Omegon Pro Ritchey Chretien 154 User manual

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Operating Manual
Full Tube Ritchey Chretien Telescopes
Omegon® Pro Ritchey Chretien
154/203/254
German Version 6.2017 Rev A 53809; 53810; 53811

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Omegon® Pro Ritchey Chretien Full Tube
Congratulations for the purchase of your new Omegon® Pro Ritchey Chretien telescope. Among the Cassegrain systems the Ritchey-
Chretien is the unrivalled champion: it is the two-mirror telescope with the highest correction level available. In contrast to other
variants, like Pressmann-Charmichel, Dall-Kirkham or the classical Cassegrain, the Ritchey-Chretien reflector shows you a completely
coma-free field full of round stars even without an additional corrector. The difference to other types of design is so significant, that
almost all large telescopes used for scientific purposes are of the Ritchey-Chretien design.
Due to the high production costs for the mirrors, real Ritchey-Chretien telescopes were, until recently, unaffordable for amateur
astronomers, because of their high price. Now we are glad to be able to present fully matured, real Ritchey-Chretien reflectors for a
reasonable price.
The Omegon® Pro Ritchey Chretien reflectors can be used for visual observations and photography without limitations, but develop
their full strength, above all, when used for Deep-Sky observation and photography. When visually panning across the milky way
they show innumerable small stars right to the edge of a well corrected eyepiece. From an aperture of 250mm a camera with APS-C
format chip will produce dot shaped stars right to the edge without the use of an additional corrector –with smaller telescopes the
remaining blurring can be completely corrected by using a field flattener.
Ritchey-Chretien units are compromising pieces of equipment for professionals, the superior image quality requires a certain amount
of adjustment work. The generously dimensioned focussing range gives the expert the opportunity to use field flatteners for large
chips, focal length reducers and focal length extension optics. There remains nothing to be desired. The included eyepiece holder is
able to carry a conventional DSLR without any problems and the size of its diameter ensures vignette free exposure of almost all
cameras. Special heavy duty eyepiece f´holders are available for extra heavy cooled cameras.
1. Included accessories:
For easy use the product is delivered with several accessory parts: Please have a look at the parts list for later
recognition.
53809 Omegon® Pro Ritchey-Chrétien 154/1370
Optical tube with 1 x fastening rail 44mm Vixen standard, 2” Crayford focuser and reducing sleeve 2”/1. 25”,
Vixen/Skywatcher finderbase.
Focuser extension sleeves: 2x 25mm, 1x 50mm
53810 Omegon® Pro Ritchey-Chrétien 203/1624
Optical tube with 1x 3”Losmandy fastening rail and 1x combination fastening rail 3”Losmandy/44mm Vixen, 2”
Crayford focuser with reducing sleeve 2”/1.25”, Vixen/Skywatcher finderbase
Focuser extension sleeves: 2x 25mm, 1x 50mm
53811 Omegon® Pro Ritchey-Chrétien 254/2000
Optical tube with 1x 3”Losmandy fastening rail and 1x combination fastening rail 3”Losmandy/44mm Vixen, 3”
Crayford focuser with reducer M74x0.75 to 2”plug-in and reducing sleeve 2”/1.25”, Vixen/Skywatcher finderbase
Focuser extension sleeves: 2x 25mm, 1x 50mm
Battery holder for tube ventilator

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2. Preparation.
Before you start using the equipment it is important to know the main control elements. There are two groups of control
elements, as shown (Fig. 1 - 6).
1) Optical tube
2) Secondary mirror
3) Secondary mirror holding screw
4) 3x secondary mirror adjustment screws
5) Secondary mirror cell with lens shade
6) Lower dovetail rails (3”Losmandy standard and
44mm Vixen standard combination rails)
7) Dovetail rail top (3”Losmandy standard)
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Figure 2
Figure 3
Figure 1

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)
8) Finderbase, Vixen standard
9) Cap nut to fasten the focuser
10) Focuser
11) Eyepiece clamping
12) Thumbscrew for 2”connection
12) Reducing adapter 2”/1.25”with protection ring and
thumbscrew
12) Coarse focusing right
13) Fine focusing
14) Coarse focusing left
15) Focuser clamping screw
16) 50mm focuser extension sleeve
17) 2x 25mm focuser extension sleeve
Figure 4
Figure 5
Figure 6

