Whats the typical maximum resolution in arcseconds of a 70mm achromat/doublet refractor?

Discuss your refractor type scopes here.
Post Reply
User avatar
realflow100 United States of America
Orion Spur Ambassador
Articles: 0
Offline
Posts: 857
Joined: Tue Jan 21, 2020 5:21 pm
4
Location: Orangeburg South Carolina USA
Status:
Offline

TSS Awards Badges

TSS Photo of the Day

Whats the typical maximum resolution in arcseconds of a 70mm achromat/doublet refractor?

#1

Post by realflow100 »


Whats the typical maximum resolution in arcseconds of a 70mm achromat/doublet refractor telescope?
I have a cheap one and ive been messing around with it
and the limit seems to be reached between 60x and 120x magnification

it looks perfect crisp sharpness at 60x but becomes noticably softer at 120x
Shouldn't it still be crisp at 140x if I follow the rule of 2x magnification per mm of aperture?
Also the contrast seems decent at 60x but the whole image looks a bit washed out/greyish at 120x
Svbony SV503 70mm ED F6 420mm FL refractor telescope (New)
Canon EOS 100D/SL1
Tamron 18-200mm F3.5-F6.3 II VC lens
canon 50mm STM F1.8
svbony 8-24mm zoom eyepiece
svbony goldline 66 degree 9mm and 6mm + 40mm plossl + 2x barlow.
svbony UHC 1.25 filter + astromania 1.25" O-3 filter + also an svbony H-B filter.
User avatar
notFritzArgelander
In Memory
In Memory
Articles: 0
Offline
Posts: 14925
Joined: Fri May 10, 2019 4:13 pm
4
Location: Idaho US
Status:
Offline

TSS Awards Badges

Re: Whats the typical maximum resolution in arcseconds of a 70mm achromat/doublet refractor?

#2

Post by notFritzArgelander »


realflow100 wrote: Tue Aug 17, 2021 3:57 pm Whats the typical maximum resolution in arcseconds of a 70mm achromat/doublet refractor telescope?
I have a cheap one and ive been messing around with it
and the limit seems to be reached between 60x and 120x magnification

it looks perfect crisp sharpness at 60x but becomes noticably softer at 120x
Shouldn't it still be crisp at 140x if I follow the rule of 2x magnification per mm of aperture?
Also the contrast seems decent at 60x but the whole image looks a bit washed out/greyish at 120x
The short answer is no, I wouldn't expect crisp views at 2x per mm.

The maximum resolution in arc seconds according to Rayleigh criterion is about 2". Using Dawes's criterion it's 1.66".

IIRC you have an f5.1 achromatic doublet so chromatic aberration is going to be a bit of a bugaboo. That's why images wash out at lower magnification. In fast refractors with terrestrial views the color smear due to chromatic aberration becomes noticeable well below 2x per mm.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
User avatar
realflow100 United States of America
Orion Spur Ambassador
Articles: 0
Offline
Posts: 857
Joined: Tue Jan 21, 2020 5:21 pm
4
Location: Orangeburg South Carolina USA
Status:
Offline

TSS Awards Badges

TSS Photo of the Day

Re: Whats the typical maximum resolution in arcseconds of a 70mm achromat/doublet refractor?

#3

Post by realflow100 »


So what would be the more typical sharp looking view but still high magnification?
it seems to be somewhere between 60x and below 120x for my scope.

I dont notice much blur problems at low magnifications. looks as sharp as naked eye for the finest details. the chromatic aberration doesnt seem to affect the clarity of the smallest details. just a little distracting.
Svbony SV503 70mm ED F6 420mm FL refractor telescope (New)
Canon EOS 100D/SL1
Tamron 18-200mm F3.5-F6.3 II VC lens
canon 50mm STM F1.8
svbony 8-24mm zoom eyepiece
svbony goldline 66 degree 9mm and 6mm + 40mm plossl + 2x barlow.
svbony UHC 1.25 filter + astromania 1.25" O-3 filter + also an svbony H-B filter.
User avatar
notFritzArgelander
In Memory
In Memory
Articles: 0
Offline
Posts: 14925
Joined: Fri May 10, 2019 4:13 pm
4
Location: Idaho US
Status:
Offline

TSS Awards Badges

Re: Whats the typical maximum resolution in arcseconds of a 70mm achromat/doublet refractor?

