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.
google.com/search?q=Newtonian+Telescope+Astrophotography+Pros+and+Cons
Astrosurf.com/luxorion/Telescope-de-Newton-II

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
google.com/search?q=Celestron+SCT+Fastar+Hyperstar+RASA+Pros+and+Cons
Astrosurf.com/luxorion/Telescopes-Catadioptriques-III

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 :
teleskop-express.de/p6972_TS-Optics-RC-Flattener-4-RCT-FullFrame-M68thread
Cloudynights.com/topic/659972-your-gso-rc-can-fully-support-52mm-imaging-circle

google.com/search?q=Ritchey-Chretien+RCT+Astrophotography+Pros+and+Cons
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 :
google.com/search?q=Newtonian+vs+Ritchey-Chretien+vs+Schmidt-Cassegrain

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 :
cloudynights.com/topic/582196-stellarvue-sv80st-sffr80-25-focal-reducer-issues
cloudynights.com/topic/588599-bad-stars-on-an-apo-refractor-whats-going-on

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
Astrosurf.com/luxorion/La-Lunette-Astronomique

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
ReducIQ1stFL2,42,93,4F/d2ndobsICDi.HtFcsrtrvlrailwtk€
RC8"42031624R!okok885424423.582"lbc50VV71
+0,851299OkOkOk6,4,15
+,6751088OkOkOk5,4,15
+0,64nwt974okokok4,8
+0,54812okokok4,1
+0,43650~~~3,2?
+,332536bbb2,69547?6-25?
rasa8"?203400???293463224.63PrimshftClt82,2
RC1032542000R!R!ok7,9110434429.743"lbc50LV162,2
+TSF42000R!R!ok7,9522,5
+0,841600R!okok6,3,15
+,6751340OkOkOk5,3,15
+0,64nwt1200okokok4,7
+0,541000okokok3,9,1
+0,43800~~~3,1?
+,332660bbb2,611244?7-27?
rasa11?279620???2,2114415233.84PrimshftClt204,4
RC1223042432R!R!R!8150494436.993"lbc50LL213,5
+TSF32432R!R!R!8523,8
+0,831946R!R!ok6,4,15
+,6741629R!okok5,4,15
+0,641459okokok4,8
+0,541216okokok4,1
+0,43973~~~3,2?
+,332803bbb2,66-21?
Units/5mmmmpixpixpixF/dmm%mmCmScormmtypkgk€
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 ? )
www.google.com/search?q=Reducer+Flattener+for+Ritchey+Chretien&tbm=isch

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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