Cassegrain/RC

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Overview

ASA produces Cassegrain and Ritchey-Chrétien telescopes in 3 sizes for serious amateurs and professionals: 400mm (16 inch), 500m (20 inch) and 600mm (24 inch). ASA also produces larger telescopes for professional use - these are not described here.

This page describes these telescopes and their installation and configuration.

Specification

Because these are semi-professional telescopes their design can be customized for each customer. The standard configuration is as follows:


400mm

500mm

600mm
Cassegrain Richey-Chrétien Cassegrain Richey-Chrétien Cassegrain Richey-Chrétien
Aperture 400mm 400mm 500mm 500mm 600mm 600mm
Focal Length 3600mm 3200mm 4500mm 4000mm 5400mm 4800mm
Focal Ratio f9 f8 f9 f8 f9 f8
Max Back Focus 400mm 400mm 365mm 365mm 360mm 360mm

The maximum back focus includes the 92mm flange attached to the back-plate.

Features common to all variants

Construction Open truss using aluminum and carbon fiber
Cooling Automated fans built in to primary mirror cell controlled by Autoslew
Focusing Motorized secondary mirror controlled by ACC

Standard Options

Mirror Covers "Barn door" motorized primary mirror covers controlled by ACC
Dovetails Dovetails for mounting on ASA and 3rd party mounts.  Also dovetails for piggy-backing secondary OTAs

Installation & Configuration

Physical installation

ASA 400/500/600 Cassegrain/Richey-Chrétien OTAs are shipped ready-assembled, normally packed vertically in a wooden crate.  If a mount dovetail plate was ordered this will normally be pre-installed by ASA and the image train flange is also pre-installed.

Image train equipment should be installed after the OTA is mounted to prevent risk of damage.

The mount should have the counterweights installed before mounting the OTA and should be positioned so that the counterweight arm is downwards and locked.  The OTA can then be safely lifted on to the mount dovetail without a risk that the mount will rotate.

The OTAs are heavy and it is highly desirable to arrange for lifting gear to be available so that they can be installed on the mount without risking damage.  The truss construction is very strong, but to minimize the risk of distortion it is desirable to attach lifting ropes near the center of gravity, where the truss tubes meet the primary mirror cell.

Balance

Balance in right ascension is achieve using the mount counter-weights and is described in the mounts section.

Balance in declination can be achieved by moving the OTA forward or backward on its mounting plate, but this should only be used for very approximate balancing as the weight of the OTA makes fine adjustment almost impossible.  A better approach is to add a second "piggy back" mounting plate, opposite the mount dovetail.  Weights can then be attached to this second mounting plate and adjusted to achieve a fine balance.

[AUTHOR'S NOTE: We need more detail here and we welcome other users' experience.  The original author has a secondary OTA piggy-backed on his 400mm Cassegrain which is attached using an adjustable dovetail and this can be easily moved to make fine weight adjustments].


Back focus

Back focus is the distance available to you for your imaging equipment. According to the ASA specifications for their Cassegrain and Richey-Chrétien telescopes the available back focus is:

400CA &v400RC 500CA & 500RC 600CA & 600RC
400mm 365mm 360mm

[These are the nominal back focus distances - the actual distance will change as the secondary mirror is moved during focusing]

However it is not as simple as that:

