7. Other Considerations
7.1 Powering the Equipment
At the very minimum, the user will need to power the mount. Other equipment may also require power, such as an astronomical CCD camera (DSLRs are powered by their own batteries), dew heaters, a laptop, etc. Generally speaking, when out in the field, equipment is powered from a 12 V battery, though power inverters can be used to power the equipment from a mains socket (if one is accessible). Going with the general assumption that equipment is to be set up in the middle of nowhere, a 12 V battery is deemed a necessity. Astronomy equipment manufacturers such as Celestron and Skywatcher sell various 12 V all-in-one batteries, which come with 7 Ah or 17 Ah of power, integrated torch, radio, car jump-start cables, etc. The 17 Ah models cost around £120 without VAT and are actually not very suitable to the job at hand.
The issue is that the 12 V batteries used are not suitable for continuous discharge, but are rather suited to short pulses to jump-start a car. Moreover, 7 Ah or even 17 Ah is not enough power when one goes beyond powering a mount. This is even more the case when one considers that the voltage drops as power is drained and to keep voltage at a good level, these batteries should not be discharged beyond 50% (70% as an absolute recommended maximum). With 17 Ah at 70% available maximum, this is actually 11.9 Ah. As an example, the Skywatcher NEQ6 Pro mount consumes 2 A of current at full speed slewing. This means the battery can run the mount at full speed slewing for 11.9 / 2 = 5.95 hours. Naturally, one does not slew at full speed the entire time and more time is spent tracking the night sky, which consumes less current, thus extending this time before the battery needs recharging. We do however need to consider that 11.9 Ah of viable power is pretty low and before you know it, the voltage drop will cause the mount's power to pulsate, making tracking less reliable.
With this example, we now include an astronomical CCD camera, with a Peltier cooling system. For example, the ATIK 383L+, which consumes 2.5 A of current at full capacity. Assuming full speed slewing and full capacity cooling of the CCD camera, this is 4.5 A of current usage. At 11.9 Ah of available power, this can run for 11.9 / 4.5 = 2.64 hours. Again, we will not necessarily be using 4.5 A of current the entire time, but one can assume the CCD camera will be using most of its rated 2.5 A most of the time (as the Peltier cooling system is left on continuously). At this stage, not only is the power drain quite large, but the voltage drop from the power drain and from the extra equipment lowers the voltage and does so quickly, making tracking on the mount unreliable.
The best kind of 12 V battery for the job is therefore Marine Deep Cycle (also known as Leisure) Batteries.
These are compact batteries that store a tremendous amount of power and are designed to be continuously discharged to provide a clean voltage supply. The above pictured 12 V battery provides a large 85 Ah of power. Going back to our example, assuming 70% discharge, this is 59.9 Ah of viable power. At 4.5 A of current usage, this gives 59.9 / 4.5 = 13.3 hours of power. Compared to the mere 2.64 hours we worked out for the 17 Ah battery by Celestron/Skywatcher, this is already one full night of imaging. Moreover, we note that this figure is practically higher as the mount will not use 2 A of current 100% of the time, nor will the CCD camera use its 2.5 A of current (though it may be close to it). 85 Ah is also not the limit for these batteries - 120 Ah and even higher can be purchased. Best of all, the above 85 Ah battery cost £90, which is £30 cheaper than the 17 Ah battery by Celestron/Skywatcher and the £30 saved can be used to buy a mains charger for the battery. If buying locally from Gibraltar, these batteries (and the mains chargers) are sold by The Battery People. International readers will be happy to know they are very much standard and can be purchased from practically any hardware/electronics store - look for Marine Deep Cycle 12 V Batteries.
Now that we have the power side of the equipment sorted, we must consider one additional hurdle. When one powers a single mount off a properly-charged battery such as the above, the mount will normally run without issues. This is usually indicated by the power LED on the mount not flashing at any point, staying on steadily throughout slewing and tracking. Unfortunately, as soon as you start connecting other equipment to the same battery, you get a voltage drop. For example, connecting the mount alongside a CCD camera to the battery. The voltage drop may not be a problem for the CCD camera's Peltier cooling system, but it is definitely for the mount. The power LED may be seen to flash randomly, particularly while slewing and tracking. This indicates the voltage is too low to power the mount reliably. A noticeable effect of this is star trailing in images, even of short exposures, due to the motors not being properly powered. There is a way around this problem without having to buy and carry a second battery - using a simple transformer!
