Overview of some accessories for your scope
Star diagonal (II)
By default most manufacturers provide a standard star diagonal with their scope to prevent some contortion when looking at objects high in the sky. Great idea ! But the quality is not always their priority and many of them give you a black plastic box containing a simple alumined prism. However, you know the star light passes through this accessory before reaching your eyes and it should be sad to loose some dozen percents of light during this process (up to 10 % for each accessory).
The best solution that ensures you keep 100 % of the incoming light is using a mirror surface with Everbrite coating which is specially designed for its high resistance to mechanical and chemical alteration (sand, moisture, etc.).
As I explain it in a report dedicated to star diagonals, models sold by Astro-Physics and Tele Vue present an average reflectivity of 99 % in white light. The cheapest model uses a 1.25" barrel ($75-134) but if your scope supports a larger one, the 2" MaxBright from Astro-Physics is the one you need ($255).
Like others manufacturers, Tele Vue also sells a set called the "Rich Field Kit" for $480, including a 2" mirror star diagonal ($190) with Everbrite coating, a 2" 55mm Plossl ($230) and a 1.25"/2" adapter. Both models will let you screw a standard 2" (48 mm) filter into the front end of the diagonal. Most of them are available from major dealers like Focuscamera.
NB. It is not necessary to "plug" the back of an SCT with a filter. The only time the back is open is when you are changing eyepieces. The 2" diagonals come with plastic caps, so you can close the back of the SCT with a cap on the diagonal when the scope is not in use.
At last, do not confuse the star diagonal showing an angle of 90° with the erecting system usually provided with spotting scopes or small telescopes like Celestron C90 or C70 that shows an angle of 45°. This accessory is almost useless to observe celestial bodies and for ergonomy reasons and compatibility with other accessories, it is by far more useful to replace it with a 1.25 or 2" diagonal.
Barlow and Powermate
A Barlow lens ($45-110) is an optical device constituted between 2 and 4 element-lenses used to diverge the incident light beam in order to increase the focal length of the primary objective while keeping the eye relief. If selected in a well-known brand, it is apochromatic without adding the slightest color. The magnification power of such a Barlow is usually between 1.5 and 4 times. In this way your f/10 scope may become a f/40 (+15 %) at its prime focus.
Various formulae can be used to calculate the Barlow focal length and magnification. Some are listed in this page from ASTUNIT website.
The combo eyepiece-Barlow presents other advantages too. For high-end wide field eyepieces this improvement is not always valid but using Orthos or Plossls, aberrations will be reduced; the edge field will be sharper. Of course with both short and long focal eypieces, this combo will get a longer eye-relief too.
As a Barlow diverges the light beam, your image, in fact the exit pupil, will be projected further out, which means the eyepiece focal length will also be through out according its focal (long focal eyepiece, greater throw out of exit pupil). This is the only known drawback of a Barlow.
Note that Tele Vue Panoptics are not "sensitive" to this effect because they include a Barlow by design. Moreover, the Tele Vue Powermate (2.5x at $175 for the 1.25" version, it is available up to 5x) prevent this eye-relief effect. Also able to recollimate the diverging light beam this accessory is telecentric (very useful for a home-maded solar telescope) and does not create vignetting.
The Powermate gives better images than a Meade Barlow for example (colorless, sharper) and can successfully be used with short focals eyepieces (one has to be silly somewhere !). The Powermate also corrects the field angles and presents a huge advantage on a Barlow when observing the sun in H-alpha light as the latest yield only a part of the field of view. All these advantages mean the Powermate can easily replace any Barlow at first run.
Using a Schmidt-Cassegrain, Maksutov or a Simak telescope, how to get a larger field of view than the one provided by your lowest and widest eyepiece ? In order to circle vast area of the sky as open star clusters or rich fields of the Milky Way, the only solution is using a small optical interface at your prime focus that shorten the focal length of your scope to "open" its relative aperture. This accessory is in fact a Shapley lens, a positive lens that one insert in front of the ocular system. Called a focal reducer, the Celestron 0.63x ($120) or the Meade 0.6x model give stunning results.
The indirect advantage of using a focal reducer is the drastic reduction of the time exposure. If you usually took your deep sky pictures at f/10 during 60 minutes, at f/6.3 the same picture will only need (10/6.3)2 = 24 minutes, almost 1/3d of the time. In other words, it will be 3 times faster ! When you have to record tens or hundreds of L, R,G and B images to stack later, you quickly realize the interest of a fast scope, hence the large market of refractors between 70-130 mm open between f/5 and f/8.
Most of focal reducers are also coma corrector to flatten the field which becomes more sensitive to such aberration. If in the past only a few manufacturers like Tele Vue provided such an accessory, today most of them as well as almost all dealers provide a focal reducer-corrector, some models being specially designed for astrophotography purposes.
Atmospheric Dispersion Corrector (ADC)
This optical accessory refocuses the incident light slightly dispersed when it passes through the atmosphere that acts as a true prism. So, at 45° of elevation, the white light (luminance) is deflected by 1.5", the blue light by 0.9" and the red light by about 0.3". The effect increases near the horizon where the dispersion reach 4" at 20° of elevation for the luminance. The ADC corrector is designed to refocus the light into the Airy disc although on the smaller instruments the corrected image may be larger than the Airy disc but at least it does not show any colored fringes. As we see below at center, thanks to the ADC the corrected images of Saturn are much sharper and it is valid for all celestial bodies.