  • Firstly, beginners may think that back focus is measured from the rear of the back plate.  Actually, for Cassegrain and Richey-Chrétien telescopes it is measured from the surface of the primary mirror (or rather where the surface would be if the hole in the mirror did not exist).   In the case of the 400CA, for example, the nominal distance from the mirror surface to the rear of the back plate is 92mm, so this immediately reduces the available back focus to 308mm.
  • Secondly, ASA Cassegrain and Richey’Chrétien telescopes are supplied with a flange that is bolted to the back of the mirror cell structure and is used to attach the image train.  The standard depth of this flange is also 92mm (from the back plate), so now you only have 216mm available back focus (308 – 92).
  • Thirdly, if you have correctors or flatteners installed these take up space (e.g. 20.4mm in the case of the ASA 4” field flattener for Cassegrain systems).  However in addition to the physical size of the corrector/flattener, their optics will usually affect the focal length of the telescope.  For example the ASA 4” field flattener reduces the focal length by 1.17mm, meaning that including it in the image train takes up 21.57mm (20.4 + 1.17) of your available back focus.
  • Finally, any other glass in the image train, including filters and the camera window, will increase the effective focal length because the refractive index of glass is different to air (see here for explanation).  Typically filters and camera windows will add 1mm each to the focal length (may be more or less depending upon the glass thickness and composition).
  • Some equipment may need to be placed at a particular distance from the focal plane.  For example, correctors and field flatteners are designed to provide optimum performance at a precise distance from the focal plane (the camera CCD).  This will usually mean that you will need an adapter between the corrector/flattener and the camera (or filter wheel if you have one installed) to ensure it is at the right distance.  This adapter will reduce the available back focus for other equipment.

If you order your Cassegrain/RC OTA from ASA or an ASA reseller and provide them with details of your proposed image train then ASA will perform the back focus calculations and can supply adapters of the appropriate size.  However it is always wise to calculate the back focus yourself in case of mistakes to avoid delay (it can take several weeks for a custom adapter to be re-manufactured).

An example back focus calculation is provided here.

Collimation

To avoid image aberrations or distortions all elements that affect the image path must be correctly aligned to each other.  This is called collimation.

Although the telescope should have been collimated at the factory it is very likely that some elements may have shifted out of alignment during delivery or storage so it is necessary to check collimation and adjust it where required.

For an ASA Cassegrain or Richey-Chrétien telescope collimation is not difficult.  It involves 4 steps:

  • Aligning the primary mirror baffle
  • Aligning the image train flange
  • Aligning the secondary mirror
  • Aligning the primary mirror

The first 3 of these can be done in daylight.  Unless you have access to specialist equipment the alignment of the primary mirror must be done at night.

Using the collimation screws

In all of the above 4 steps, collimation is achieved by adjusting 3 sets of screws, mounted at 120 degrees to each other to form a triangle.

Each set of screws comprises 3 individual screws.  The central screw is a “pull” screw – when rotated clockwise it pulls the element towards you.  The two other screws are “push” screws – when rotated clockwise they push the element away from you.

So if you want to push the element away you first loosen the pull screw by the required amount then you tighten the push screws to lock it in position.

If you want to pull the element towards you then you first loosen the 2 push screws by the required amount then you tighten the pull screw to lock it in position.

Normally it should only be necessary to adjust 2 of the 3 sets of screws to achieve the desired alignment.  If you have difficulty visualising that then imagine a triangular plate, mounted on legs at its 3 corners.  By altering the length of 2 of the legs you can point the plate in any direction – you do not need to adjust the 3rd leg.

With practice, adjusting the screws to correctly align an element is a quick process.  However for beginners it is best to only adjust 1 set of screws at a time and observe the effect.  Make sure that the 2 sets of screws you are not currently adjusting are not excessively tight – this will avoid stressing the element.

Required tools

You will need a set of hex keys (Allen keys) to adjust the collimation screws.  You will also need a laser collimator (e.g. the Howie Glatter collimator) and a draw tube to hold the laser which is compatible with the image train flange on the telescope (ASA can supply the draw tube and laser collimator).

Preliminary steps

Except for primary mirror collimation (which requires the full image train to be installed), the other 3 collimation steps (baffle, image train flange and secondary mirror) are best carried out with the image train removed – leaving just the image train flange in place.

It is not necessary to remove the back-plate.

Aligning the primary mirror baffle

The baffle is glued to a triangular plate inside the back-plate.  The 3 sets of baffle collimation screws are accessible on the rear of the image train flange and are labelled “BA” (the other 3 sets labelled “IT” are for the image train, see the next section).

