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.

Are simple Newtonians the best compromise between very Expensive APO Refractors
and expensive Astrographs ? Anyway, if you are to image in a well lit city at sea side, as
I am, the Atmospheric Conditions will ruin about 80% of these Huge Price Gaps :

Obstruction Losses vs Telescope Type
Click Image, Ctrl+ to enlarge more

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 4400euro

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 : UV, Blue, Green, Red, IR focus Exactly at the Same Spot
Plus 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.

- TSRCFlat3 2.5" Flattener from TS-Optics can be added to the 10" and 12" RCT
with 3" Focuser to enlarge their Corrected Image Circle from ~44 up to ~52mm :

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 :

Some useful tips about Refractors before buying for Astro-Photography

You should never buy a 2 - 3 Elements Refractor, even labelled "APO", without buying it
Together with its dedicated or best suited Corrector-Flattener - take advice from regular
users of both, or at least from your reseller. Else, you risk to be very, very disappointed :

Even very well corrected for Lateral Chromatic Aberration Triplets could show very high
Longitudinal Chromatic Aberration. Again, a well suited Flattener could partially remedy.
Narrow Band / RGB filters are mandatory to Optimize such a Refractor - No OSC use.
200k Euro Super-ED APO Refractor = 900 m Focus Offset from Blue to Red :
APM APO Refractor 304/2280 CNC LW II

high Longitudinal Chromatic Aberration
Click Image

A 4 Elements Petzval Refractor design shows a perfect 44-67mm Flat Field without any
Corrector-Flattener. FPL51-53 special glass, will make it very expensive, but will avoid
Lateral Chromatic Aberration. Longitudinal Chromatic Aberration will be limited to 200-
150 m
in the best cases, here are LoCA values for some FSQ-106 Petzval Refractors
to balance with 2.4 m pixel pitch modern BSI sensors : SamsungNXminiTopIR.html

FSQ-106 Longitudinal Chromatic Aberration
Click Image, Ctrl+ to enlarge more

A standard optical glass Petzval will show only the Brightest Stars bloated, especially in
the Blue Layer. This can be greatly improved using Narrow-Band filters or RGB filters,
should at least use Minus-Violet filter. Here is an OSC stack as is = without any filter :
M42 Orion, Running Man, Nikon D600 + Vixen NA 120/800 : 6x 8 min at ISO-200
Click image to enlarge

OSC : One Shot Colors with a usual RGGB Bayer Filtered camera as opposed to :
LRGB : 3-4 groups of shots for Luminance and/or Red, Green, Blue Colors,
with a Mono(Pan)Chromatic camera - Refocused carefully for each filter...
EAA : Electronically Assisted Astronomy via Narrow-Band filtering :
Ha, OIII, SII filters used with a Mono(Pan)Chromatic camera,
Refocused for each filter (unless using parfocal filters ?)...

Ritchey-Chretien +Flattener +Reducer -vs- Celestron-Rasa

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 ).

TSF TS-Optics RC Flattener TSRCFlat3 for 2,5"+ Focuser
Reducer 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- +Flattener +Reducer 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 (Good Seeing condition).
Formulas to get the Maximum Usable FL from different Pixel-Pitch, Seeing conditions :
- Ok Seeing
= 2-4" FWHM. Maximum-FL = Pixel-Pitch x 309,3975
- Good Seeing = 1-2" FWHM. Maximum-FL = Pixel-Pitch x 618,795
- Exceptional Seeing : 0,5-1" FWHM. Max-FL = Pixel-Pitch x 1237,69
F/d : Focal Length / Primary Mirror diameter.
2nd : Secondary Mirror diameter.
... /!\ 2nd value for RCT-8", RCT-10" in their last row.
Obs : Obstruction Ratio = Secondary Mirror diameter / Primary Mirror diameter.
... /!\ 2nd value for RCT-8", RCT-10" in their last row.
IC : Image Circle.
Di.Ht : Tube Diameter x Height.
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 up to 0,6x.
Focal Reduction vs Distance Adjust
Click Image.

...to be continued... © Dima Lootvoet 2010-2021
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