#4

Post by notFritzArgelander »


realflow100 wrote: Tue Aug 17, 2021 7:53 pm So what would be the more typical sharp looking view but still high magnification?
it seems to be somewhere between 60x and below 120x for my scope.

I dont notice much blur problems at low magnifications. looks as sharp as naked eye for the finest details. the chromatic aberration doesnt seem to affect the clarity of the smallest details.
That’s the right range. Trial and error for each individual scope is best. Assuming a perfect 70mm f5 I’d guess 70x is safe with maybe 100-110x if well made.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
User avatar
WilliamPaolini United States of America
Saturn Ambassador
Articles: 9
Offline
Posts: 303
Joined: Tue May 25, 2021 8:57 pm
2
Location: Virginia, USA
Status:
Offline

TSS Awards Badges

Re: Whats the typical maximum resolution in arcseconds of a 70mm achromat/doublet refractor?

#5

Post by WilliamPaolini »


realflow100 wrote: Tue Aug 17, 2021 3:57 pm Whats the typical maximum resolution in arcseconds of a 70mm achromat/doublet refractor telescope?
I have a cheap one and ive been messing around with it
and the limit seems to be reached between 60x and 120x magnification

it looks perfect crisp sharpness at 60x but becomes noticably softer at 120x
Shouldn't it still be crisp at 140x if I follow the rule of 2x magnification per mm of aperture?
Also the contrast seems decent at 60x but the whole image looks a bit washed out/greyish at 120x
The answer to your question really just depends as it will vary for the scope depending on many things, like the objects you are looking at, the sky conditions (seeing, transparency, altitude of observing site), and of course the particular optics. With Apochromats I find that the view stays nicely crisp for planetary with exit pupils as low as 0.65mm (1.5x/mm or 40x/in) when the optical figure is descent and the atmosphere is cooperating. So with your aperture that would be about 108x if its color correction was fairly perfect as with an apochromat, less since you are working with an achromat. Remember, as the exit pupil gets smaller the view gets dimmer and the contrast appears lessened. So you are noticing this effect when you say it looks high contrast at 60x (1.16mm exit pupil) but washed out at 120x (0.58mm exit pupil). Given that the scope in question is an achromat and not an apochromat, the resulting color blur will mean that you will not be seeing as crisp of a rendition at the same magnifications an apochromat will, so you need to reduce expectations some. But you can experiment and use a green or red filter and am sure the views will appear crisper as a result since you are limiting the color blur by removing some of the spectrum. If viewing the Moon instead of a planet, with its very bright and high contrast features you will find you can reach a lot more magnification and still see a crisp appearing view. With my apochromats while a planet (especially Jupiter with its more subtle features) I like to go no lower than a 0.65mm exit pupil, the Moon I find can take even a 0.25mm exit pupil (4x/mm or 100x/in) and still show very well.

Since the seeing and transparency and altitude and site darkness can have quite an impact on the outcomes, you might want to try instead seeing how much magnification you can put the scope thru with daytime observing in the back yard. So pick a familiar target like a flower no more than 25 meters or yards away, closer preferred, and see how much magnification the view can take BEFORE it looks "off" (i.e., not as sharp or contrasty as naked eye up close). This I find is a much better way to evaluate what a particular scope can do as it can often take weeks or months to find an evening when the atmosphere is really good enough to let an optic strut its stuff.

As far as the resolution of the 70mm scope, that would be about 1.65 arcsec (as already mentioned by others). However, I would still take that with a grain of salt as that is assuming no color blur. So given you are working with an achromat I'd say maybe expect a resolution not quite that and perhaps closer to 2 arcsec unless you have a filter on the scope to reduce the spectrum and thereby the amount of color blur coming through as well. So if the figure of the scope is diffraction limited and you have a green filter like a #56 or #58 then likely you will get much cleaner looking splits of close doubles nearer the theoretical 1.65 arcsec for your 70mm aperture scope.
Last edited by WilliamPaolini on Tue Aug 17, 2021 10:09 pm, edited 2 times in total.
-Bill

