4 Inch Field Corrector for Newtonians and up to 60mm field diameter

The corrector is the bigger version of the 3 inch corrector that was introduced a few years ago.
Since the CCD cameras have again been getting bigger in size, some customers have asked for an even bigger corrector which is now available.

This corrector has a 4" barrel and is suitable for CCD cameras up to 60mm field diameter, however the best performance is achieved on 50mm field. To achieve a similar resolution on an even bigger field this corrector uses an ED lens. This together with the big lens diameters (90mm) makes this corrector rather expensive but it will be the best solution possible for nearly any Newtonian telescope and large field diameters.

The corrector decreases the focal length by less than 2%.

The housing ends with 6 metric M3 threads on D=96mm.

The focus is moved OUT by appr. 17mm

The back-focus is around 60mm slightly depending on the focal ratio. Please see table below for further details.

Please note the following distances for different mirrors:

A=Distance from Flange to Focus

B=Distance from Flange to primary mirror vertex (middle of the reflecting surface)

Diameter

Focal length

Focal Ratio

B

C=FocalLength-B

A

250

875

3.5

832.3

42.8

58.0

250

1000

4

962

43.5

58.7

250

1250

5

1205.5

44.5

59.68

300

1050

3.5

1006.2

43.8

59.0

300

1200

4

1155.6

44.4

59.6

300

1500

5

1454.8

45.2

60.4

400

1200

3

1156.3

43.7

59.0

400

1400

3.5

1354.9

45.1

60.2

400

1600

4

1554.4

45.5

60.75

400

2000

5

1954.1

45.9

61

600

2100

3.5

2053.7

46.3

61.4

600

2400

4

2353.4

46.6

61.7

1000

4000

4

3952.5

47.5

62.55

As you can see from the table, the distance from corrector flange to focus varies slightly depending on the mirror diameter and f-number. If you can't find your f-number or aperture you can make a guess by averaging close examples.

If you build your own system with this corrector you have to make an adapter, that puts your CCD in the right distance from the corrector. This should be accurate by +-1mm if you want to get the optimum performance. Also take filter thickness into account than add 1/3 of filter thickness to the true distance.

Then construct your telescope such that the corrector flange will be appr. 45mm inside the original focal point and allow some focusing range from that point. If you want to avoid the corrector to stand inside the mirror aperture the original focus should be at least 190mm above the mirror lightpath (for example 190 + 200mm = 390mm above the tube center for a 400mm mirror).

Please don't ask me for spot diagrams for your private mirror focal ratio (like asking for f/3.7 if there is a f/3.5 and f/4 on the web-site). There won't be any magical happening between the other focal ratios so there is no need to ask for this.
Also please don't ask me where to put your secondary or your focal point if you did not understand the above table.

I would love to answer all your questions but I have not enough time in my live, sorry.

 

Examples

The corrector can be used between f/3 and f/5.5. The corrector has been optimized for a field diameter of max. 60mm. In the below examples you can see the performance and vignetting (assuming that you use a large enough secondary mirror) for different examples. You can see that even with f/3 and a D=60mm sensor the corrector works very well.

400mm f/4 Spotsizes

400mm f/4 Vignetting

400mm f/3 Spotsizes

400mm f/3 Vignetting

More Spotdiagrams and vignetting:

250mm f/3.5    Vignett
250mm f/4    Vignett
250mm f/5    Vignett
300mm f/3.5    Vignett
300mm f/4    Vignett
300mm f/5    Vignett
400mm f/3    Vignett
400mm f/3.5    Vignett
400mm f/4    Vignett
400mm f/5    Vignett
600mm f/3.5    Vignett
600mm f/4    Vignett
1m f/4    Vignett

as can be seen from the calculations, the same corrector can be used on a large range of Newtonian mirrors from 250mm to 1m diameter. Spotsize is very small, concentrating 80% in 1 Pixel even for cameras with small pixel size.

How to order:
please contact

ASA Astrosysteme GmbH

Galgenau 19

A-4212 Neumarkt

Austria

 

german http://www.astrosysteme.at/de/korrektoren.html

 

englisch http://www.astrosysteme.at/eng/correctors.html

 

 

They also sell great ready-to-go telescopes that use these correctors !