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Reviews

A 150 mm Achromat and a 120 mm Apo, a personal review.

by John Baars

Performance of and differences between a 150mm widefield achromat and a 120mm apochromat.

A personal review.

Apples and pears are both fruit. Comparisons between the two are normally not done. Different taste, shape and bite. Still, we do it all the time. Consumers know the differences. Why bother for a difference test?

Do we really know differences we usually take for granted? We say we do. But do we really?

Underlying comparison was made out of curiosity. What are the real differences we ( I ) take for granted every time I use my telescopes? Are they as severe as common knowledge tells us? Some differences were obvious, some were not

The idea of testing and comparing the two was born during a long period of clear nights, approximately twelve or so. I got used to using one telescope and the other every other day. After a while I took them both out. For comparing them. No scientific aspirations. Just for fun. From the amateurs point of view. If I write them down, would anyone be interested? I thought it could be fun to share my findings with you. Maybe my writings here could be a decision point for someone. Here we go.

Meet the instruments

150 mm f/5 Achromat Skywatcher Startraveller.

120 mm f/7.5 Apochromat Skywatcher Evostar.

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Some technical specs:

Instrument

120 f/7.5 ED Apochomat

150 f/5 Achromat

Strehl

0.96

Quite some quality differences established in official tests. From 0.33 to 0.8. The instrument discussed has a Strehl between 0.7 and 0.75

Naked Eye Limited Magnitude 4.5 (light polluted skies)

Under Bortle 4 skies

12.9 - 13

not measured

13,2

13,4

RC- index (Rest Chromatic index according to W. Rohr)

Apo value = <1.0

Achro = >2.0 and up.

0.7

14.7

Is this chromatic aberration, or Chromatic Aberration?

F in mm.

900

750

Peak/Valley wave error

0.2 or better

Approximately 0.3

Size Airy disc in mm.

0.009

0.007

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Eyepieces used.

- Baader Hyperion 32 mm, this eyepiece is not capable of handling a f/5 light-cone, no round stars at the edge of the field. Even the f/7.5 cone of the 120mm is a bit to steep.

- Eudiascopic 30 mm, works OK in the 120 f/7.5, but no good in the 150 f/5.

- Panoptic 24 mm, works fine in both.

- Leica 8.9- 17.8 zoom ASPH, works excellent in both.

- Fujiyama 6 mm, works very good in both. A barlowed 12mm Brandon is on par.

- Pentax XO5, works excellent in both.

- Pentax XO2.5 works excellent in both.

- Zeiss barlow 2X. Outstanding quality. Works as 2,33X in combination with the Leica as a result of a larger projection distance, due to the adapter.

The Leica, in combination with the barlow, could take the 150 mm up to 197X and 236X in the 120 mm. Which lead to exit pupils of 0.76mm in the achromat and 0.51 mm in the apo. An extension tube can take magnification further up for use on very close double stars.

Comparisons were attempted while having an equal exit pupil on both instruments. Which means equally bright / dark sky background. As the 120 could be taken further up, the sky background could be darkened even more. Exit pupils of 0.4 mm to 0.3 mm. Of course I tried. In most cases this did not lead to better views except for double stars or some planetary nebulae. On the other hand, the possibility of zooming meant that magnification could be adjusted to reach the most favorable view given the atmospheric circumstances, such as transparancy.

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Eyepieces, Magnification, Field of view, Exit pupils.

120 mm f/7.5 Apochromat

150 mm f/5 Achromat

Eyepiece

Magnification

Field of view in degrees

Exit pupil in mm

Magnification

Field of view in degrees

Exit pupil in mm

32 mm Hyperion

( seldom used )

28

2.6

4.3

--

--

--

30 mm Eudiascopic

30

1.7

4

--

--

--

24 mm Panoptic

38

1.8

3.2

31

2.2

4.8

17.8 - 8.9 mm Leica zoom

51 - 101

1.1 – 0.74

2.3 – 1.2

42 - 84

1.3 – 0.9

3.6 – 1.8

2,33X Barlowed Leica Zoom

118 - 236

0.47 – 0.3

1 – 0.5

98 - 197

0.57 – 0.4

1.5 – 0.8

5 mm Pentax XO5

180

0.23

0.66

150

0.28

1

2X Barlowed 6mm Fujiyama

Planetary nebulae & doubles

300

0.14

0.4

250

0.17

0.6

2,5 mm Pentax XO2.5

Planetary nebulae & doubles

360

0.11

0.33

300

0.14

0.5

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Recommended exitpupils.