U.S.A.F. Veteran - Visual Amateur Astronomer since 1966 - Fully Retired since 2019
8" f/5 Newt - Lunt 152 f/7.9 - TSA 102 f/8 - Vixen 81S f/7.7 - P.S.T. - Pentax 65ED II - Nikon 12x50 AE
Pentax XWs - Baader Morpheus - Takahashi LEs - Edmund RKEs - BST Starguiders - 6ZAO-II/5XO/4Abbe
PM and Email communications always welcomed
User avatar
realflow100 United States of America
Orion Spur Ambassador
Articles: 0
Offline
Posts: 857
Joined: Tue Jan 21, 2020 5:21 pm
4
Location: Orangeburg South Carolina USA
Status:
Offline

TSS Awards Badges

TSS Photo of the Day

Re: Whats the typical maximum resolution in arcseconds of a 70mm achromat/doublet refractor?

#6

Post by realflow100 »


Hmm so for being a super cheap achromat somewhere just above 60x or so is not too bad then?
This test is in the daytime by the way. Not at night yet.
Hasn't gotten to a clear night yet since I got my new diagonal and barlow just earlier today.
its not a triplet/apochromat. just a cheap doublet/achromat.
i also modified it and removed the cheap clear plastic spacer that was spacing the lens elements. and put 4 small much thinner pieces of paper in it instead. the edges of the field of view are significantly sharper too.
and I also re-did the baffling inside so it doesnt stop down the aperture. also shortened the focuser tube as it was unneccessarily long.
I got a decent 30$ star diagonal also. and a decent 21$ 2x barlow.

I can see the grating in the platform a person would stand on at the top of a cell tower like a half a mile to a mile away (Not quite sure but its pretty far away.)
Svbony SV503 70mm ED F6 420mm FL refractor telescope (New)
Canon EOS 100D/SL1
Tamron 18-200mm F3.5-F6.3 II VC lens
canon 50mm STM F1.8
svbony 8-24mm zoom eyepiece
svbony goldline 66 degree 9mm and 6mm + 40mm plossl + 2x barlow.
svbony UHC 1.25 filter + astromania 1.25" O-3 filter + also an svbony H-B filter.
User avatar
notFritzArgelander
In Memory
In Memory
Articles: 0
Offline
Posts: 14925
Joined: Fri May 10, 2019 4:13 pm
4
Location: Idaho US
Status:
Offline

TSS Awards Badges

Re: Whats the typical maximum resolution in arcseconds of a 70mm achromat/doublet refractor?

#7

Post by notFritzArgelander »


WilliamPaolini wrote: Tue Aug 17, 2021 9:52 pm
realflow100 wrote: Tue Aug 17, 2021 3:57 pm Whats the typical maximum resolution in arcseconds of a 70mm achromat/doublet refractor telescope?
I have a cheap one and ive been messing around with it
and the limit seems to be reached between 60x and 120x magnification

it looks perfect crisp sharpness at 60x but becomes noticably softer at 120x
Shouldn't it still be crisp at 140x if I follow the rule of 2x magnification per mm of aperture?
Also the contrast seems decent at 60x but the whole image looks a bit washed out/greyish at 120x
The answer to your question really just depends as it will vary for the scope depending on many things, like the objects you are looking at, the sky conditions (seeing, transparency, altitude of observing site), and of course the particular optics. With Apochromats I find that the view stays nicely crisp for planetary with exit pupils as low as 0.65mm (1.5x/mm or 40x/in) when the optical figure is descent and the atmosphere is cooperating. So with your aperture that would be about 108x if its color correction was fairly perfect as with an apochromat, less since you are working with an achromat. Remember, as the exit pupil gets smaller the view gets dimmer and the contrast appears lessened. So you are noticing this effect when you say it looks high contrast at 60x (1.16mm exit pupil) but washed out at 120x (0.58mm exit pupil). Given that the scope in question is an achromat and not an apochromat, the resulting color blur will mean that you will not be seeing as crisp of a rendition at the same magnifications an apochromat will, so you need to reduce expectations some. But you can experiment and use a green or red filter and am sure the views will appear crisper as a result since you are limiting the color blur by removing some of the spectrum. If viewing the Moon instead of a planet, with its very bright and high contrast features you will find you can reach a lot more magnification and still see a crisp appearing view. With my apochromats while a planet (especially Jupiter with its more subtle features) I like to go no lower than a 0.65mm exit pupil, the Moon I find can take even a 0.25mm exit pupil (4x/mm or 100x/in) and still show very well.

