So what am I missing, between large aperture reflector versus smaller refractor?
- cumbrianwolf
- Moon Ambassador
- Articles: 0
- Posts: 72
- Joined: Tue Dec 07, 2021 5:10 pm
- 2
- Location: Wigton, Cumbria. UK
- Status:
Offline
So what am I missing, between large aperture reflector versus smaller refractor?
My telescope happens to be an astro imaging Newtonian at 254 mm f/ 4, this was originally chosen for dual use as in some imaging and some visual. So apart from the weight reduction, what are the pros and cons of a smaller refractor versus a larger reflector?
- JayTee
- Universal Ambassador
- Articles: 2
- Posts: 5642
- Joined: Thu Apr 25, 2019 3:23 am
- 5
- Location: Idaho, USA
- Status:
Offline
-
TSS Awards Badges
TSS Photo of the Day
Re: So what am I missing, between large aperture reflector versus smaller refractor?
Not only do we consider the f/ number, to me the more important number is the focal length. The focal length determines your image scale (based on which Imaging camera you use) and your guide scope suitability. A small ish refractor with a corresponding short focal length is going to show a bigger chunk of sky than your Newtonian. We like these because the smaller refractors are great for imaging the bigger nighttime objects, like Andromeda, the Pleiades, and other large astro features.
∞ Primary Scopes: #1: Celestron CPC1100 #2: 8" f/7.5 Dob #3: CR150HD f/8 6" frac
∞ AP Scopes: #1: TPO 6" f/9 RC #2: ES 102 f/7 APO #3: ES 80mm f/6 APO
∞ G&G Scopes: #1: Meade 102mm f/7.8 #2: Bresser 102mm f/4.5
∞ Guide Scopes: 70 & 80mm fracs -- The El Cheapo Bros.
∞ Mounts: iOptron CEM70AG, SW EQ6, Celestron AVX, SLT & GT (Alt-Az), Meade DS2000
∞ Cameras: #1: ZWO ASI294MC Pro #2: 662MC #3: 120MC, Canon T3i, Orion SSAG, WYZE Cam3
∞ Binos: 10X50,11X70,15X70, 25X100
∞ EPs: ES 2": 21mm 100° & 30mm 82° Pentax XW: 7, 10, 14, & 20mm 70°
Searching the skies since 1966. "I never met a scope I didn't want to keep."
∞ AP Scopes: #1: TPO 6" f/9 RC #2: ES 102 f/7 APO #3: ES 80mm f/6 APO
∞ G&G Scopes: #1: Meade 102mm f/7.8 #2: Bresser 102mm f/4.5
∞ Guide Scopes: 70 & 80mm fracs -- The El Cheapo Bros.
∞ Mounts: iOptron CEM70AG, SW EQ6, Celestron AVX, SLT & GT (Alt-Az), Meade DS2000
∞ Cameras: #1: ZWO ASI294MC Pro #2: 662MC #3: 120MC, Canon T3i, Orion SSAG, WYZE Cam3
∞ Binos: 10X50,11X70,15X70, 25X100
∞ EPs: ES 2": 21mm 100° & 30mm 82° Pentax XW: 7, 10, 14, & 20mm 70°
Searching the skies since 1966. "I never met a scope I didn't want to keep."
- cumbrianwolf
- Moon Ambassador
- Articles: 0
- Posts: 72
- Joined: Tue Dec 07, 2021 5:10 pm
- 2
- Location: Wigton, Cumbria. UK
- Status:
Offline
Re: So what am I missing, between large aperture reflector versus smaller refractor?
How wide-field are we referring to, as I thought my scope was wide atJayTee wrote: ↑Fri Jan 14, 2022 6:17 pm Not only do we consider the f/number, to me the more important number is the focal length. The focal length determines your image scale (based on which imaging camera you use) and your guide scope suitability. A small ish refractor with a short focal length is going to show a bigger chunk of sky than your Newtonian. We like these because the smaller refractors are great for the bigger nighttime objects, like Andromeda, the Pleiades, and other large astro features.