The main object in aligning the baffle is to ensure that it points to the center of the secondary mirror to avoid vignetting.  Check this by placing your eye at the focal plane position (approximately 200mm behind the image train flange) and look to see whether the secondary mirror appears to be centered in the baffle tube.

A secondary objective, where the optional primary mirror covers are installed, is to ensure that the baffle does not catch on the covers as they open and close.

If primary mirror covers are installed it is best to slightly open them before adjusting the collimation screws.  Otherwise, if you adjust the screws excessively this may force the baffle against the mirror covers and may cause it to become detached (it is only glued to its triangular plate).

Use the BA collimation screws as described above to adjust the baffle as necessary until it is correctly aligned.

Aligning the image train flange

The image train flange projects through the back-plate and provides the mounting point for the image train.  The image train flange can be collimated using the 3 sets of screws labelled “IT”.  The objective is to ensure that the image train is aligned to point exactly at the center of the secondary mirror.

Install the draw tube on the image train flange, ensuring that it is tightened correctly.

Install the laser collimator in the draw tube, tighten it in position and turn it on.  The laser beam should fall on the secondary mirror, near the recessed spot at the center.

Now adjust the IT collimation screws as described above to adjust the image train flange so that the laser beams falls precisely in the middle of the recessed spot at the center of the secondary mirror.

You have not quite finished.  It is possible that the laser itself is not collimated.  To check this, rotate the laser collimator in the draw tube through a complete circle, pausing every 90 degrees and re-tightening it in the draw tube (it is important to re-tighten).  Check that the laser beam still falls in the middle of the spot after each 90 degree rotation.  If the laser beam seems to rotate as the laser is rotated then this indicates the laser itself needs to be collimated.  A good quality laser should have collimation screws that can be adjusted to ensure it is perfectly aligned.  After collimating the laser re-check whether the laser beam still falls precisely in the middle of the recessed spot at the center of the secondary mirror and readjust the collimation screws if necessary.

Aligning the secondary mirror

After aligning the image train flange as described above, the laser beam should be falling directly on the recessed spot at the center of the secondary mirror and will be reflected back towards itself.

Look from the secondary mirror towards the laser to see where the reflected beam is falling.  A mirror may help with this.  Be very careful not to look directly into the laser beam.

If the secondary mirror is properly aligned you will not be able to see the reflected beam as it will fall directly on top of the laser, you will just see an even glow around the laser.  If you can see the reflected beam to one side of the laser then you need to adjust the secondary mirror collimating screws until the reflected beam coincides with the laser beam.

The three sets of collimating screws are located to the rear of the secondary mirror cell and are adjusted as described above.

Aligning the primary mirror

[AUTHOR'S NOTE: The primary mirror alignment process is essentially the same for Newtonian Astrographs and Cassegrain/Richey-Chrétien telescopes so the following section can probably be expanded and put on a separate page, linked to from both here and the Astrographs page].

This step has to take place at night with clear visibility and reasonable seeing.  This process assumes you are using a camera, although you can use an eyepiece if one is available. 

While alternative methods exist, the most basic approach is as follows:

  • Work on a night with reasonably good seeing and allow the equipment to reach thermal equilibrium before starting (to avoid air currents and thermal distortions confusing the process).
  • Choose a bright star near the zenith.  The reason for choosing the zenith is to reduce atmospheric interference and to ensure the weight of the primary mirror is falling equally on the collimation screws.
  • Center the star on the camera CCD and defocus it.
  • The star should have a “donut” appearance with a dark center surrounded by a fuzzy light (the dark center is the obstruction caused by the secondary mirror).
  • Collimation is achieved by adjusting the primary mirror collimation screws until the dark center is exactly in the middle of the “donut”.  Make sure you re-center the star after each adjustment.

FAQs

Question Answer

See also

Autoslew, ACC and Sequence software pages for the installation and operation of the software required to operate these telescopes.

Mounts pages for the integration of these telescopes with ASA mounts.