Focusmax 

HerH

Buying the correct Telescope & CCD camera

Field of View

The field of view (FOV) that your camera will see through a given telescope is determined by physical size of the CCD sensor and the focal length of the telescope.

Note that this has nothing to do with the number of pixels.

A CCD that has 512 x 512 pixels (262,144 pixels )that are 20 microns (4.47 x 4.47 uM) square will have exactly the same field of view as a CCD with 1024 x 1024 (1,048,576 pixels) that are 10 microns (3.16 x 3.16 uM) square even though the latter CCD has four times as many pixels.

This is also why binning 2x2 or 3x3 affects resolution but does not affect the field of view of the CCD. Larger CCDs have larger fields of view at a given focal length.

You can change the field of view of a CCD only by changing the focal length of the telescope, by using a focal reducer you shorten the effective focal length of the telescope.

You can increase the field of view (and make the image brighter in the process). By using a barlow or eyepiece projection you effectively lengthen the focal length of the telescope and decrease the field of view (and make the image dimmer in the process).

In order to determine the field of view for a given CCD, note the CCD's length and width dimensions (or diagonal) in millimeters and use the formula to determining the field of view for that CCD through any telescope as follows:

(135.3 x D ) / L = Field of View in arcminutes

where D is the length or width dimension of the CCD in millimeters, and L is the focal length of your telescope in inches.

You can use the same formula to find the diagonal field of view if you know this dimension. So, for example,

if you wanted to know the diagonal field of view of the STF-8300 when attached to a 5" F/6 telescope you would first determine the focal length of the telescope by multiplying its aperture, 5 inches, by its focal ratio, 6, to get its focal length, 30 inches.

The diagonal dimension of the CCD is 22.5 mm. To calculate the field of view multiply 135.3 x 22.5 = 3,044 and then divide by 30 = 101.5 arcminutes. By way of comparison, the diagonal field of view of the STXL-11002M through the same telescope would be 135.3 x 43.6 = 5,912.6 divided by 30 = 197.1 arcminutes, about twice the field of view.

PART 2

The image scale of your system (FOV) is dependent upon only two factors- your CCD camera's pixel size, and your telescope's effective focal length.

A low number for image scale (like 0.5 arcsec/pixel) means high resolution, and a high number for image scale (like 10 arcsec/pixel) means lower resolution.

This makes sense- if 10 arcsec worth of data (light) are represented by only one pixel, you have essentially crammed all of that detail into one point! Calculation of image scale (in units of arcsec/pixel) is easy:

FOV = 206 x pixel size (in microns) / effective focal length (in mm).
= 206 x 20 / 1200
= 3.43 arcsec / pixel

So what is the best image scale to aim for (in other words, what pixel size should we want in our CCD camera, for a given focal length telescope)?

Surprisingly, it's not 3.43"/pixel at my site. The Nyquist theorem suggests that in order to efficiently record this information and convert it into digital format, our system should be sampling the image more aggressively, by operating at an image scale of about 1/3 times the seeing,

For example, 1/3 x 3.43", or 1.17"/pixel. (Actually, it should be 1/3.3 times the seeing, but who's counting).

This means that my CCD camera/scope combination should ideally have an image scale of 1.17"/pixel, in order to take full advantage of my (suboptimal) seeing conditions and produce a final resolution of approximately 3.5"/pixel.

For example: Exchanging Camera

I have a Takahashi 106ed , focual lenght is 530mm, and the F is a 5, apature is 106ed. My camera is a SX964, but I am thinking in buying
a FLI 16803 for deepspace narrowband with a larger FOV. Lets do some numbers of both combos

Current setup:

SX694 Pixel size : 4.54um
Focal Length (mm):530

=206x4.54/530= 1.76" arcsec per pixel resolution, if we apply the 1/3 = 0.59

Remember what I said :A low number for image scale (like 0.5 arcsec/pixel) means high resolution, and a high number for image scale (like 10 arcsec/pixel) means lower resolution.

I have an excellent 0.6 and to take full advantage of a high resolution you will need from 0.5 to 1

Thinking to change camera to 16803.

The FLI 16803 pixel size is : 9um

same scope : 206x9/530=3.49" arcsec /pixel divide by 1/3 = 1.16

This combination will result in under smapling, which means that it will result in softer images, althrough the wide field is great. You can put a barlow 1.5 to low to increase resolution (lower the 1.16) but why do that, you will reduce the FOV.