Pictured above attached to the side of the mount, the transformer takes in the 12 V supply from the battery and steps up the voltage to any selected one (this one transforms to 15 V, 17 V, etc all the way up to 24 V). The transformer is actually a universal laptop charger for cars. It is universal because it allows the user to set the appropriate voltage for their specific laptop and use one of eight connectors to plug into the laptop. The fact that these come with around eight connectors allows the user to use the one that fits his/her mount's power connector most snugly, preventing power loss through a loose connection. Mounts tend to operate well between 12 V and 15 V, so setting the transformer to 15 V even when the mount is connected by itself to the battery is not a problem. It is when the CCD camera with its Peltier cooling system goes on the battery as well that this becomes a necessity - to provide a clean voltage to the mount that allows for reliable operation. Users who power even more equipment from the same battery, such as dew heaters, tend to increase the setting all the way up to 17 V (once everything is connected and powered, of course). These transformers (universal laptop chargers for cars) are very cheap at under £10 including delivery and are easily found online through eBay. Hardware/electronics stores may also have them as their main purpose is for laptops and are therefore popular accessories.
7.2 Focusing Precisely
It goes without saying that photography requires precise focusing for fine detail to be discernible in images. However when your target is so faint that it requires semi-long exposures just to see anything, you might soon lose patience trying to reach perfect focus. Fortunately, optically-speaking, everything we see in the night sky is effectively at the same focal point in your optical system, meaning that achieving perfect focus on a bright star such as Vega or Sirius would mean having perfect focus for your target object, faint as it may be. Keep in mind that you cannot just put in an eyepiece into the telescope, focus that and believe that your DSLR or CCD camera will also be focused. You must achieve focus with the optical train (focal reducer, filters, filter wheel, camera, etc) as it is to be used for actual imaging. This means taking very short exposures on a bright star such as Vega or Sirius and fine-tuning focus until you reach that perfect focus.
There is a danger of chasing the seeing here. If you set a very low exposure time, such as a fraction of a second, the star may appear perfectly focused and then in a subsequent exposure, not perfectly focused. This is due to the atmospheric distortions due to air turbulence in the Earth's atmosphere. The trick is to set an exposure time of around two seconds or a tad more in order to average out this apparent motion and distortion in each of your test exposures for focusing. One can of course now question how you decide when perfect focus is indeed reached. This is essentially when the star you are aiming at is as small as possible in your image (smallest Full-Width Half-Maximum, or FWHM). Some astrophotographers use motorised focusers, which have a small motor that precisely turn the focus knobs on the telescope.
Powered by USB, this can be automated by the image capture software as it can move the focuser until the smallest FWHM is achieved, thus achieving perfect focus. There are inherent problems with this method of focusing precisely however. Mainly, the need for a motorised focuser, an extra USB device, waiting for the image capture software to focus it for you and actually achieving perfect focus is not as quick as it may sound when it is an automated process. These can be expensive (£100 and more!) and to put it pessimistically, it adds another thing that can go wrong to the whole equation of astrophotography. How else is one supposed to achieve perfect focus however!? The answer is simple - Bahtinov Masks!
To the left is the standard design for a Bahtinov Mask, originally designed by Russian astrophotographer Pavel Bahtinov. This mask is pre-cut to suit the specifications of the telescope being used in terms of aperture and focal ratio. The effect of placing this mask in front of the opening to the telescope is star diffraction spikes. Stars are purposely diffracted to appear as a pair of crossed spikes with a central spike going through them.
The star shown above has been diffracted by a Bahtinov Mask. The idea is that when the star is out of focus, the central spike appears off-centre and toward the left or right, depending on whether the focuser is too far in or too far out. When perfect focus is achieved however, the central spike appears precisely centred through the two crossed spikes. At this point, the focus can be locked on the focuser, the Bahtinov Mask can be removed, and imaging of your target can begin with confidence. The best part of using a Bahtinov Mask for precise focusing is that it is extremely effective and extremely fast. The process involves simply slewing to a bright star, popping the mask on the telescope, focusing roughly, taking very short exposures (of around two seconds) continuously while tweaking focus and once focus is reached, removing the mask and slewing to your intended target. Actually achieving focus can be done in under a minute. Image capture software can help with this process as it can strongly magnify on a specific star that appears in your image.