Since the seeing and transparency and altitude and site darkness can have quite an impact on the outcomes, you might want to try instead seeing how much magnification you can put the scope thru with daytime observing in the back yard. So pick a familiar target like a flower no more than 25 meters or yards away, closer preferred, and see how much magnification the view can take BEFORE it looks "off" (i.e., not as sharp or contrasty as naked eye up close). This I find is a much better way to evaluate what a particular scope can do as it can often take weeks or months to find an evening when the atmosphere is really good enough to let an optic strut its stuff.

As far as the resolution of the 70mm scope, that would be about 1.65 arcsec (as already mentioned by others). However, I would still take that with a grain of salt as that is assuming no color blur. So given you are working with an achromat I'd say maybe expect a resolution not quite that and perhaps closer to 2 arcsec unless you have a filter on the scope to reduce the spectrum and thereby the amount of color blur coming through as well. So if the figure of the scope is diffraction limited and you have a green filter like a #56 or #58 then likely you will get much cleaner looking splits of close doubles nearer the theoretical 1.65 arcsec for your 70mm aperture scope.
An excellent and detailed response.
Scopes: Refs: Orion ST80, SV 80EDA f7, TS 102ED f11 Newts: AWB 130mm, f5, Z12 f5; Cats: VMC110L, Intes MK66,VMC200L f9.75 EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker IV, Vixen Porta II, Celestron CG5
User avatar
John Baars Netherlands
Co-Administrator
Co-Administrator
Articles: 5
Online
Posts: 2724
Joined: Sat May 11, 2019 9:00 am
4
Location: Schiedam, Netherlands
Status:
Online

TSS Awards Badges

TSS Photo of the Day

Re: Whats the typical maximum resolution in arcseconds of a 70mm achromat/doublet refractor?

#8

Post by John Baars »


realflow100 wrote: Tue Aug 17, 2021 9:55 pm (...)
I can see the grating in the platform a person would stand on at the top of a cell tower like a half a mile to a mile away (Not quite sure but its pretty far away.)
This kind of resolution is not measured in arcseconds but in lines/mm, it is called linear resolving power. *1 For astronomical purposes it seldom is discussed. It is the reason why a telescope can "see" the Cassini division on Saturn, while this division is well under the angular resolving power.
Linear resolving power SL and the angular resolving power SA are related through the focal length, f, of the optical system .
SL= 206265 divided by SA.f
In your system:
SL= 206265 / (1.66X360) = 345 lp/mm
Low contrast and seeing ( turbulence) while observing the grating in the platform normally destroy the maximum abilities of the telescope. High contrast details on the Moon at high altitude are another story and sometimes details can be seen that are narrower than the angular resolution predicts.



*1 Rutten and Venrooij, Telescope optics, chapter 18
https://www.telescope-optics.net/telesc ... lution.htm
Refractors in frequency of use : *SW Evostar 120ED F/7.5 (all round ), * Vixen 102ED F/9 (vintage), both on Vixen GPDX.
GrabnGo on Alt/AZ : *SW Startravel 102 F/5 refractor( widefield, Sun, push-to), *OMC140 Maksutov F/14.3 ( planets).
Most used Eyepieces: *Panoptic 24, *Morpheus 14, *Leica ASPH zoom, *Zeiss barlow, *Pentax XO5.
Commonly used bino's : *Jena 10X50 , * Canon 10X30 IS, *Swarovski Habicht 7X42, * Celestron 15X70, *Kasai 2.3X40
Rijswijk Public Observatory: * Astro-Physics Starfire 130 f/8, * 6 inch Newton, * C9.25, * Meade 14 inch LX600 ACF, *Lunt.
Amateur astronomer since 1970.
Post Reply

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

Register

Sign in

Return to “Refractor type Telescopes”