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Newtonian, Celestron, RCT, Lenses

Newtonian vs SCT vs RCT to AstroPhotography

Usual Newtonian for Astro-Photography

Historic, good, simple Newtonian Telescope is keeping some Real advantages.
-Simple Newtonians without Coma Corrector for small sensor imagers have less
Obstruction than both RCT, SCT. My 2003 Sky-Watcher 203/1000 has 26%. 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

-The Simple Newtonian design without Coma-Corrector is 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 Fastar, Hyperstar, Rasa.
search.for/?q=Newtonian+Telescope+Astrophotography+Pros+and+Cons
Astrosurf.com/luxorion/Telescope-de-Newton-II

Eos 20d + Kenko 2x Converter + Sky-Watcher 200/1000, 1/125s ISO 400 F/10

Half Moon: Eos 20d, Kenko 2x, SkyWatcher 200/1000

Click Image, Ctrl+ to Enlarge

Is simple Newtonian the best Compromise between Expensive APO Refractor
and Costly Astrograph, anyway, if you are to image in a well lit city at sea side,
as I am, Harsh Atmospheric Conditions will ruin about 75% of their Huge price-
gap Bonuses.

SCT ruined by its Rudimentary Focuser for Astro-Photo.

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

Celestron Focus-Shift, Mirror-Flop: emphasized by the C8's 2m FL !

Click Image, Ctrl+ to Enlarge

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 or 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 ~500€ 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.

Astro-Photography is enough work to get rid of all Chromatic Aberrations.

Click Image, Ctrl+ to Enlarge

- TSRCFlat3 2.5" Flattener from TS-Optics can be added to the 10" + 12" RCT
with 3" Focuser to enlarge their Corrected Image Circle from ~44 up to ~52mm :
teleskop-express.de/p6972-TS-Optics-RC-flattener-4RCT-Fullframe-M68thread
cloudynights.com/topic/659972-your-gso-rc-can-fully-support-52d-imaging-circle

search.for/?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 :
search.for/?q=Newtonian+vs+Ritchey-Chretien+vs+Schmidt-Cassegrain

Main Cons against Catadioptrics: RCT are often over 40% Obstructed !

Click Image, Ctrl+ to Enlarge

Camera Lenses and Catadioptrics for Astro-Photography

You should never buy Camera Lenses w/o giving them a try against Bright Stars.
Some are well Corrected for AC, Fringing w/ High Contrast Lights, other aren't.
Ex.: Canon EF-200 f/2.8 LII is very good for Astro. vs Nikons 180 f/2.8 ED show
AC, Fringes around Bright Stars ! Despite they both have very good reputation.

Some Samyang lenses are Very Good too, especially 24mm f1.4 + 85mm f/1.4.
The 35 f/1.4, the 135 f/2 are Simply Excellent for All Purposes + Astro-Photos.:
In-Depth Tests, Comparison of Samyang 35mm f1.4 vs Sigma f1.4, Nikon F/2

Six 35mm F/1.4 prime lenses gathered from Lens-Tip.com.

Click Image, Ctrl+ to Enlarge

The 12 Podiums+ from the table above:
- Stars-Coma. Sigma, Samyang, Zeiss...
- Astigmatism. Samyang, Sigma, Canon...
- ChromaAber Sigma, Samyang, Canon...
- APScDistors Sigma, Zeiss, Sony, Samyang...
- FFDistorsion Sigma, Sony, Zeiss, Samyang...
- Ctrl1.4Resol. Sigma, Canon, Zeiss, Samyang...
- CtrlMaxResol Sigma, Zeiss, Nikon, Samyang...
- Price!-Price! Samyang, Sigma, Canon...
- 1.4Vigneting Samyang, Zeiss, Sony...
- F/2Vigneting Samyang, Zeiss, Sony...
- 2.8Vigneting Zeiss, Samyang, Sony...
- F/4Vigneting Zeiss, Samyang, Nikon...

Catadioptric lenses to be reserved for Moon + Planetary shots after trying their
Focusing easyness. Their Central Obstruction and f/5.6-8 usual Aperture falling
down to T/8-11 ruin their Contrast Power. So that Focusing them on Deep Sky
Objects become a real pain. Some get good pictures of Mars, Moon, Jupiter
and Saturn, when using them...

Some useful tips about Refractors before Astro-Photo.

