Calculations

Various calculations associated with the hardware.

 

 

Links

 

 

 


Back Focus Review

Provided below is a review of calculating the Back Focus.

 

References

 

 

C-11 Imaging Train

Type

Recommended

Nominal Back Focus (NBF)

ESATTO

Focuser

Visual Back

Adapter

Tilt

Plate

Off Axes

Guider

Filter

Wheel

Camera

ZWO 6200

Total

Primary Focus

146.05mm

67mm

16.5mm

5mm

17.5mm

22mm

12.5mm

140.5mm

Focal Reducer

146.05mm?

67mm

16.5mm

5mm

17.5mm

22mm

12.5mm

140.5mm

HyperStar

 

 

Spacer

Filter Tray

Adjustment

Ring

 

 

Camera

QHY 128C

 

HyperStar

 

55mm

18mm

17mm

6mm

N/A

N/A

18mm

 

59mm

 

Note the ESATTO Focuser has a 15mm travel range, assuming we start at dead center of travel range, we have a +/- adjustment range of about 7mm.

 

 

 

 

 


Image Scale Calculations

Image scale calculations require you know the Focal Length of your optical train.  This can be rather difficult with the various possible hardware configurations.  It may be easier and more accurate to obtain the Image scale by taking a photo with the configuration and obtain the Image Scale from Astronetry.net, and back-calculate the Focal Length.  This is how the values below were calculated.

 

Image Scale: How to determine Image Scale for your setup

P" =  (8S)/FL

 

Focal Length: How to determine Image Scale for your setup

FL =  (8S)/P"

 

 

Where :

P = Image scale/pixel in Arc seconds

S = Size of pixel in microns

FL = Focal Length in inches

 

 

Under/Over sampled?  

Typical Seeing Conditions = 2 arc Seconds this means Ideal image scale should be 1.0 to 1.5 arc seconds/pixel.

 

In the table below purple values represent calculated values while green values represent measured or provided by vendor.

 

Telescope Configuration

Focal Length (FL) (Inches)

FL (mm)

Sensor Pixel Size (S)

 (Microns um)

Measured Image Scale (P")

(arcsec/pixel)

Under/Over

Sampled

Measured Image Size

(arcmin or Deg)

Comments

C-11 HD | ZWO ASI-6200 |

110.24"

2800 mm

3.76 um

0.280

OverSampled

44' 39.1" x 29' 47.2"

Ideal 0.67" - 2"

C-11 HD | Reducer | ZWO ASI-6200 |

77"

1960 mm

3.76 um

0.393

Slight OverSample

62' 39.5" x 41' 47.9"

 

C-11 HD | HyperStar | ZWO ASI-6200 |

21.26"

 540 mm

3.76 um

*1.4

OverSampled

 

 

C-11 HD | ZWO ASI-6200 | Bin 2x2 |

110.24"

2800 mm

7.52 um

 

 

 

Ideal 0.67" - 2"

C-11 HD | Reducer | ZWO ASI-6200 | Bin 2x2 |

 

77"

1960 mm

7.52 um

0.783

Good

62.5 x 41.7

 

C-11 HD | HyperStar | ZWO ASI-6200 | Bin 2x2 |

21.26"

540 mm

7.52 um

*0.55"

 

Slight Oversample

 

 

C-11 HD | QHY128 |

110.24"

2800mm

5.97 um

0.444

Oversampled

44.9' x 30.2'

Typical Primary Configuration

C-11 HD | Reducer | QHY128

77"

1960 mm

5.97 um

*0.62

Oversampled

*60' x 45'

Typical Reducer Configuration

C-11 HD | HyperStar | QHY128

21.26"

540mm

5.97 um

*2.25

Undersampled

*3.8 x 2.5

Typical HyperStar Configuration

C-11 HD | TeleVue Barlow 2X | QHY128

95.329"

2,422mm

5.97 um

0.501

Oversampled

25.3' x 17'

 

C-11 HD | TeleVue Barlow 4X | QHY128

 

 

5.97 um

 

 

 

 

Takhashi FS-60C | ZWO 17mm Mini

13.7478"

349mm

5.86 um

3.41

Undersampled

110' x 69'

Standard Finderscope Configuration

TPO UtraWide 180 F/4.5 | ZWO 17mm Mini

7.087

180mm

5.86 um

6.73

Undersampled

3.62deg x 2.27deg

Standard Finderscope Configuration

 

 

 

 


Dither and Drizzle

 

Resources:

 

Dither is a technique used when capturing images in astrophotography to help create a better signal-to-noise ratio and is the process of slightly shifting the pointing direction of your telescope between exposures.

 

Drizzle is a technique applied after images have been captured and was developed by NASA to reconstruct undersampled images.  In the Image below, the Top Left image represents the original object, the Bottom Left is the captured image, and the Bottom Right is the captured image with Drizzle applied to it.  This technique is applied to images when processing (IE in Pixinsight)

 

Ref: Drizzle: Reconstructing Undersampled Images - A.S. Fruchter & R.N. Hook

 

 

 


Exposure Calculations

Image scale calculations require you

Links

 

Camera

Bin

 

Comments

ZWO ASI6200 Pro

1x1

  • Sensor Size: 36mm x 24mm

  • Resolution: 9576 x 6388

  • Pixel Size: 3.76um

  • Download Time: 0.5s

  • Full Well: 51ke

  • Read Noise: 1.2-3.5e

  • ADC Bits: 16Bit ADC

  • Dynamic Range: 80db 14571:1

  • DR Stops: 14 bits

  • Dark Current (0 C): 0.003 e/p/s

 

ZWO ASI6200 Pro

2x2

  • Sensor Size: 36mm x 24mm

  • Resolution: 4788 x 3194

  • Pixel Size: 3.52um

  • Download Time: 0.5s

  • Full Well: 200ke

  • Read Noise: 2.4-7e

  • ADC Bits: 16Bit ADC

  • Dynamic Range: 80db 2857:1

  • DR Stops: 14.8 bits

  • Dark Current (0 C): 0.012 e/p/s

  • S/N Ratio = 2x original

 

 

 

 

 

Astronomy

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