- JayTee
- Universal Ambassador
- Articles: 2
- Posts: 5642
- Joined: Thu Apr 25, 2019 3:23 am
- 5
- Location: Idaho, USA
- Status:
Offline
-
TSS Awards Badges
TSS Photo of the Day
Re: So what am I missing, between large aperture reflector versus smaller refractor?
Th f/ number is a function of speed. It determines the duration of your image. An f/ 4 scope will function twice as fast as an f/ 8 scope. The interesting thing about astrophotography is that exposure duration has many different factors involved, more than just the f/ number. I will get some links for you to read on the trade-off between f/ number and exposure duration.
∞ Primary Scopes: #1: Celestron CPC1100 #2: 8" f/7.5 Dob #3: CR150HD f/8 6" frac
∞ AP Scopes: #1: TPO 6" f/9 RC #2: ES 102 f/7 APO #3: ES 80mm f/6 APO
∞ G&G Scopes: #1: Meade 102mm f/7.8 #2: Bresser 102mm f/4.5
∞ Guide Scopes: 70 & 80mm fracs -- The El Cheapo Bros.
∞ Mounts: iOptron CEM70AG, SW EQ6, Celestron AVX, SLT & GT (Alt-Az), Meade DS2000
∞ Cameras: #1: ZWO ASI294MC Pro #2: 662MC #3: 120MC, Canon T3i, Orion SSAG, WYZE Cam3
∞ Binos: 10X50,11X70,15X70, 25X100
∞ EPs: ES 2": 21mm 100° & 30mm 82° Pentax XW: 7, 10, 14, & 20mm 70°
Searching the skies since 1966. "I never met a scope I didn't want to keep."
∞ AP Scopes: #1: TPO 6" f/9 RC #2: ES 102 f/7 APO #3: ES 80mm f/6 APO
∞ G&G Scopes: #1: Meade 102mm f/7.8 #2: Bresser 102mm f/4.5
∞ Guide Scopes: 70 & 80mm fracs -- The El Cheapo Bros.
∞ Mounts: iOptron CEM70AG, SW EQ6, Celestron AVX, SLT & GT (Alt-Az), Meade DS2000
∞ Cameras: #1: ZWO ASI294MC Pro #2: 662MC #3: 120MC, Canon T3i, Orion SSAG, WYZE Cam3
∞ Binos: 10X50,11X70,15X70, 25X100
∞ EPs: ES 2": 21mm 100° & 30mm 82° Pentax XW: 7, 10, 14, & 20mm 70°
Searching the skies since 1966. "I never met a scope I didn't want to keep."
- Lady Fraktor
- Universal Ambassador
- Articles: 0
- Posts: 9988
- Joined: Mon Apr 29, 2019 9:14 pm
- 4
- Location: Slovakia
- Status:
Offline
Re: So what am I missing, between large aperture reflector versus smaller refractor?
