensors so limiting their field of view. That in the Unistellar EVScope 2 gives a field of view of 45.2 x 33.9 arc minutes – and thus can just encompass a full Moon.
The Unistellar EVScope 2 camera
The pixel size of its 7.7 megapixel sensor is just 2.4 microns. Each pixel will subtend 1.1 arc seconds on the sky. The nominal resolution of a 114 mm aperture telescope is ~1.2 arc seconds so, under typical seeing conditions (the atmosphere limits the resolution to 2-3 arc seconds), the sensor will nicely sample the image produced by the telescope. I have included these details to help define a suitable astro camera to be used in the system.
[Note: I have found it very difficult to get correct information about the sensor and have used plate solving of its images to help deduce the actual specification.]
The proposed camera for use with the Celestron 130 SLT
There is one obvious choice – the Altair Hypercam Fan-cooled 183C PRO Colour Astronomy Imaging Camera at ~£500. This 20 megapixel camera (5440 x 3648 pixels) has 2.4 micron pixels which subtend 0.76 arc seconds on the sky. ( (pixel size in microns/ focal length) *206.3) As the nominal resolution of the 130 SLT is ~1 arc second, the sensor, under perfect conditions, would under-sample the image produced by the telescope. But, under typical seeing conditions, it would nicely sample the image. It has a sensor size of 13 x 9 mm having a diagonal of 15.86 mm rather than the 8.6 mm of that in the EVScope 2. Coupled to the 130 SLT this gives a field of view of 69 x 46 arc minutes, substantially larger than that of the EVScope 2. The increase in the field of view is not quite as much as the increase in sensor size would have one believe as the EVScope 2 has a shorter focal length than the 130 SLT. The image below gives an accurate representation of the relative fields of view.
A cooled version is available for £749.
Attaching the camera to the 130 SLT
My 130 SLT has an interesting eyepiece attachment system. A 1.25 inch barrel tube (to accept a 1.25 inch eyepieces) is attached to the wide barrel of the rack and pinion focuser. (It may well be that later models sport a 2 inch barrel tube. – most images on the web show a 1.25 inch eyepiece adapter but one shows a 2 inch adapter.) In either case a suitable t-mount adapter could be used to attach the camera to the focuser but I found that I was not then able to achieve infinity focus with my dedicated astro camera – as may well be the case with other dedicated cameras such as the 183C PRO. However, the 1.25 inch (or 2 inch) eyepiece tube adapter actually uncouples from the focuser leaving behind a M42 threaded barrel onto witch the camera can screw directly. So no adapter is required saving some money and plenty of inward travel is available to reach infinity focus. Problem solved.
Two pleasing results
Firstly, the system did not give rise to any significant vignetting and there was no evidence of coma towards the corners of the field of view – this is where a smaller sensor camera helps.
The Imaging Software.
The software package that I and many imagers use is SharpCap:
The basic program is free, but I think the Pro version may be required at a cost of ~£12 per annum. (If the 183C PRO were purchased from Altair Astro, a free licence for 1 year is provided.)
This will obviously require the use of a laptop. Whether its cost should be included in the total cost of the system is a moot point. The EAA scopes require either a smartphone or tablet. One can purchase a refurbished i5 laptop for ~£200 so any additional cost is not great. There may be a further expense if the imaging session is to last for several hours away from mains power. One can purchase a 1,500 W 12 volt to AC inverter and a ‘Jump Start’ battery for around £100 in total.
Using SharpCap Live Stacking
When SharpCap is opened, select the camera and adjust the exposure and gain until a passable image is seen. The exposure needs to be short so the stars are not trailed – perhaps 20 seconds. The gain must be increased until a reasonable number of stars are visible in the frame – if not Live Stack cannot align them.
When one is happy with the captured images, one can select Live Stack in Tools. SharpCap will start integrating the captured frames and so the observed stacked image will improve – quickly at first and then more slowly. [To reduce the noise by a factor of 2 one needs 4 times as many frames so x2 noise improvements are seen when 4,16, 64, 256 … frames have been taken.] The ‘Black Level’ and ‘Mid Level’ sliders in the Live Stack display can be adjusted to give the best ‘stretched’ image. One can adjust the colour balance with the colour sliders at the lower left of the control panel. [The button below the red slider, if clicked upon, will do an auto colour balance. At any point the captured image can be saved either as a .png file showing the ‘as seen’ stretched result or an un-stretched .fits file result for further processing in a free program like “Siril”.
As a demonstration of Live stacking
The first figure below shows the first frame of a set of 16. The second after 4 frames have been stacked and the third after 16 frames have been stacked. The improvement is quite surprising. [This individual frame, though quite noisy, is probably better that one would normally have.]
This result was obtained after ‘tweaking’ in Photoshop.
Finding faint objects using plate solving
This is a method used by the EAA imaging telescopes to enable the ‘homing in’ onto faint objects that may well not be seen in the finderscope or single captured images. There are two ways that the 130 SLT can use plate solving.
Starsence Autoalign
As bought, the 130 SLT cannot do this but, at a cost of ~£325, it is possible to purchase the Celestron Starsence accessory which couples to the mount and achieves this. This seems expensive, but a new handset is included which contains the stellar data base and software to enable plate solving. One simply presses ‘Align’ and the telescope spends a while observing star fields to align itself. When objects are targeted the telescope will then use plate solving to make the fine pointing adjustments.
Using a Plate solve program and a BBC Basic Program
One can download into the laptop the program ‘PlateSolve 2.28:
https://planewave.com/download/platesolve2/
and the UCAC3 star catalogue:
https://planewave.com/download/ucac3-catalog/
Extract the star catalogue into the C drive.
One takes and saves an image which is opened in PlateSolve.
The approximate RA and Dec of the object needs to be entered along with the field size – for this telescope/camera combination this is 69 x 46 arc minutes.
Clicking on ‘Plate Match’ will, when plate solved, give the central coordinates of the field. This can be compared with the desired coordinates and you should be able to see in which way to adjust the mount Az and El to centre on the object.
A BBC Basic Program
I have written a BBC Program to aid in this. First download the BBC Basic emulator for Windows:
https://www.bbcbasic.co.uk/index.html
Then from my article:
Copy and paste the BBC program. To make it simple it asks separately for the Hours and Minutes and Degrees and Minutes of the desired object and that of the result of the plate match.
Having entered these it give the RA and Dec that the mount should be slewed to – even though these are not the object coordinates. Assuming the object is then centred in the field of view (slight adjustments may need to be made) then use ‘Synchronise to Target’ and the correct RA and Dec will appear.
This whole process does not take too long and I think should be tried before a StarSence system in purchased.