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3. Commissioning
a) Mounting the tube
The telescope is equipped with a rail (8” and 10” versions with two rails), which enable secure
fastening of the telescope tube on a mount. In order to do so open the dovetail clamping on your
mount so far, that the rail can be inserted into the receptacle. Make sure that the rail is properly
seated in the receptacle –with larger tubes you will have no direct vision to the receptacle of the
mount, because the tube is in the way. In such a case it may happen that one assumes that the tube is
correctly clamped, even though it is canted in the receptacle. If this is the case, the tube will most
certainly fall out at some time and hit the ground of the observer. Depending on the size of the tube,
not only the tube will be destroyed, but it will cause further damage or injuries.
Once you have made sure that the rail is perfectly seated in the receptacle, just clamp the rail down
with the clamping facilities of the mount. If you intend to balance the tube, just slacken the clamping
so that the tube can be moved –but will not fall out.
If you perform this step for the first time you should ask a second person for assistance and to check
whether everything is fitted correctly. Practice this tube installation for a few times –later you must
be able to do this work in the dark and without any help.
b) Using focuser and spacer sleeves
Your Ritchey-Chretien telescope is equipped with a focuser for focusing the image. Several spacer
rings for installation between focuser and tube are also included. This design gives you the benefit of
being flexible when it comes to usable accessories. Depending on the aperture, your Omegon Ritchey-
Chretien telescope is either fitted with a 2” or a 3” focuser and the matching spacer sleeves.
At the first glance spacer sleeves appear to be very impractical –wouldn’t it be much easier to simply
make the tube of the eyepiece holder longer and do without spacer sleeves. However, this would be
of disadvantage, because the longer eyepiece holder tube would bend more extremely under load –
this is why we decided on a short eyepiece holder tube with approx. 50mm draw tube travel and the
use of several spacer sleeves. The fact that the large diameter of the spacer sleeves prevents
vignetting is an additional advantage. The rear end of the tube features a large thread. This thread
carries the focuser, and the spacer sleeves also use this thread. When working in the dark please
make sure not to cant the spacer sleeves –this would damage the thread.
The focuser has a big adjusting wheel for coarse focusing on either side, and a smaller black
adjustment wheel for fine tuning on one side. Apart from this there is an additional knurled screw
under the adjusting unit to fix the eyepiece holder and a pressing screw for the adjusting unit. This
pressing screw should never be loosened completely to avoid slipping of the eyepiece holder. Do not
apply force to turn to a certain position, if something blocks the eyepiece holder. Over the course of
time the feed shaft would damage the running surface and the eyepiece holder would no longer work
correctly. Should the eyepiece holder be blocked, do not continue to turn to the same direction, but
check, whether the eyepiece holder is blocked or has reached its end position. Or may be its only a
tightened eyepiece holder thumbscrew. In order to rule out any operating faults when working in the
dark, you should become familiar with the eyepiece holder and its control elements during the
daytime. The focuser can be removed from the tube by simply unscrewing the silver fastening ring on
the tube side end of the focuser. At this point one can install one or several spacer rings between
focuser and tube in order to match the position of the focuser to your own accessories. Depending on
whether to use a diagonal mirror for visual observations or directly attach a camera for
astrophotography, just use a appropriate spacer sleeve to achieve a convenient focal position and the
best possible stability. When photographing with a DSLR without any additional accessories, one will

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most certainly use all spacer rings, when photographing with a focal reducer and a camera with
additional filter wheel, spacer rings may not be necessary at all for focusing.
c) Adjustment of optics
You can generally adjust the telescope by using a star. However, we highly recommend the purchase
of adjusting accessories for this telescope type.
What does adjustment mean and why does a reflector need to be adjusted from time to time?
The light gathering system of you telescope consists of two mirrors: The large mirror at the bottom end of the
telescope, which collects the light and the smaller secondary mirror, which directs the light to the eyepiece,
where it is then available for watching. The tilt and the distance of both mirrors to each other and to the
eyepiece holder thereby is a decisive factor for the performance of your telescope. However, a telescope with
precision ground mirrors will still produce a very poor image, if it is deadjusted. Each of the two mirrors is
therefore movably mounted and can be precisely tilted and displaced. While the distance between main
mirror and secondary mirror is of almost no importance in some telescope types, such as e.g. the spherical
Schmidt-Cassegrain telescope or the very popular Newton telescopes, with the Ritchey-Chretien telescope
you must take care not to change the distance between the two mirrors when you make any adjustments.
What needs to be adjusted?
The adjustment aims at the alignment of both telescope mirrors and the eyepiece holder, so that the centres
and focal points of the mirrors are on one common axis that runs through the middle of the tube, the so
called optical axis. Apart from that, the centre of the eyepiece holder tube must also match the optical axis.
What needs to be accounted for?
In principle there are three components which can be tilted to two directions and displaced along s convoy.
The correct order of adjustment processes is of utmost importance, otherwise one will never get finished. Pre-
adjustment work usually takes place in a brightly lit room, the final fine tuning is then made on a star via
photo, or observation with high magnification.
What tools are needed?
A Ritchey-Chretien telescope responds more sensitively to any de-adjustment than any other telescope type,
but achieves a better image quality when correctly adjusted. We therefore highly recommend the purchase of
a laser pointer and a collimation eyepiece. The following description explains the adjustment using these
tools. These tools are generally not required –but without these, a simple adjustment may take several
nights, even for experienced users.
For the following procedure we will use:
33141 Omegon Collimation Eyepiece
4577 Omegon Newton Laser Pointer 1.25'' with sight glass

