C8 - Focal Reducer/Corrector f/6.3

Motivation | Photos | Preliminary Conclusions | Links | Appendix: FOV Simulations | Appendix 2: Data

In progress

On this page, I present the Celestron SCT reducer/correcor f/6.3 for Celestron and Meade Schmidt-Cassegrain telescopes and my first experiences with it.

 

Motivation

The Celestron C8 has a very long focal length and a correspondingly small field of view. So my idea was to buy a focal length reducer, for example the Celestron f/6.3 reducer/corrector, in order to get larger fields of view via a shorter focal length. When I asked my astronomy dealer Mr. Kloß about this, he gave me the short answer: "Visually, this does not help at all!" And if you look at the data in the appendix, you will understand his answer. Since the reducer/corrector is only suited to 1.25" accessories, there are no long eyepiece focal lengths available. Moreover, long focal length 1.25" eyepieces vignet anyway (from a focal length of about 26 mm on, depending on the eyepiece type). So I refrained from buying one initially.

But actually I had bought the C8 to replace two of my existing telescopes, the Skymax-127 and the Explorer 150PDS. My wife vetoed the sale of the Skymax-127, especially since it is suitable for travelling and can be used as a spotting scope. So I wanted to sell at least the Explorer, but then it came to my mind that I need it for the Atik Infinity camera. So what should I do? Determined to sell the Explorer, my only option was to try something with the C8, and the most obvious thing to do was to acquire an f/6.3 reducer/corrector. With 1280 mm, the focal length is still above the maximum recommended 1000 mm for the Atik Infinity, but my hope was that the camera might still come into focus, because the C8 has a lot of reserve. So I ordered a reducer/corrector, the Chinese version, as it turned out afterwards.

 

Photos

Unpacking...

Reducer/corrector in the box next to the 1,25" Visual Back

Box opened, reducer/corrector taken out

Caps removed from the reducer/corrector

Ditto, closer view

The lenses of the reducer/corrector

Ditto

Connecting the Reducer/Corrector to the C8

Reducer/corrector screwed to the C8 tube

Ditto, closer view

1,25" visual back screwed to the reducer/corrector

Ditto, closer view

Zenith mirror attached

Ditto, closer view

Eyepiece (1,25", 24 mm) inserted

Complete view on Star Discovery mount and with power supply

Eyepiece (1,25", 32 mm) inserted

 

 

Ditto, closer view

 

Photos of the C8 with attached Atik Infinity (Demo Photos)

Atik Infinity mounted to thes C8 directly (without zenit mirror)

Ditto from farther away

Ditto, complete view

 

Visited Sky Objects

So far, I have visited the following sky objects with the Celestron C8:

Highlights

 

First Experiences...

In progress

Connecting the reducer/corrector to the C8 went without any puzzles or problems on my first attempt. Also the "dry test" with the Atik Infinity camera was hassle-free.

The view through the eyepieces (24 mm and 32 mm) looked good, until I pointed the telescope at a crane. I found the observed pincushion distortions (see the photos above) quite strong! Probably, I will hardly notice them in the sky, both visually and with the Atik Infinity...

Pincussion distortions when using the 32 mm eyepiece...

...and also when using the 24 mm eyepiece

The target photographed with a camera (section)

Photos: View through two eyepieces with the reducer/corrector

More to come, after I tried the reducer/corrector on the night sky, both visually and with the camera!

Magnification and Exit Pupil

If you look at the table for the exit pupil in the appendix below, it is not recognizable at first sight that, at the same magnification, it is the same for the C8 with and without reducer/corrector. Of course, the formula for the exit pupil clearly show this, whereas the table rather hides this. Of course, the corresponding eyepiece focal lengths are shorter with reducer/corrector, because it creates a shorter focal length.

The First Night: Comparison with and without Reducer/Corrector (January 16, 2020)

On January 16, 2020, I had the first opportunity to compare the C8 with and without reducer/corrector on some DSO. The comparison objects were M 1, M 35, M 36, M 37, M 38, M 42/43 and M 45. With the reducer/corrector I used my 24 mm Televue eyepiece (1.25", 65° apparent angle of view => 1.22° angle of view, EP = 3.81) without reducer/corrector my 40 mm Lacerta eyepiece (2", 68° apparent angle of view => 1.34° angle of view, EP = 4.0). Overall, the field of view was very similar in both conditions.