Recommended exit pupils

In mm

Moon, planets

2 - 0,5

Double stars

1,5 - 0,3

Planetary nebulae

1 - 0,3

Galaxies with low surface brightness

2 - 1

Galaxies with high surface brightness

2 - 0,8

Emission nebulae with OIII

5 - 4

Emission nebulae with UHC

4 - 3

Extended objects

5 - 4

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Here are my findings.

I listed the objects / subjects in random order. Each item can score 5 points. I considered a 10 point scale, but that does not give better insight. If the score was even I divided equally. One could argue about some points given but the overall picture is clear. Some observations like the one of Jupiter and the Veil observation were done in July / August 2019.

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Subjects

120 Evostar

Points

150 Skytravel

Points

Remarks/ comments

Extended Nebulae, Fields.

1

4

Rosette as a whole, Veil nebula. The wide field of view gives an enormous impression. Much dark sky around the object. The 150 mm wins this one.

Details visible in bright DSO’s.

3

2

M42, NGC 2392. Whisps and brightness differences.

Visibility faint Messier DSO’s under LP skies.

2,5

2,5

M105, NGC3384, M65 etc in Leo, M51, Messier 84 - 88 , M104 etc. Small differences seen can be allocated to differences in transparency of the sky.

Small bright DSO’s, Planetary nebulae.

3

2

M94, NGC6210, several planetaries. 120mm permits higher magnification and reveals a tad more details.

Double stars, sharpness of individual stars.

4

1

120mm needlepoint sharp stars, relative less sensible for seeing.

Cleaner split is obvious: Propus, F star in Trapezium, Izar, Sirius B, Delta Cygni, STF2094.

As expected the star test reveals a more textbook diffraction pattern in the 120mm. Even if the usual CA in the f/5 is ruled out by a green filter.

Open Clusters.

2

3

Calculated 150mm gain in magnitude ( 0,48 ) is partly lost on sharpness (0,2 gain observed) and partly on glare. On the other hand big open clusters fit in easier in the FOV. of the 150 mm.

Limiting Magnitude.

2

3

Only just. Less in 150 than calculated. Observed gain in the 150 mm is 0,2 Magnitudes..

Big Globulars.

2.5

2.5

In the 120mm the core of M3, M92, M13 seems better resolved, more contrast; a bit sharper. In the 150 they seem a tad brighter though. This is at exitpupil 0.6mm and magnification 200X.

The 150mm peeks at exitpupil 0.7mm and magnification 200+. At equal 0.6mm exitpupils the image in the 150mm seems a little bit darker.

Deepest larger and faint DSO’s.

2

3

On the limits of visibility.The difference with the background sky is barely visible. Techniques: hood/ dark adapted eye, averted vision, slewing/movement, zooming, observation at highest point.

M101, M109, M108, NGC 4566, NGC6781, NGC6742

Moon.

4

1

Details, sharpness, contrast, seeing, color. Quite a difference. The 120 shows 6 versus 2 craterlets on Plato floor. Far better defined ejecta. Darker shadows.

Planets.

4

1

150mm is definitely no Jupiter telescope. Saturn is somewhat better. Mars is more satisfying. Barely differences on Uranus or Neptune.

Filters.

1

4

150mm permitting larger exit pupil at same magnification. Light-consuming filters are easier to use. Low magnification is easier obtained in 150 mm.

Wide Field.

2

3

2.6 degrees vs 3.1 degrees with the available 31mm Baader eyepiece. No top images though with the Baader. More often used is 1.7 degrees in the 120 and 2.2 degrees in the 150 mm.

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You want to add the points up? Be my guest. But it doesn’t give you insight in the differences I tried to establish.

In my humble opinion the main differences we all know ( widefield, planets , double stars and Chromatic aberration) are quite clear.

The visibility of faint DSO ‘s under LP skies surprised me a bit. The 120 does a better job than I expected. Would you say the 120 does a better resolving job on big Globulars? I certainly didn't when I started this little game. The world wide advertised wide field view of the 150 was not as spectacular as I thought it to be. On the whole other differences were not as large as I expected they would be. The 150 does a better job (exept for planets) than most people think. On the other hand the 120 is quite capable of showing DSO’s in comparison with her bigger brother.

Thanks for reading.

John Baars.

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