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Procedure:
A) Adjustment with laser pointer
The baffle, that carries the main mirror, connects the eyepiece holder of the RC with the main mirror. We
will now adjust the eyepiece holder in such a way, that it points “straight” to the secondary mirror. Take
the front lid off the telescope and look on the main mirror under an oblique angle. Apart from the main
mirror and the baffle, you can also see the reflection of the secondary mirror with its holder in the main
mirror. You see a small circle in the middle of the secondary mirror. This is the centre marking of the
secondary mirror. Now insert the 4577 Omegon laser pointer into the eyepiece holder and switch it on.
Under ideal conditions you will now see the laser reflex in the centre marking of the secondary mirror,
and the laser point will also be visible in the sight glass of the laser pointer. The laser point will not
perform a considerable movement when turning the focuser forth or back or when turning the laser. And
now step by step.
1) Checking the adjustment of the laser.
Just like any other piece of optical equipment, the laser pointer will also get de-adjusted. This is generally
no problem, one simply readjusts the laser. However, it would be fatal to use a deadjusted laser to adjust
a telescope. This is why the laser is always checked first. Insert the laser into the eyepiece holder to do so.
For the following procedure make sure that the laser sits level on the eyepiece holder without any tipping
movement. Now rotate the laser –if the laser is correctly adjusted, the reflected laser in the sight glass
will not move. If it moves, adjust the laser as described in the operating instructions for the laser.
2) Adjusting the eyepiece holder using the laser.
Make sure that the pressing screw of the eyepiece holder is slightly tightened, so that the eyepiece holder
moves neatly without skewing or slipping. Now move the eyepiece holder to approx. the middle of its
adjustment range (25 on the scale). Insert the laser and then focus forward and back. Watch the laser
reflex on the secondary mirror by looking into the telescope from the front. The laser reflex should not
move and be exactly in the middle of the secondary mirror mark. Should the reflex on the secondary
mirror move, you must enhance the adjustment of the eyepiece holder, to make sure that the laser point
will stay in place when focusing. Now adjust the unit consisting of eyepiece holder and main mirror to the
secondary mirror, so that the laser points exactly into the centre mark of the secondary mirror. On the 6”
and 8” Ritchey-Chretien the associated adjustment screws are located directly on the bottom of the
eyepiece holder and are slightly difficult to reach. On the 10” model the adjustment screws are located on
the outside of the tube and are very easy to reach. The procedure is the same for all models: Turn the
small grub screws to press the mirror slightly forward, then turn then round head socket head cap screws
to fix this setting. Now move the laser beam into the middle of the secondary mirror centre marking.
3) Adjusting the secondary mirror.
Now use the socket head cap screws on the secondary mirror holder to guide the laser beam back into
the sight glass of the laser pointer. Here are no pairs of adjusting and counter screws –each screw is
countered by the two other screws. If you want to slight tighten one screw, you must first slightly loosen
the other two screw. Do not loosen the central Philips head screw in the holder! At the end of the
adjustment process the reflected laser should be in the middle of the secondary mirror centre marking, as
well as in the middle of the sight glass of the laser pointer and should not excessively move when
focusing. The coarse adjustment is thus finished –fine tuning can now take place on a star, but is normally
not required.