To reduce the effort, I first observed all objects with the reducer/corrector and then without. Thus I cannot really speak of a "comparison". I did not notice any significant differences between the two configurations, except that without reducer/corrector the image seemed a little larger due to the larger apparent field of view, which I found pleasant. The Crab Nebula M 1 was very faint in both configurations, but maybe a little brighter with reducer/corrector - maybe a consequence of the aperture ratio of f/6.3 (on the other hand, the EP was slightly larger without reducer/corrector).

Overall, I was very satisfied with the results of the reducer/corrector in this comparison. I also did not notice the strong pincushion distortions that I had found during daylight. But without straight lines in the field of view finding distortions is difficult...

This practical test, of course, confirmed what I had already heard and what the technical data state: Visually, the Reducer/Corrector does not offer any advantages, at least not with respect to a larger field of view (the 1.25" 32 mm eyepiece comes close to the 2" 40 mm eyepiece, but still has a smaller field of view).

If, on a certain occasion, you want to observe only with 1.25" eyepiece and can do without the highest magnifications, then the reducer/corrector is definitely a simple and efficient solution because you need not handle heavy and large 2 accessories and can nuse the 1.25" zenith mirror..

A First Nightly Test with the Reducer/Corrector at the Atik Infinity (March 15, 2020)

On 15.3.2020, I made a first "night test" with the Atik Infinity camera at the Celestron C8. I tested the following two configurations: (1) f/6.3 reducer/corrector and 1.25" zenith prism, (2) f/6.3 reducer/corrector, 0.5x TS focal reducer (screwed into the camera) and 1.25" zenith prism. The telescope was mounted on the Star Discovery mount, which I operated with the SynScan WLAN module via the SynScan app on the iPhone. Here are some of the "best" photos I took during this test session:

         

M 41 (missed, above of M 41), f/6.3 reducer/corrector

 

M 41 (missed, above M 41), f/6.3 reducer/corrector and 0.5x TS focal reducer

 

NGC 2264 (missed, to the left of NGC 2264), f/6.3 reducer/corrector and 0.5x TS focal reducer

   

M 42/43, f/6.3 reducer/corrector

 

M 42/43, f/6.3 reducer/corrector and 0.5x TS focal reducer

 

M 42/43, f/6.3 reducer/corrector and 0.5x TS focal reducer

   

NGC 2024, f/6.3 reducer/corrector

 

NGC 2024, f/6.3 reducer/corrector

 

NGC 2024, f/6.3 reducer/corrector and 0.5x TS focal reducer

All in all I found the results, apart from those for M 42/43, fairly disappointing, but I was not that much in practice with the camera anymore... Faint objects like the Flame Nebula NGC 2024 were very pale, which did not get much better in post-processing (even though they were saved as 16 bit TIFF files). I got vignetting with the additional 0.5x TS focal reducer, which is more or less annoying depending on the object. On the other hand, you get a much larger field of view (about 0.8° against 0.4°, thus about 48' against 24'; the latter fov does not even capture the moon as a whole).

 

Preliminary Conclusions

In preparation

It is too early for first conclusions...

 

Links

 

Appendix: FOV Simulations

Field of View Simulations for Various DSO

For the C8 with a f/6.3 reducer, I used the astronomy.tools Field of View tool to demonstrate how certain Messier objects fit the field of view of the Atik Infinity camera. In addition, I calculated thisd for the Explorer 150PDS which I had used with the camera in the past. Finally, I also calculated the field of view in my 10 mm TeleVue Delos eyepiece (72° apparent field of view), which has a comparable field of view to the Atik Infinity camera.

Here are the color codings for the different configurations of my equipment:

Note that the field of view is considerably smaller for the C8 even when used with a f/6.3 reducer/corrector than that of the Explorer 150PDS.