Bahtinov Masks need to be made for the specific telescope being used for imaging. For this reason, you will need to stock a few if you have several telescopes that you use for imaging. They may also be used for reaching perfect focus visually with an eyepiece. StarSharp is a popular manufacturer of Bahtinov Masks and pre-make them for popular telescopes. Morris Engraving do however custom-make them for you, providing you give them the focal length, aperture and outer diameter of your telescope. They have an eBay store that sell lots of Bahtinov Masks for popular telescopes and are happy to receive custom orders at no extra charge. Buying a telescope should also involve you buying a matching Bahtinov Mask - keep this in mind! Do not worry however, because Bahtinov Masks tend to cost between £15 and £30, depending on size (even custom-made ones).
7.3 Placement of Filters
Consideration of where the filters go in your optical train is important for a number of reasons. This does vary if you have a DSLR or One Shot Colour (OSC) CCD camera, or a monochrome CCD camera, since the latter requires filters to reproduce colour. Generally speaking, an optical train involves the following, in order:
Telescope ---> Light Pollution Suppression Filter ---> Focal Reducer / Coma Corrector ---> Spacer Rings ---> Off-Axis Guider ---> Filter Wheel / Filter Holder ---> Imaging Camera
This can of course vary if for example you are not using an Off-Axis Guider (OAG), but is generally set out as above. A spacer ring may go between the filter wheel / filter holder and imaging camera, if you are to reach simultaneous focus on the imaging camera and autoguiding camera connected to the OAG, but these are specifics to different equipment combinations. In general we can assume the light pollution suppression filter, if any is used, goes at the very front of what goes into your focuser. This is therefore threaded to the the focal reducer / coma corrector being used with the telescope, which in their own right are usual accessories used in astrophotography. Spacer rings are put in place to ensure the imaging camera's CCD sensor is precisely a certain distance away from the focal reducer / coma corrector, which tends to be 55 mm for most but varies depending on actual focal reducer / coma corrector being used (manufacturers will state the optimum distance). This distance does of course take into account the depth of the CCD sensor within the actual imaging camera (for DSLRs this is 45 mm but for CCD cameras, it tends to vary between 13 mm and 18 mm). If an OAG is not used, the thickness of the missing OAG is usually taken into account in extra spacing from the spacer rings. The autoguiding camera branches off the optical train here, connecting to the OAG on its side, looking at a prism. The filter wheel / filter holder is relevant for those using monochrome CCD cameras as these filters are required to reproduce colour in final images. Finally, we have the imaging camera at the back of the optical train.
As above, we see the light pollution suppression filter is placed in front of the entire optical train. In theory it can go behind the focal reducer / coma corrector, but it would add spacing that would have to be taken off by removing (or using thinner) spacer rings. It also seems sensible to reject the wavelengths corresponding to light pollution before it enters the optical train at all so placing the light pollution suppression filter at the very front is generally a good idea. For those using a monochrome CCD camera and an OAG, it is an extremely good idea to place the filters behind the OAG and not in front. The reason for this is simple. Imagine you are imaging in narrowband, say Hydrogen-Alpha with a 7 nm bandwidth. If this filter is in front of the OAG, the autoguiding camera will essentially be looking at the narrowband image, which given the nature of autoguiding and therefore short exposures, is a terrible idea. It may work for broadband (LRGB) filters, though not ideal, but can be problematic in narrowband. The filters used for imaging particular parts of the spectrum must therefore be placed behind the OAG. Those not using an OAG at all can ignore this requirement as the autoguiding will obviously be handled by a second telescope or guidescope with an entirely different optical train.