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 µm Focus Offset from Blue to Red :
APM APO Refractor 304/2280 CNC LW II

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 µm in the best cases, here are LoCA values for some FSQ106 Petzval Refractors
to balance with 2.4 µm pixel pitch Modern BSI sensors : Samsung-NXmini-TopIR.html
Or even with 1.55 µm pixel pitch Modern BSI sensors : Pentax-Q10-Astro-IR.html

High End Refractor's Huge LoCa to compare w/ Newtonian-RCT's Null NoCa

FSQ-106 Longitudinal Chromatic Aberration

Reflectors' Null NoCa. - Click Image, Ctrl+ to Enlarge

A Petzval Refractor will always be from 1 up to 2 stops Slower than a F/4 Newtonian :
Camera Simulator, Exposure Calculator, LV from -49 up to 69 Ev
Maybe you can compensate for this by shooting with a More Sensitive CMos sensor :
Astro Photography Sensor Sensitivity ranking tools

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 Color 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 ?)...

RCT +Flattener +Reducer -vs- Celestron-Rasa

Camera equivalent Exposure calculator: 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

RCT: GSO-Kepler-Orion-TS- +Flattener +Reducer -vs- SCT-Fastar-Rasa
Reduc	IQ	M1	FL	2,4	2,9	3,4	f/d	M2	obs	IC	Di.Ht	Fcsr	trvl	rail	wt	k€
RC8"	4	203	1624	R!	ok	ok	8	85	42	44	23.58	2"lbc	50	VV	7	1
+0,8	5		1299	Ok	Ok	Ok	6,4									,15
+,67	5		1088	Ok	Ok	Ok	5,4									,15
+0,6	4	nwt	974	ok	ok	ok	4,8
+0,5	4		812	ok	ok	ok	4									,1
+0,4	3		650	~	~	~	3,2									?
+,33	2		536	b	b	b	2,6	95	47?			6-25				?
rasa8"	?	203	400	?	?	?	2	93	46	32	24.63	prim	shif	Clt	8	2,2
RC10	3	254	2000	R!	R!	ok	7,9	110	43	44	29.74	3"lbc	50	LV	16	2,2
+TSF	4		2000	R!	R!	ok	7,9			52						2,5
+0,8	4		1600	R!	ok	ok	6,3									,15
+,67	5		1340	Ok	Ok	Ok	5,3									,15
+0,6	4	nwt	1200	ok	ok	ok	4,7
+0,5	4		1000	ok	ok	ok	3,9									,1
+0,4	3		800	~	~	~	3,1									?
+,33	2		660	b	b	b	2,6	112	44?			7-27				?
rasa11	?	279	620	?	?	?	2,2	114	41	52	33.84	prim	shif	Clt	20	4,4
RC12	2	304	2432	R!	R!	R!	8	150	49	44	36.99	3"lbc	50	LL	21	3,5
+TSF	3		2432	R!	R!	R!	8			52						3,8
+0,8	3		1946	R!	R!	ok	6,4									,15
+,67	4		1629	R!	ok	ok	5,4									,15
+0,6	4		1459	ok	ok	ok	4,8
+0,5	4		1216	ok	ok	ok	4									,1
+0,4	3		973	~	~	~	3,2									?
+,33	2		803	b	b	b	2,6					6-21				?
Units	/5	mm	mm	pix	pix	pix	f/d	mm	%	mm	Cm	scor	mm	typ	kg	k€

Commas used instead of dots for better visibility.

IQ : Image Quality from 1 up to 5. Image Quality lowers in 2 cases : too much FL
... for Usual Seeing + smallest Pixel Pitch or... too much Focal Reduction Factor.
M1 : Primary Mirror diameter ; nwt : 2 configurations Close to my Newtonians.
FL : native Focal Length. Resulting Focal Length = native FL* Reduction Factor.
2,4 - 2,9 - 3,4 : Pixel Pitch of Sensor Used. R!: Reserved 4 Exceptional Seeing,
... won't happen here at Sea Side : Cold North Wind blowing over Warm Sea !
... Ok = Good ; ok : ok ; ~ : average ; b : blurred borders Good seeing condition.
Formulas to get a Max. 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.
M2 : 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 cm.
Fcsr: Focuser Size, Type. Off Axis Focus Knob to hold 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 Focus 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 hretien 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 )
search.for/?q=Reducer+Flattener+for+Ritchey+Chretien&tbm=isch

0,5x nomin. Focal Reducer's Operating Distance up to 0,6x

Click Image, Ctrl+ to Enlarge
...to be continued...