Quite significant actually, a 254mm f/ 4 with a 25mm eyepiece has a TFOV of 1.34°
My 80mmf/ 6.9 using a 25mm eyepiece has a TFOV of 2.5°
The maximumFOV of your 254mm is 2.69°, my 80mm is 4.9°
My 80mm
The maximum
Gabrielle
See Far Sticks: Elita 103/1575, AOM FLT 105/1000, Bresser 127/1200 BV, Nočný stopár 152/1200, Vyrobené doma 70/700, Stellarvue NHNG DX 80/552, TAL RS 100/1000, Vixen SD115s/885
EQ: TAL MT-1, Vixen SXP, SXP2, AXJ, AXD
Az/Alt: AYO Digi II, Stellarvue M2C, Argo Navis encoders on both
Tripods: Berlebach Planet (2), Uni 28 Astro, Report 372, TAL factory maple, Vixen ASG-CB90, Vixen AXD-TR102
Diagonals: Astro-Physics, Baader Amici, Baader Herschel, iStar Blue, Stellarvue DX, Tak prism, TAL, Vixen
Eyepieces: Antares to Zeiss (1011110)
The only culture I have is from yogurt
See Far Sticks: Elita 103/1575, AOM FLT 105/1000, Bresser 127/1200 BV, Nočný stopár 152/1200, Vyrobené doma 70/700, Stellarvue NHNG DX 80/552, TAL RS 100/1000, Vixen SD115s/885
EQ: TAL MT-1, Vixen SXP, SXP2, AXJ, AXD
Az/Alt: AYO Digi II, Stellarvue M2C, Argo Navis encoders on both
Tripods: Berlebach Planet (2), Uni 28 Astro, Report 372, TAL factory maple, Vixen ASG-CB90, Vixen AXD-TR102
Diagonals: Astro-Physics, Baader Amici, Baader Herschel, iStar Blue, Stellarvue DX, Tak prism, TAL, Vixen
Eyepieces: Antares to Zeiss (1011110)
The only culture I have is from yogurt
- cumbrianwolf
- Moon Ambassador
- Articles: 0
- Posts: 72
- Joined: Tue Dec 07, 2021 5:10 pm
- 2
- Location: Wigton, Cumbria. UK
- Status:
Offline
Re: So what am I missing, between large aperture reflector versus smaller refractor?
I am starting to grasp it once again, something I need to look into more.Lady Fraktor wrote: ↑Fri Jan 14, 2022 6:34 pm Quite significant actually, a 254mm f/4 with a 25mm eyepiece has a TFOV of 1.34°
My 80mm f/6.9 using a 25mm eyepiece has a TFOV of 2.5°
The maximum FOV of your 254mm is 2.69°, my 80mm is 4.9°
- JayTee
- Universal Ambassador
- Articles: 2
- Posts: 5642
- Joined: Thu Apr 25, 2019 3:23 am
- 5
- Location: Idaho, USA
- Status:
Offline
-
TSS Awards Badges
TSS Photo of the Day
Re: So what am I missing, between large aperture reflector versus smaller refractor?
Here is my favorite field of view calculator. Make sure you select Imaging mode. Input your numbers, all of them, to see what your FOV is for any astro object you have selected. Go ahead and try different focal lengths and you will see how the imaging scale changes and how imaging scale is not directly dependent on your f/ number
https://astronomy.tools/calculators/field_of_view/
https://astronomy.tools/calculators/field_of_view/
∞ Primary Scopes: #1: Celestron CPC1100 #2: 8" f/7.5 Dob #3: CR150HD f/8 6" frac
∞ AP Scopes: #1: TPO 6" f/9 RC #2: ES 102 f/7 APO #3: ES 80mm f/6 APO
∞ G&G Scopes: #1: Meade 102mm f/7.8 #2: Bresser 102mm f/4.5
∞ Guide Scopes: 70 & 80mm fracs -- The El Cheapo Bros.
∞ Mounts: iOptron CEM70AG, SW EQ6, Celestron AVX, SLT & GT (Alt-Az), Meade DS2000
∞ Cameras: #1: ZWO ASI294MC Pro #2: 662MC #3: 120MC, Canon T3i, Orion SSAG, WYZE Cam3
∞ Binos: 10X50,11X70,15X70, 25X100
∞ EPs: ES 2": 21mm 100° & 30mm 82° Pentax XW: 7, 10, 14, & 20mm 70°
Searching the skies since 1966. "I never met a scope I didn't want to keep."
∞ AP Scopes: #1: TPO 6" f/9 RC #2: ES 102 f/7 APO #3: ES 80mm f/6 APO
∞ G&G Scopes: #1: Meade 102mm f/7.8 #2: Bresser 102mm f/4.5
∞ Guide Scopes: 70 & 80mm fracs -- The El Cheapo Bros.
