Ritchey-Chretien + Reducer-Flattener = Zoom ?

Study : Newtonian vs SCT vs RCT for Astro-Photography

Usual Newtonian for Astro-Photography

Historic, good and simple Newtonian Telescopes are keeping some Real advantages :
- The simplier Newtonians without Coma-Corrector for small-sensor imagers have less
Obstruction than both RCT or SCT. My ~2003 SkyWatcher 203/1000 has 26%, and
my ~2007 SkyWatcher 254/1200 is only 23% obstructed, fine for high micro-contrast.
Astro-Photography with first a S.W. 200/1000, then a S.W. 254/1200 Newtonian
- These simplier Newtonian designs without Coma-Corrector are Far Less Expensive !
- When pointing toward zenit, the Camera is at the height of my eyes ! Not at the height
of my knees and Under the RCT or SCT, except in Fastar, Hyperstar or RASA config.

SCT ruined by its Rudimentary Focuser for Astro-Photography

Celestron SCT in Fastar or Hyperstar or RASA configurations have a Real advantage
over other Catadioptrics : a very Wide Aperture that allows Short Exposure durations.
However, it has to be admitted : these configurations are reserved for Wide-Field only.
And it's very difficult for me to imagine doing Astro-Photography from Celestron SCT.
From their former SCT design to their latter RASA design via their EdgeHD design in
Fastar or Hyperstar configurations, Celestron SCTs' drawbacks are - alas - the same :
- Focus Shift due to the Off-Axis Focusing Force applied to the Heavy Primary Mirror.
- Mirror Flop due to the Off-Axis Focus Knob trying to maintain that heavy 1st Mirror.
- Bigger cameras or imagers can't simply be used with the Celestron SCT-8, 11" too ?
... in Fastar or Hyperstar or RASA configs, with a camera larger than the 2nd mirror.
- Heat produced by the Camera or Imager's Cooler in front of the Schmidt Corrector
... in Fastar or Hyperstar or RASA configurations, some additional Turbulence to fear.
- 12V, RMT, USB Cable to the Camera, Imager stay in front of the Schmidt Corrector
... in Fastar or Hyperstar or RASA configurations, some additional Blur to fear (F/2) ?
- No Reducer nor Extender available in Fastar or Hyperstar or RASA configurations...
- SCTs in Fastar or Hyperstar or RASA configs are only for Wide-Field Astro-Photo.
- No more RASA-9"25 and the RASA-11" is Way Overpriced for me at 4400 !

Very Versatile Ritchey-Chretien for Astro-Photography

Why would I go Ritchey-Chretien RCT after so many years using Newtonian for Astro
Photography ? Because Newtonians and Ritchey-Chretien are the only instruments that
are 100% pure Reflectors : IR, Red, Green, Blue, UV focus exactly at the same place !
And Ritchey-Chretien Telescopes have many other big advantages over Newtonians :
- RCT are smaller and lighter than Newtonians for the same Primary Mirror diameter.
- 3 Reducer-Flatteners for ~400€ and the FL of your ~1000€ 8" RCT can be adjusted
from 1624, its native FL, down to 812 mm in 5 Steps, a very useful feature, see table 1.
Could even be 6 steps down to 650 mm if a good quality 0,4x Reducer was available ?
- Buy a Wide Reducer (2"), even for a Small Sensor, to keep Image Quality at its best.
- Without Flattener, their Image Quality is already better than Newtonians', versus with
it, their Image Quality is up to the Best Instruments available for Astro-Photography !
- The Focal Reducers included in both ranges 0,63...0,67x and 0,75...0,8x are mostly
Flatteners too : an interesting feature to achieve Perfect Pictures, see the point above...
Astrosurf.com/luxorion/Telescope-Ritchey-Chretien-IV : Unfair CDK vs RC spots
AstroSysteme.com/ASA_comparo_RC_vs_CDK : RC vs CDK from ASA...
Alluna-Optics.com/Ritchey-Chretien-or-DK : RC vs DK from Alluna to balance

Web informations, comparisons available for these Astrographs :

Ritchey-Chretien+Reducers -vs- Celestron-RASA in figures

Camera Equivalent Exposure calculation : Camera-Simulator, Exposure-Calculator
NX-mini-imx183=4min F/4,8 iso400 = F/5,4 iso500 = F/6 iso640 = F/8 iso1100
( my Newtonians = F/5 - F/4,8 ).

Reduc: 0,8~,75=F/6 - ,75+,67=0,5=F/4 - ,75+0,5=,375=F/3 - ,67+0,5=,335=F/2,7
Ritchey-Chretien GSO-Kepler-Orion-TS- + Reducer-Flattener vs SCT-RASA
Commas used instead of dots for better visibility.

IQ : Image Quality from 1 to 5 out of 5. Image Quality lowers in 2 cases : too much FL
... for Usual Seeing and smallest Pixel Pitch or... too much Focal Reduction Factor.
1st : Primary Mirror diameter ; nwt : 2 configurations close to my Newtonians.
FL : native Focal Length or resulting Focal Length = native FL x Reduction Factor.
2,4 - 2,9 - 3,4 : Pixel Pitch of the Sensor used. R! = Reserved for Exceptional Seeing.
... Won't happen here at sea side, the Cold Wind from North blowing over Warm Sea !
... Ok = Good ; ok = ok ; ~ = average ; b = blurred borders (Ok~Good usual Seeing).
F/d : Focal Length / Primary Mirror diameter.
2nd : Secondary Mirror diameter.
... /!\ 2 values found for RCT-8" and RCT-10"
Obs : Obstruction Ratio = Secondary Mirror diameter / Primary Mirror diameter.
... /!\ 2 values found for RCT-8" and RCT-10"
Lng : Tube Length.
Dtr : Tube Diameter.
Fcsr : Focuser Size, Type. Off-Axis Focus Knob to Maintain the Primary SCT Mirror.
Scor : Number of Configurations with IQ > 2/5 - Sum of these IQs > 2/5 (last line).
trvl : Focuser's Travel mm. Off-Axis Focusing Action Applied to Primary SCT Mirror.
... Hence its Focus Shift and Mirror Flop problems.
rail : Vixen and / or Losmandy rail.
wt : Tube weight kg.
k€ : Price~ in Euro x 1000.

Reducer : which Reducer-Flattener is attached to the Ritchey-Chretien Telescope.
... ,8 - ,75 - ,67 - ,6 - ,5 - ,4 - ,375 - ,33 : Reduction ratios found as is or resulting...
...from Adjusting their Distance to the Sensor ( or possibly combining 2 of them ? )

Changing Operating Distance of a 0,5x nominal Focal Reducer to 0,6x.
Focal Reduction vs Distance Adjust
Click Image, Ctrl+ to enlarge more

...to be continued... © Dima Lootvoet 2010-2020
Main Page