The ADC corrector is offered by two manufacturers, ZWO that offers a model at $128 and Optikesysteme Gutekunst that offers a very high-end "Pro" model at... 7150 € VAT excluded as well as a compact (slim) model at 3500 € VAT excluded. Note that the two models sold by Professional Telescopes (and distributed by APM) are in fact the Gutekunst models. The price difference is explained in particular by the fact that the prism of ZWO ADC is polished at λ/10 at 632.8 nm while Gutekunst models are polished at λ/30 RMS. But it is not obvious that the difference is visible on amateur pictures. In any case, the one who is ready to invest more than 7000 € in such an accessory can afford a telescope of very large diameter equipped with an adaptive optics much more effective.
With or without Off-axis Guider
How to control the tracking accuracy of your scope once you placed your camera at the prime focus and you have depressed the flexible for a long exposition ? Surely not using the finderscope !
Several systems can be used to split your light beam, from the simple optical system using a small mirror diagonal to the optoelectronic probe like the Ferrier Instruments model, without speaking of CCD adaptive optic or using a secondary CCD fixed in parallel to the first.
The cheapest method is the "off-axis" guider, an easy way to control de visu the tracking rate of your scope and make some corrections if needed. You have to use together an Off-axis Guider ($80-170) and a reticle Guiding eyepiece, for example a Meade wireless 9 mm Plossl ($149) or the Orion wireless 12.5 mm Plössl ($129) to track with great accuracy your object and detect its slightest shifts.
The largest Off-axis guider is the Lumicon 2" Giant Easy-Guider ($395), a master piece usually fixed on large scopes and able to support flexion of heavy accessories like flip-mirror finder and CCD camera. The Giant Easy-Guider is a focal reducer and does flatten the field too. Its lens diameter is 80 mm. It does an excellent job visually and photographically, stars are pinpoints, without vignetting even with a Tele Vue 55 mm Plossl.
Many thing can be said about guiders. The smallest models split the light cone using a small internal star diagonal, other use a full aperture semi-reflective lens, including or not a focal reducer. High-end models as the SBIG one take advantage of CCD adaptive optic using a small detector to track a specific star and drive the scope, others are fixed on rotative plate to easily selecting the guided star in periphery of the field and use helicoidal focusers.
But the best solution is working without off-axis guider at all. If you are fan of CCD, you can for example use a SBIG ST-7EA ($2950), a special CCD device combining the popular Texas Instruments camera head TC-211 and the Kodak KAF-0400 detector for auto-guiding. This combination eliminates the need for an optional off-axis guider. However a good finder is always useful.
If we forget the cheapest and smaller focusers that git, jump and slide when you try to use them, sturdy models are often expensive and manufactured by only one designer. Celestron, Intes, Meade, Starmasters, Takahashi or Vixen for example sell such accessories. But most of them do not sell a focuser as a separate part except if you come with a broken one and a proof of buy.
The accuracy of a focuser is limited by the design of its Archimedes screw that insures the assembly shift. Turning the handle of a straight focuser of one ten of a turn shifts your focus quite a bit, the order of 5mm or so which is much too long for an accurate focusing.
The only solutions to improve the focusing are using micrometric handles as on microscopes or by changing the vertical slide into a helicoidal one. This is the concept used successfully by the Crayford focuser, available at Van Slyke Engineering or Markus Ludes's APM Telescopes among others. It is much like a rack and pinion focuser but rather than using gears it uses rollers and friction to move the drawtube.
How it is strange using an SCT to shift the primary mirror position to focus the scope ! To eliminate this mirror shift and inherent problems due to its weight, the Crayford focuser is a solution as the JMI NGS-F electrical focuser ($250). This last prevents vibrations, allowing a much more accurate focusing. This is in my humble opinion one of the most useful accessory you will never regret. The NGS-F focuser provides also a 2" adapter for large optics.
At last, if you want to be under cover of temperature changes, you can buy a temperature compensating focuser, like the model TCF-S from Optec (about $900 with accessories, also sold by Company 7). It is a high end motorized Crayford style focuser. It is available with a barrel of 1.25", 2" and even 3" for some specific scopes from Celestron and Meade.
This accessory being dedicated to CCD users mainly, it adds some weight at the back of your scope (up to 5 kg added to other accessories). As a standard 8" scope supports about 3-4 kg only, the maximum depending on the balance and the distance to rear cell, use the TCF-S preferably with robust scopes (OTA and mount), so usually larger than 250 mm (8.5") of aperture.
How to remove an incrusted dust grain or a thumbprint from a lens like an eypiece without a suited tool ? The LensPen is the accessory you need. This small accessory the size of a pencil is a must you should keep near your scope. Stand-alone, it is equipped with a brush and a magic liquid container. It prevents disasters using inappropriate liquids, clothes or an air brush to cleanup your lenses ($9).
Zoom 8-24 mm
Do you like flying near the Moon surface or diving into the Milky Way ? Yes, this is quasi the impression I feel using the Tele Vue zoom 8-24 mm as I explained in this dedicated zoom report. This 8-24 mm zoom is a very useful accessory that gathers in one eyepiece all the ones you need although it is limited in field size and overall performances.
There are three other models sharing similar peformances, the 8-24 mm from Vixen, Tele Vue and Meade, these two last being 20 % more expensive ($160-210). In addition, the Tele Vue model has a thicker eyecup which does not fall down easily as the Meade one, it has less focal marks but a much better support from Tele Vue than all other models which is in my humble opinion the most important thing after the product quality. A must that we can recommend to all users even those using a 60 mm scope. Recently for the VinoVue users Tele Vue provides a 8-24 mm zoom with click-stop.