∞ Mounts: iOptron CEM70AG, SW EQ6, Celestron AVX, SLT & GT (Alt-Az), Meade DS2000
∞ Cameras: #1: ZWO ASI294MC Pro #2: 662MC #3: 120MC, Canon T3i, Orion SSAG, WYZE Cam3
∞ Binos: 10X50,11X70,15X70, 25X100
∞ EPs: ES 2": 21mm 100° & 30mm 82° Pentax XW: 7, 10, 14, & 20mm 70°
Searching the skies since 1966. "I never met a scope I didn't want to keep."
- cumbrianwolf
- Moon Ambassador
- Articles: 0
- Posts: 72
- Joined: Tue Dec 07, 2021 5:10 pm
- 2
- Location: Wigton, Cumbria. UK
- Status:
Offline
Re: So what am I missing, between large aperture reflector versus smaller refractor?
Hmm, I see how I could be missing out as most DSO are usually large, and it saves the effort of having to think about mosaics if I wanted to image a large emission nebula for instance. A smaller refractor was also a consideration as it reduced the effort of set up and of course the total weight on the mount.
- JayTee
- Universal Ambassador
- Articles: 2
- Posts: 5642
- Joined: Thu Apr 25, 2019 3:23 am
- 5
- Location: Idaho, USA
- Status:
Offline
-
TSS Awards Badges
TSS Photo of the Day
Re: So what am I missing, between large aperture reflector versus smaller refractor?
We almost always recommend a small refractor as the "beginners" setup for AP . Besides the benefits you list above, more importantly, a small, short FL imaging frac allows for larger guiding errors before they are visible on the image itself, this is a function of imaging scale. They also have the advantage because of their lower f/ number they can use shorter exposure times creating a smaller window for errors to occur. Meaning, you keep more of your light subs for processing. They "cut" you some slack so you can "cut" your teeth on AP procedures without losing too much hair!
You could easily use your TS 80 as an imaging scope. Go on craigslist and find an "El Cheap" 50 - 70mm frac as your new guide scope. Now you have shortFL needed to learn the ropes. Technically, you could use your Newt as a guide scope but that introduces more problems than it solves. Typically you'd like your guide scope to have a shorter FL than your imaging scope.
You could easily use your TS 80 as an imaging scope. Go on craigslist and find an "El Cheap" 50 - 70mm frac as your new guide scope. Now you have short
∞ Primary Scopes: #1: Celestron CPC1100 #2: 8" f/7.5 Dob #3: CR150HD f/8 6" frac
∞ AP Scopes: #1: TPO 6" f/9 RC #2: ES 102 f/7 APO #3: ES 80mm f/6 APO
∞ G&G Scopes: #1: Meade 102mm f/7.8 #2: Bresser 102mm f/4.5
∞ Guide Scopes: 70 & 80mm fracs -- The El Cheapo Bros.
∞ Mounts: iOptron CEM70AG, SW EQ6, Celestron AVX, SLT & GT (Alt-Az), Meade DS2000
∞ Cameras: #1: ZWO ASI294MC Pro #2: 662MC #3: 120MC, Canon T3i, Orion SSAG, WYZE Cam3
∞ Binos: 10X50,11X70,15X70, 25X100
∞ EPs: ES 2": 21mm 100° & 30mm 82° Pentax XW: 7, 10, 14, & 20mm 70°
Searching the skies since 1966. "I never met a scope I didn't want to keep."
∞ AP Scopes: #1: TPO 6" f/9 RC #2: ES 102 f/7 APO #3: ES 80mm f/6 APO
∞ G&G Scopes: #1: Meade 102mm f/7.8 #2: Bresser 102mm f/4.5
∞ Guide Scopes: 70 & 80mm fracs -- The El Cheapo Bros.