M 8

M 11

M 13

M 16

M 17

M 27

M 31

M 33

M 44

M 56

M 81

M 92

FOV Comparison with and without Reducer/Corrector

Moreover, I simulated, which FOV can be achieved using my 1.25" and 2" eyepieces, the first ones for use with the reducer/corrector, the second ones for use without it. In the end, there is little difference between the two configurations, and this is another proof that the reducer/corrector does not allow for a wider FOV- as I had hoped for...

Left: 1.25" eyepieces (with reducer/corrector)

Right: 2" eyepieces (no reducer/corrector)

 

Appendix 2: Data for Celestron C8 with f/6.3 Reducer/Corrector

Observation-Relevant Data (in Comparison with my 6" Newton Tube)

Telescope

Focal
Length
(mm)

Aperture
(mm)
Focal
Ratio
Light
Gathering
Power
Maximum+
Minimum*
Maximum*
Minimum+
Usable Magnification
Usable Focal Length
of Eyepiece (mm)
Factor/Exit Pupil (mm) >
Manuf.
1.5
2
6.5
7
6.5
7
1.5
2
Celestron C8 (Schmidt-Cassegrain)
2032
203
10
841
305
406
31.26
29.03
65.0
70.0
6.7
5.0
Cel. C8 (Schmidt-Cassegrain) (Red.)
1280
203
6.3
841
305
406
31.26
29.03
41.0
44.1
4.2
3.2
Skymax-102 (Maksutov-Cassegrain)
1300
102
12.75
212
153
204
15.69
14.57
82.9
89.3
8.5
6.4
Skymax-127 (Maksutov-Cassegrain)
1500
127
11.81
329
191
254
19.54
18.14
76.8
82.7
7.9
5.9
Explorer PDS150/Dobson 6"
750
150
5
459
225
300
23.08
21.43
32.5
35.0
3.3
2.5

*) Calculated for an exit pupil of 6.5 mm and 7 mm
+) Factor 1.5 or 2 for Dobsonian/Newtonian telescopes; in general, the lower value of 1.5 is used for Newtonian telescopes; if the manufacturer specified a different magnification, it is also listed (some manufacturer provide considerably higher numbers...).

Recommendations for the Focal Lengths of Eyepieces for Celestron C8 with f/6.3 Reducer/Corrector (Following My Recommendations and Those of Others)

Criteria Exit Pupil
Focal Length of Eyepiece
Category Application Area
from...to
Calculated
On the Market
Examples
Existing
Maximum FOV Search
7
10
44-63**
40
56*
---
Minimum Magnification / Large FOV Overview, large-area nebulae
4.5
6.5
28-41
25-40
28, 32, 40
32
Normal Magnification Large-area, faint nebulae; nebulae, open star clusters
3.5
4
22-25

20-25

20, 24, 25
24
Best for many objects, e.g. for most galaxies, and mid-size DSO
2
3
12.5-19
12.5-18
12.5, 15, 16, 18
16
Maximum Magnification / Maximum Resolution "Normal" upper magnification limit; globular star clusters
1
1.5
6.3-9.5
6.5-9.5
6.5, 7, 9
7, 10
Maximum perceptibility of small, low-contrast details; planetary nebulae, small galaxies;
maximum magnification for moon and planets
0.6
0.8
3.5-5
3.5-5
3.5, 4, 6
4
Separation of narrow double stars, small planetary nebulae;
perception of faintest details
0.4
0.5
2.5-3.2
2.5-3.5
2.5, 3, 3.5
---

*) Partly available as 2" eyepiece; **) typically no suitable 1.25" eyepieces available; problems with viewing at 40 mm; italic: not possible; red: magnification too high; blue: commercial focal lengths

Visual Power (Magnification) and Other Data for Different Focal Lengths of Eyepieces (Mostly My Current Eyepieces)

Note: These tables include the StarTravel 120 refractor, a TSWA32 eyepiece (2", 32 mm focal length, 70° viewing angle) that I borrowed together with the StarTravel 120, a 18 mm eyepiece (2", 82° viewing angle), a 38 mm eyepiece (2", 70° viewing angle), and a 56 mm eyepiece (2", 52° viewing angle).

I did not include the Celestron 25 mm Plössl- eyepiece, which is delivered with the C8, because I gave it away...