If you when to buy a larger pixel camear think first that the telescope might not be the correct one.

FLI / Takahashi CCA250 Set-Up

The extension from preciseparts is here, this adapter ( the piece between the MMOAG and the telescope rear end ) will give me the 159mm to reach the back focus distance. Here are some images of the imaging train completed. Next week some testing on the focuser, CCD and filter wheel.

Takahashi 106ed Image train with a FLI Atlas Focuser

Cleaning your Filter Wheel

Having a good cleaned filters and or the lenses is highly important for the benefit of quality images, the example below shows a set of flats stacked to create a master Flats, the dust particles are present everywhere impossible to eliminate. Its time now to clean all the filter wheel and filters.

New Takahashi CCA 250 

My new Takahashi CCA250 is here in my apt in Spain, it has been 4 months waiting for building & delivery.

Specifications

Takahashi CCA 250 mm Tri-Focal Astrograph Telescope OTA

• The Takahashi CCA-250 Tri-Focal Astrograph functions at f/5, f/8, and f/3.9 (with 6345 Flattener) and has an aperture of 250 mm (9.84")
• The CCA-250 Tri-Focal Astrograph was produced by Takahashi to replace the Baker Ritchey Chretien BRC-250
• The CCA-250 rear assembly includes a large camera rotator and can operate an electric rotator for remote operation
• The CCA-250 Tri-Focal Astrograph design includes a carbon fiber tube allowing for light weight and stable focus
• The CCA-250 by Takahashi has a D-series (Losmandy) -compatible dovetail for easy mounting.

QSI 632 to Takahashi 106ed & Reducer 0.73x 

Back Focus Distance

Building the Image Train

Reducer 0.73x

Setup

Reducer & Extension made from PreciseParts

Setup with Reducer & Extension

New make to measure OAG for the SX Lodestar

Extension made to measure from PreciseParts

Color Ratio Calculator

To test the relative color balance of your imaging system, you will need to image a solar analog star and measure the variation in brightness through each filter.  This is easily done.  Imager Bart Declerq recommends imaging an out-of-focus G-type star, preferably near the zenith.  If no star is available so high in the sky, an atmospheric extinction correction factor can be applied using the chart shown in the next section. 

Measurement of the star brightness can be done using the Information tool in  Maxim DL function in other image processing software.

20 second exposure

Take one exposure through each color filter, red, green, and blue.

Choose an exposure that yields a brightness between 10,000 to 50,000 ADU(bright enough for a good signal but not saturated).

Use the identical exposure for each filter.

Measure the average brightness of the out-of-focus star in each image.  The value should be slightly different based on the characteristics of the CCD chip and filter set.  For example, the measured value of the star might be as follows:

Red Value:  19,000
Green Value:  25,000
Blue Value:  14,000

The color ratios are determined as follows:

Red Correction Factor = 1/(Red Value/Maximum Value)
Green Correction Factor = 1/(Green Value/Maximum Value)
Blue Correction Factor = 1/(Blue Value/Maximum Value)

In the above example, the green value is the maximum value so the correction factors would be:

Red Factor = 1/(19,000/25,000) = 1/0.76 = 1.32
Green Factor = 1/(25,000/25,000) = 1/1 = 1.00
Blue Factor = 1/(14,000/25,000) = 1/0.56 = 1.79

These value yield the 1.3:1.0:1.8 RGB ratio used on the Whirlpool Galaxy example image above.  Most cameras have the greatest sensitivity in green or red and therefore green or red is normally the basis for comparison, but some cameras (notably the popular ST-2000) have higher blue sensitivities and might yield a ratio more like 1.7:1.3:1.0 in RGB.

You might use exposure times of 130 minutes, 100 minutes, and 180 minutes in red, green, and blue, respectively, to obtain proper color balance. 

using the green (1) as the base

Green = 1

Blue = 1.8

Red = 1.3

If taking 10 frames x 600 second, I use the same ratio to apply the difference exposure time.

For example

Green =  1 = 10 x600 seconds  = 6000 /60 = 100 Mins

Blue = 1.8  = 10 x 1080 seconds or 18 x 600  = 10800 =180 mins

Red = 1.3  = 13 x 600 = 7800 sec = 130 mins

Summery

Color  Factor

Green                   Blue                       Red

1                             1.8                          1.3

Instead of changing the color ratio you can change the exposure time which will do the same effect as changing the ratio

Exposure

100 min                180min                 130min