∞ Mounts: iOptron CEM70AG, SW EQ6, Celestron AVX, SLT & GT (Alt-Az), Meade DS2000
∞ Cameras: #1: ZWO ASI294MC Pro #2: 662MC #3: 120MC, Canon T3i, Orion SSAG, WYZE Cam3
∞ Binos: 10X50,11X70,15X70, 25X100
∞ EPs: ES 2": 21mm 100° & 30mm 82° Pentax XW: 7, 10, 14, & 20mm 70°
Searching the skies since 1966. "I never met a scope I didn't want to keep."
- SkyHiker
- Local Group Ambassador
- Articles: 0
- Posts: 2293
- Joined: Sat May 11, 2019 8:40 pm
- 4
- Location: Santa Barbara, CA
- Status:
Offline
-
TSS Photo of the Day
TSS Awards Badges
Re: So what am I missing, between large aperture reflector versus smaller refractor?
Let's see if I can remember. Check Wikipedia "eyepiece". If
F = focal length of objective
f = focal length of eyepiece
d = sensor diameter (AP ) or eyepiece field stop (visual)
D = objective diameter
FR = focal ratio (x in F/ x)
with everything in mm, then
FR = F/ D
Magnification M =F/ f
TFOV (in radians) = 2*atan(0.5d/F) or, approximately, d/F
AFOV (in radians) = 2*atan(0.5d/f) or, more approximately, d/f
Dawes limit (arc seconds) = 116/D
degrees = (180/pi ) * radians
The light flux is inversely proportional to the F ratio squared, the exposure time is proportional to the F ratio squared so aF/ 4 we're pretty good compared to many fracs at F/ 7.5 or so. Not good when compared to a C11 with hyperstar at F/ 2 or a Celestron Raza at F/ 2. A factor 4 is pretty significant. But those reflectors can't handle filter wheels.
Generally for a 10" Newt compared to an average refractor, it has an advantage of F ratio, resolution based on Dawes limit and magnification. The resolution is deteriorated by the secondary obstruction and bumps in the mirror surfaces. Aside from that, the improved resolution according to Dawes maxes out at about 8"aperture due to turbulence on average nights. So a smaller frac might have an equivalent resolution on most nights.
F = focal length of objective
f = focal length of eyepiece
d = sensor diameter (
D = objective diameter
with everything in mm, then
Magnification M =
TFOV (in radians) = 2*atan(0.5d/F) or, approximately, d/F
Dawes limit (arc seconds) = 116/D
degrees = (180/
The light flux is inversely proportional to the F ratio squared, the exposure time is proportional to the F ratio squared so a
Generally for a 10" Newt compared to an average refractor, it has an advantage of F ratio, resolution based on Dawes limit and magnification. The resolution is deteriorated by the secondary obstruction and bumps in the mirror surfaces. Aside from that, the improved resolution according to Dawes maxes out at about 8"
... Henk. Telescopes: GSO 12" Astrograph, "Comet Hunter" MN152, ES ED127CF, ES ED80, WO Redcat51, Z12, AT6RC, Celestron Skymaster 20x80, Mounts and tripod: Losmandy G11S with OnStep, AVX, Tiltall, Cameras: ASI2600MC, ASI2600MM, ASI120 mini, Fuji X-a1, Canon XSi, T6, ELPH 100HS, DIY: OnStep controller, Pi4b/power rig, Afocal adapter, Foldable Dob base, Az/Alt Dob setting circles, Accessories: ZWO 36 mm filter wheel, TV Paracorr 2, Baader MPCC Mk III, ES FF, SSAG, QHY OAG-M, EAF electronic focuser, Plossls, Barlows, Telrad, Laser collimators (Seben LK1, Z12, Howie Glatter), Cheshire, 2 Orion RACIs 8x50, Software: KStars-Ekos, DSS, PHD2, Nebulosity, Photo Gallery, Gimp, CHDK, Computers:Pi4b, 2x running KStars/Ekos, Toshiba Satellite 17", Website:Henk's astro images
Create an account or sign in to join the discussion
You need to be a member in order to post a reply
Create an account
Not a member? register to join our community
Members can start their own topics & subscribe to topics
It’s free and only takes a minute