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B) Adjustment with collimation eyepiece
We proceed iteratively. The aim is to achieve concentric circles around middle of the field of view
identified by the reticule of the collimation eyepiece.
Figure 7: View through the collimation eyepiece
i.
ii.
iii.
iv.
v.
vi.
vii.
This is what you should see when looking through a well adjusted Ritchey-Chretien telescope with the 33141
Omegon Collimation Eyepiece. You will see the following pieces:
i. The grey slanted cross is the reticule of the collimation eyepiece.
ii. The small quarter circles at the intersection of the reticule are parts of the small circle
that marks the centre of the secondary mirror.
iii. The bright area in the middle around the small circle is the image of the bright area in
the collimation eyepiece
iv. The black border around the bright area is the secondary mirror holder with lens shade.
v. The bright area around the black border is the image of the main mirror, which is
separated into four segments by the 4 black secondary mirror spider vanes.
vi. The thin black border around the main mirror is the main mirror holder
vii. Right at the outside the stray light in the bright room shows the distance between
main mirror and tube
The adjustment of the telescope generally follows the same sequence as with the laser pointer. You must
therefore always make sure to loosen one or two counter screws, before tightening any other screw. Once the
adjustment process is finished, all counter screws must be tightened. Please make sure that the screws are
only tightened hand-tight! They are adjustment screws for high precision equipment –brutal force is strictly
prohibited.
C) Fine tuning on a star. The last few fractions of a millimetre that still separate you from the perfect
adjustment by following the procedure described above, can be adjusted by
Figure 7

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using an artificial or real star. The following, extremely enlarged picture shows the ideal star, as it
should be visible in the centre of the field of vision in the telescope –a round bright circle, the so-
called Airy disc, with one or several concentric, round diffraction rings. Please note, that this
picture will normally not be visible, even with a perfectly adjusted optical system –the star disc
will dance and drift because of air turbulence. It is therefore necessary to keep looking through
the eyepiece in order to be able to judge whether one sees a round, symmetric image with air
turbulence, or a one or two side distorted image. A deadjusted star will most likely appear as
shown in the picture at the left –a squeezed ellipse, that will turn by 90° during focusing. During
fine tuning one will use photos or look through the eyepiece to adjust the equipment so, that the
star in the middle of the image looks like the one in the upper picture.
Solar warning!
Do not use this telescope to watch
the sun!
Watching the sun without special
filter causes immediate and
irreversible blindness!
Do not leave children on their own
with the telescope without
supervision!

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Appendix A: Technical Data
53809 Omegon Pro Ritchey-Chrétien 154/1370
Aperture 154mm
Focal length 1.370mm
Aperture ratio f/9
Tube construction Full tube type
Optics type Ritchey-Chretien Cassegrain with two hyperbolic mirrors
Mirror substrate BK-7/H-K9L
Obstruction caused by secondary mirror holder 72mm
Lens shades 7 pieces internally
Tube diameter 191mm
Tube length without focuser 410mm
Tube length with standard focuser 490mm
Tube weight without focuser 4.600g
Tube weight with focuser 5.400g
Focuser 2”Crayford focuser with 10:1 gear reduction and reducer from
2”t0 1.25”, fully rotating
Focuser connection thread M90x1mm
Focuser adjustment travel 34mm
Focuser extension sleeves 2x 25mm; 1x 50mm
Finderscope Optional
Finderbase Vixen/Skywatcher
Focal distance from end of tube 240mm
Focal distance above the retracted focuser 237mm
Ventilator No ventilator

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53810 Omegon Pro Ritchey-Chrétien 203/1624
Aperture 203mm
Focal length 1.625mm
Aperture ratio f/8
Tube construction Full tube type
Tube material Steel
Optics type Ritchey-Chretien Cassegrain with two hyperbolic mirrors
Mirror substrate Quartz crystal with extremely low thermal length extension
Obstruction caused by secondary mirror holder 95 mm(47% of diameter, 22% of area)
Lens shades 10 pieces internally
Tube diameter 229mm
Tube length without focuser 470mm
Tube length with standard focuser 560mm
Tube weight without focuser 6.730g
Tube weight with focuser 7.450g
Fastening One attachment dovetail rail 3”Losmandy format, one
fastening rail combination for 3”Losmandy and 44mm Vixen
universal dovetail receptacles
Focuser Detachable Crayford focuser with linear guidance with 10:1
gear reduction and reducer from 2” t0 1.25”, fully rotatable
Focuser connection thread M90x1mm
Focuser adjustment travel 50mm
Focuser extension sleeves 2x 25mm; 1x 50mm
Finderscope Optional
Focal distance from end of tube 237mm
Focal distance above the retracted focuser 140mm
Ventilator No ventilator

