Package Description
The package we are offering is ideal for the astrophotographer who does not have a permanent observatory and therefore has to set up and dismantle the telescope on a regular basis. It is designed to be easy for one person to handle alone. Based on Celestron's excellent C8 optical tube assembly (OTA) it incorporates a Hyperstar prime focus corrector to provide wide-field imaging at f/2, a very fast focal ratio that reduces exposure times quite dramatically.
The wide field enables objects to fit onto small CCD detectors that would otherwise have to be assembled from a mosaic of several images. The short focal length reduces tracking accuracy requirements and simplifies alignment. In fact, for many objects the short exposures would allow an alt-az mount to be used and there is no need for accurate polar alignment.
The package includes an HEQ5 mount which is more than capable of handling the C8 and additional equipment such as a piggy-backed guide scope. We are offering the option of upgrading this to an EQ6 in case you want a mount that is capable of taking a larger telescope as well.
Why choose an 8-inch telescope for imaging
There is a general misconception in astronomy that the bigger the aperture the better. It is true that a bigger aperture collects more light and has a greater theoretical resolving power. There are some objects and features that can only be seen with very large apertures, but there are many good reasons for choosing a suitable aperture, rather than the biggest you can afford.
One of the most obvious reasons for choosing a smaller aperture is the convenience of having something that can be handled and stored easily. An 8-inch SCT is one of the most popular quality telescopes because it offers a good balance between size of aperture and convenience of handling. It is also well-matched to the seeing conditions experienced in the UK and will often show as much detail as a larger aperture, except for brief periods when the atmosphere steadies for a few moments. Visual observers can take advantage of these steady periods but often have to spend a long time at the eyepiece waiting for those few 'magic moments'.
When imaging, it is usually the average seeing that governs the overall result, especially if longer exposures are used, and larger apertures don't offer the full advantage that theory would predict. However, when using Hyperstar to reduce exposure times it is sometimes possible to take a series of quite short exposures and reject those where the seeing is poor. By adding the remaining frames it is possible to produce images that are better than the average seeing. This technique is often used by planetary observers whose targets are quite bright. Hyperstar makes it feasible for many deep-sky objects as well.
What is Hyperstar and Why use it?
Most SCTs come with a focal ration of f/10. This is produced by the combination of the primary mirror and secondary working together. For imaging extended objects such as galaxies and nebulae it is desirable to have a faster focal ratio. The most common is f/6.3 which can be produced by adding a focal reducer into the light path. This usually fits immediately behind the primary mirror cell and has the effect of making the field of view wider and reducing the tracking accuracy required. Some manufacturers produce SCTs designed specifically for imaging with a native f/6.3 focus. These are not very popular because they require a larger central obstruction.
It is possible to buy focal reducers that provide a final focus of f/3.3. These have a very short back-focal distance and can only be used for imaging and not for visual use. They do reduce exposure times significantly and allow larger objects to fit onto small CCD chips. It is not feasible to go faster than f/3.3 by adding a focal reducer at the Cassegrain focus of an SCT.
Professional large telescopes often use the prime focus of the the primary mirror for imaging and this is what Hyperstar allows the amateur to do. Hyperstar is a small lens assembly that fits in place of the usual secondary mirror to produce a very fast focus at the front end of the telescope tube. This very fast focal ratio of f/2 reduces exposure times dramatically and allows wider fields of view.
Celestron designed a system called Fastar and produced OTAs that were 'fastar compatible'. This made it easy to swap the secondary mirror for a prime focus corrector and to swap back again. Starizona took this basic concept and used modern materials and designs to produce Hyperstar, essentially a better type of Fastar. Hyperstar incorporates its own collimation adjustments so there is no need to disturb the collimation of the telescope's secondary mirror. When this is put back in place there is rarely a need to adjust collimation.
See our Hyperstar page for more details
What other equipment would I need to add to the package?
The basic package consists of:
- Celestron Carbon Fibre C8 Optical Tube Assembly
- C8 Hyperstar
- HEQ5 Equatorial Mount with Synscan Computer Control
Apart from a camera and a power supply, the package contains all that is needed for basic astrophotography. Other accessories can be added to make observing more convenient and more effective but we have kept the package to the basic minimum because customers sometimes have the other equipment that is needed.
If we were starting from scratch and putting together a comprehensive package these are the items we would include.
- Starlight Xpress SXV M25 One-shot colour CCD Camera
- Laptop Computer
- Celestron Power Tank
- Flexible Dew Shield
- Dew Heater and Controller
- Piggy-backed Short-tube Refractor for Autoguiding
- Autoguiding Camera
- Feathertouch Focuser
- Microtouch Autofocuser
As an alternative to a CCD camera it is quite feasible to use a digital SLR and to dispense with the laptop. This would not be as sensitive as the dedicated CCD camera but would still produce impressive results.
|