Now that we have discussed in detail how an optical train is set out and why, we will cover the details of the filter wheel / filter holder. In theory, the filter can be threaded in its own right between the OAG and the imaging camera, without anything holding it. However, filters are thin pieces of glass (usually 2 mm thick) held in thin, threaded metal rings (unless they are unmounted, in which case this argument is invalid). An imaging camera can be quite heavy and this does place unnecessary strain on the filter itself. Personally I would find it a bit unnerving to hold my imaging camera off a thin filter, not only for the filter itself, but the imaging camera as well. This means we need some equipment to hold the filter inside it and have threads to go to to spacer rings / OAG on the front and the imaging camera on the rear. A filter wheel is a common accessory and the following is an example of ATIK's offering, the ATIK EFW2, which costs about £390 without VAT:
A filter wheel is quite simply that - a carousel wheel inside a USB-controlled and 12 V-powered accessory that is able to thread itself in the front and rear to your spacer rings / OAG and imaging camera. Different carousels can generally be purchased in order to thread different-sized filters into the system, be they 1.25" or 2", for example. The above shows the carousel for 2" filters, holding up to five at a time. Though there are also manual filter wheels that are not USB-controlled or 12 V-powered (admittedly less popular in terms of market choice), the advantage of an electronic filter wheel such as the ATIK EFW2 is that the user can set in the image capture software which filter to use for which sequence of images. This allows fully automated imaging as it can be set off to run for several hours, imaging through multiple filters. This may or may not be something useful, as astrophotographers may wish to dedicate individual nights to particular filters and may therefore not benefit from the ability to sequence filters automatically. Moreover, a filter wheel is not a small accessory and can be bulky and therefore has to be accommodated into the optical train. A filter wheel tends to be about 20 mm thick, which plus the filter adds about 21 mm optical distance from the focal reducer / coma corrector to the imaging camera's CCD sensor. This has to be factored into the use of spacer rings to make sure the imaging camera's CCD sensor is the prescribed distance away from the focal reducer / coma corrector (usually 55 mm).
For users who do not wish to spend as much money on a filter wheel, do not need automatic sequencing or cannot accommodate the optical thickness or size of the filter wheel in their optical trains, there are alternatives. Filter holders, usually called filter drawers or quick filter changers, are extremely simplistic accessories. They are basically a tiny box that threads itself to the spacer rings / OAG on the front and the imaging camera on the rear, with a drawer that can be loosened and taken out of this box. The drawer in its own right contains a thread that allows you to put a filter into it. The following is an example of Teleskop Express' offering, which costs about £70 without VAT:
The product in question is viewable at this link. This is as simple and small as it gets. What's more, you can buy extra individual drawers and for different-sized filters (e.g. 1.25" or 2"). Filters can then be manually but extremely quickly exchanged. This accessory adds very little optical distance to your optical train, almost no weight, is very small and is much cheaper than any electronic filter wheel available. It can certainly suffice to place the filter behind the OAG and in front of the imaging camera, without hanging the imaging camera on the filter itself. This is an accessory that I personally use all the time and should be considered alongside filter wheels.
7.4 Fighting Dew
A common issue in humid places, including Gibraltar, is the formation of dew. When a surface cools below the dew-point temperature of the air, water vapour from the air condenses on the surface and forms water. This is of course a big problem when it comes to setting up a telescope for visual or imaging use as throughout the night, the telescope lens/mirrors can cool quickly and below the dew-point temperature of the air. This simple hurdle can be prevented however. Some telescope optical designs are more susceptible than others when it comes to dew, as the lens/mirrors are more exposed to the air. This is covered in detail in the telescope section.
Preventing the formation of dew can be done via one of two ways, or both of course. The simplest one is preventing the lens/mirrors of the telescope being so exposed to the air. This is done by placement of what are called dew shields.
Dew shields are very simple rolled sheets of foam that are placed at the front of the telescope, which removes much of the contact between the telescope lens/mirrors and the air. Dew shields also act as protection from indirect sources of light going into the telescope, which can help improve image contrast. The above pictured dew shields (to the left) are by Astrozap and are priced between £15 and £50 without VAT depending on size (for telescope aperture). It must be noted that Refractors are normally designed with integrated extendable dew shields. These may or may not suffice, depending on design and local air conditions.
The second method of preventing dew is by using dew heaters. These are thin heating strips that strap around the outer diameter of the telescope, where the lens/mirror concerned is. They are 12 V-powered and provide some heating that stops the lens/mirror temperature falling below the dew-point temperature of the air, thus preventing dew entirely.
These are generally more expensive than dew shields (relative to size) and need a 12 V supply to operate. Prices for the Astrozap ones (pictured left) range between £20 and £50 without VAT depending on size (for telescope aperture). They are also available in small sizes for eyepieces. Naturally however, it must be noted that dew heaters do not shield the telescope from indirect sources of light.
Moreover, for users of telescopes that have a lens element at the front, such as Refractors or Schimdt-Cassegrains, Astrozap also make heated dew shields. These combine both elements together into one product, where the heating strip is at the rear, hence why they are suitable for telescopes with a lens element at the front.