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53811 Omegon Pro Ritchey-Chrétien 254/2000
Aperture 254mm
Focal length 2.000mm
Aperture ratio f/8
Tube construction Full tube type
Tube material Steel
Optics type Ritchey-Chretien Cassegrain with two hyperbolic mirrors
Mirror substrate Quartz crystal with extremely low thermal length extension
Obstruction caused by secondary mirror holder 110mm(43% of diameter, 18.75% of area)
Lens shades 7 pieces internally
Tube diameter 299mm
Tube length without focuser 625mm
Tube length with standard focuser 722mm
Tube weight without focuser 14.610g
Tube weight with focuser 15.600g
Fastening One attachment dovetail rail 3“ Losmandy format, one
fastening rail combination for 3” Losmandy and 44mm Vixen
universal dovetail receptacles
Focuser Detachable 3” Crayford focuser with linear guidance with 10:1
gear reduction and reducer to 2” and from 2” t0 1.25”,
eyepiece side thread M74x0.75mm, fully rotating
Focuser connection thread M117x1mm
Focuser adjustment travel 50mm
Focuser extension sleeves 2x 25mm; 1x 50mm
Finderscope Optional
Focal distance from end of tube 242mm
Focal distance above the retracted focuser 138mm
Ventilator 3 pieces installed, 12V

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Appendix B: Recommended accessories
33141 Omegon Collimation Eyepiece
4577 Omegon Newton Laser Pointer 1.25”with sight glass
51284 Astro Physics 0.67x Reducer 2”
32974 Omegon red dot finder Deluxe
47014 Omegon LED finder

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Appendix C: Practical hint: Cleaning the optical system
After some time at the latest any star gazer will notice that pollution has spread over the optical surfaces of his
telescope.
The following is generally valid: small dust particles and other minor contamination has no significant effect on the
performance of an optical system and may thus remain on optical surfaces!
Any cleaning is work and bears risks and should therefore and should therefore be carried out at rarely as possible! In
addition to the certainty that regular cleaning will leave scratches in the optical system, there is also the risk of the
optics dropping down, especially when wet cleaning!
The most frequent types of pollution on telescopes are dust and pollen, on eyepieces grease and residues from lacrimal
fluid. When using a strong lamp, pollution will practically always be visible, even on completely new optics.
When should a telescope be cleaned?
The optics should be cleaned if the picture negatively affected when looking through. With extremely polluted optics
bright objects, such as planets, show a light ring, similar to a light dew deposit. Only then optics needs to be cleaned –
not if dust or small dirt particles can be seen on the mirror.
There are only a few exceptions from this rule:
1) Contamination by pollen. Pollen contains sugar and is degraded by bacteria, which settle on the optics. These
organisms exude acid containing substances, which could attack the optical system. If a yellowy deposit is
detected on the optics after a star gazing session in spring, the optical system should be cleaned.
2) Large area contamination. If beverages or other fluids have accidently come into contact with the optical system,
the optics must be cleaned Even if these fluids were clear, components in these fluids or there degradation
products may attack the coating of optical components.
3) Eyepiece lenses since the optical components in eyepieces are near focus, bigger dust particles will become
visible, e.g. dirt on eyelashes will disturb the image. Eyepieces must therefore be cleaned more frequently.
Cleaning should be performed as follows:
1) Brush off coarse dirt particles with a soft brush
2) If possible spray-clean the optics with distilled water, e.g. with plant sprayer. Make sure that no water enters into
the optical system, use e.g. a kitchen towel to wipe it off the outer border of the optics. Eyepieces are cleaned
with the eye lens pointing down, so that the cleaning fluid cannot enter into the space between the lenses.
3) Do not wipe off the few drops that will still be on the optics after cleaning, suck them off with a soft cloth.
4) Only wipe if it is absolutely necessary –do not apply pressure! Always make sure that the cloth is clean and
suitable for this purpose, e.g. the 21290 Omegon® cleaning cloth or the 47315 Omegon® SPUDZ micro-fibre
cleaning cloth.
5) Only use cleaning fluid if distilled water is not able to remove the dirt. Ideal for this purpose are special fluids, e.g.
the fluid contained in the 5551 Omegon® 5 in 1 cleaning set. If you want to mix your own cleaning fluid you must
always make sure, to solely use clean components from a chemist shop. As an example, a solution with isopropyl
alcohol and/or Ethanol is suitable for cleaning the optics, but the use of spirit instead of the higher quality Ethanol
will sustainably ruin the coating of the optics. Re-coating such a single item will normally cost you a four digit sum
of money –one should therefore always try to avoid such damage. The use of fragrances as frequently used in
commercial glass cleaners is strictly prohibited. These any many other substances will leave a film on the surface,
which is not or hardly visible when just examining the object. However, when looking through the optics, the
image will be heavily affected. The same applies for the use of the more reasonably priced demineralized water
from a DIY-shop instead of the real distilled water from a chemist. One should in any case try and test the fluids
and methods on an e.g. glass table to find out if any residues will remain after drying.