Telescope
Further Data
Focal Length of Eyepiece (mm)
Magnification
 
Focal Length of
Telescope (mm)
4
7
10
16
18
24
26
28
32
32
35
38
40
56
PS 72/432
432
108.00
61.71
43.20
27.00
24.00
18.00
16.62
15.43
13.50
13.50
12.34
11.37
10.80
7.71
ST120
600
150.00
85.71
60.00
37.50
33.33
25.00
23.08
21.43
18.75
18.75
17.14
15.79
15.00
10.71
TLAPO1027
714
178.50
102.00
71.40
44.63
39.67
29.75
27.46
25.50
22.31
22.31
20.40
18.79
17.85
12.75
150PDS
750
187.50
107.14
75.00
46.88
41.67
31.25
28.85
26.79
23.44
23.44
21.43
19.74
18.75
13.39
Skymax-127
1500
375.00
214.29
150.00
93.75
---
62.50
---
---
46.88
---
---
---
---
---
C8
2032
508.00
290.29
203.20
127.00
112.89
84.67
78.15
72.57
63.50
63.50
58.06
53.47
50.80
36.29
C8 (Red.)
1280
320.00
182.86
128.00
80.00
---
53.33
---
---
40.00
---
---
---
---
---
 
True Field of View (°)
Apparent Field of View (°) >
82
82
72
82
82
65
70
56
52
70
69
70
68
52
 
Focal Length of
Telescope (mm)
4
7
10
16
18
24
26
28
32
32
35
38
40
56
PS 72/432
432
0.76
1.33
1.67
3.04
3.42
3.61
4.21
3.63
3.85
5.19
5.59
6.16
6.30
6.74
ST120
600
0.55
0.96
1.20
2.19
2.46
2.60
3.03
2.61
2.77
3.73
4.03
4.43
4.53
4.85
TLAPO1027
714
0.46
0.80
0.91
1.84
2.07
2.18
2.55
2.20
2.33
3.14
3.38
3.73
3.92
4.08
150PDS
750
0.44
0.77
0.96
1.75
1.97
2.08
2.43
2.09
2.22
2.99
3.22
3.55
3.63
3.88
Skymax-127
1500
0.22
0.38
0.48
0.87
---
1.04
---
---
1.11
---
---
---
---
---
C8
2032
0.16
0.28
0.35
0.65
0.73
0.77
0.90
0.77
0.82
1.10
1.19
1.31
1.34
1.43
C8 (Red.)
1280
0.26
0.45
0.63
1.03
---
1.22
---
---
1.30
---
---
---
---
---
 
Exit Pupil (mm)
 
Focal Ratio
4
7
10
16
18
24
26
28
32
32
35
38
40
56
PS 72/432
6
0.67
1.17
1.67
2.67
3.00
4.00
4.33
4.67
5.33
5.33
5.83
6.33
6.67
9.33
ST120
5
0.80
1.40
2.00
3.20
3.60
4.80
5.20
5.60
6.40
6.40
7.00
7.60
8.00
11.20
TLAPO1027
7
0.57
1.00
1.43
2.29
2.57
3.43
3.71
4.00
4.57
4.57
5.00
5.43
5.71
8.00
150PDS
5
0.80
1.40
2.00
3.20
3.60
4.80
5.20
5.60
6.40
6.40
7.00
7.60
8.00
11.20
Skymax-127
11.81
0.34
0.59
0.85
1.35
---
2.03
---
---
2.71
---
---
---
---
---
C8
10
0.40
0.70
1.00
1.60
1.80
2.40
2.60
2.80
3.20
3.20
3.50
3.80
4.00
5.60
C8 (Red.)
6.3
0.63
1.11
1.59
2.54
---
3.81
---
---
5.08
---
---
---
---
---

Blue: Equipment borrowed for comparison purposes; gray: sold equipment; italic: 2" eyepieces

Magnification: Yellow: low (30-50 x); magenta: medium (80-100 x); violet: high (150-200 x - and more); red: beyond maximum usable magnification.
Exit pupil: Values in magenta cells are either too small (< 1 mm) or too large (> 6.4/7 mm); yellow background: best for galaxies (about 2-3 mm).

 

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15.08.2020