Winchester Electronics: Optec MAXfield FAQ

Optec MAXfield FAQ

What is the MAXfield?

The MAXfield telecompressor is a three-element lens system designed to work specifically with Schmidt-Cassegrain f/10 telescopes (SCT's) and medium to small format CCD cameras. The telecompression ratio is 0.33x, so an f/10 system becomes f/3.3 and an f/11 system becomes f/3.6. Basically, the MAXfield is a three-part system - a telescope mount threads onto the 2-inch thread on the back of the SCT - a lens mount which holds the lens system - and a camera mount which adapts the lens system to a particular camera. The picture below shows the three parts of the MAXfield telecompressor system.

The Optec MAXfield tele-compressor system.

In some cases the telescope mount is not needed. Since the lens system will fit into any 2-inch focuser, you may be able to order the MAXfield 2" (#17402) and save a little money.

Can the MAXfield be used with any telescope other than an f/10 SCT?

The MAXfield will work fine with an f/11 C-14 SCT provided the telescope has enough back-focus. Of course at 0.33x, an f/11 system becomes f/3.6. Any SCT with an f-ratio of 10 or higher should work fine. Takahashi has an f/12 SCT available that should work with the MAXfield provided there is enough back-focus. The MAXfield has been used successfully with a classical cassegrain, but has not been tested on a Maksutov cassegrain. The design corrects for the coma of an SCT which tends to greater than a Maksutov (which is essentially coma-free).

O.K., how much is "enough" back-focus?

Our calculations indicate that the required back focus is about 195mm. There is a fairly easy way to check this distance with your own telescope. First remove the visual back and place a blank white card 195mm (about 7 5/8") behind the last thread of the telescope's threaded mount. If you can achieve focus from an object at infinity (the moon works well for this), then your telescope has "enough" back focus.

Can I couple the MAXfield with an f/6.3 telecompressor to get an incredible f/2 system?!?

No. (It may work mechanically, but optically you'll really be disappointed.)

How does the MAXfield attach to the back of the telescope?

When the standard MAXfield is specified (#17400) the MAXfield is shipped with a telescope mount which threads directly onto the rear cell of the SCT. The MAXfield lens assembly will fit directly into a 2" focuser such as the motorized NGF-S by JMI. In fact, owners of the NGF-S can save a few dollars by ordering the MAXfield 2" (#17402) which does not include the telescope mount.

Explain the difference between the MAXfield 2" and the standard MAXfield again.

The standard MAXfield includes both the telescope mount and the lens assembly while the MAXfield 2" includes only the lens assembly. In the drawing below the telescope mount is shown in red, the lens assembly is light blue and the camera mount is purple. The camera faceplate (ST-7 in this case) is yellow. Note that the 2" filters (shown in blue) thread into the front of the lens assembly.

How much vignetting will there be using the MAXfield?

Of course, that depends on a number of factors. The MAXfield will provide an 11mm diameter unvignetted image which will cover most chips in the small to medium size class. In practice, you may see some vignetting depending on the telescope as well as the camera. With our ST-6 camera (TC-241 CCD with a diagonal of 11mm) we see about 8% vignetting using the standard 2" rear cell mount on our 10" SCT. The standard SCT rear cell thread is an (approximately) 2-inch diameter male thread. On the 10" and larger class telescopes the rear cell can be removed to expose an even larger 3-inch diameter thread. Using the JMI Large-Format Adapter with an NGF-S focuser, we virtually eliminated all vignetting in our system.

Is vignetting a problem?

Not really, since good flat-fielding will remove the negative effects of a vignetted image.

Why are different mounting plates needed for different CCD cameras?

Each CCD camera has a different specification for important parameters such as optical distance to the CCD chip and camera faceplate mount. Even different models from the same manufacturer may place the CCD chip at a different depths within the camera head even though the physical mount is the same (i.e. cameras using a T-mount thread).

What is meant by optical distance to the CCD?

The optical distance takes into consideration the index of refraction of any glass that may be in the optical path. For instance, nearly all cameras has some sort of cover glass used to seal the CCD chamber. The thickness of this glass may vary from manufacturer to manufacturer (and even between models of the same manufacturer) and must be compensated for in the mounting plate. Basically, the introduction of flat glass into the optical path will increase the optical distance to the chip. In other words, even though the CCD may be a known physical distance from the faceplate, the optical distance will usually be further. It is this optical distance that we are most concerned with.

The T-thread seems to be a new standard with CCD cameras. Why can't you have just one mounting plate?

Again, the reason is that the optical distance to the CCD varies with different CCD cameras. This difference in spacing can usually be accommodated for by simply re-focusing at f/10. However, at f/3.3 focusing is critical. Moving the CCD chip as little as 1mm can degrade the edge sharpness. The images below demonstrate how critical the focusing can be. The image on the left was made at the optimal focus. The image on the right was made after increasing the spacing by just 1mm. [Note the star images in the lower left corners of each image.]

Image taken at the proper optical distance. (left) --- Same image with CCD moved just 1mm. (right)

The images above were taken by Dennis di Cicco for his review
of the MAXfield in the Fall 1995 issue of CCD Astronomy magazine.

If spacing is so critical, exactly what is the optimal distance from the MAXfield to the CCD chip?

For best performance and to achieve a magnification of 0.33x, the CCD should be placed at an optical distance of 29.7mm behind the center of the rear lens element of the MAXfield. Of course, each of the MAXfield camera mounts ensures this optical distance is maintained. Simply order the correct mounting kit with your MAXfield to match your CCD camera.

Are there any cameras that cannot be used with the MAXfield?

Cameras using CCD chips larger than 11mm (diagonal measurement) would certainly be vignetted. Below is a table showing diagonal measurements for Starlight Xpress cameras.

Camera	CCD Sensor	Diagonal Size
HX5	ICX084AL	7.6 mm
HX9	ICX085AL	11.1 mm
MX5 series	ICX055BL,BK	6.1 mm
MX7 series	ICX249AL,AK	10.2 mm
MX9	ICX083AL	10.9 mm
SXV-H9	ICX285AL	11.2 mm

Speaking of filters, can filter wheels be used with the MAXfield?

Unfortunately, with the limited back focus there isn't room for filters behind the MAXfield lens system. The front of the 2" lens assembly, however, is threaded for manual insertion of 48mm filters. Optec filters are available for tri-color work, but any 48mm photographic or deepsky/LPR filter should fit into the front of the MAXfield. Of course, with the Starlight Xpress one-shot colour cameras (MX5-C, MX7-C) this is not a problem!

Exactly how are the 2" filters used with the MAXfield?

After attaching a mounting plate to your camera, the lens mount is secured to the camera mounting plate with setscrews. (These setscrews allow rotation of the camera relative to the telescope for composing an image.) This camera/MAXfield assembly can now be inserted into the telescope mount (or NGF-S focuser) and is held in place with thumbscrews. By holding the camera and loosening the thumbscrews, you can withdraw the camera and telecompressor to change filters. Thread a new filter onto the front of the lens mount and re-insert the whole assembly back into the telescope mount. The whole process takes about 30 seconds.

Changing filters sounds like a pain. How do you insure proper registration of the camera between filter changes?

When changing filters it is necessary to remove the camera and MAXfield from the telescope mount. A small registration pin protrudes from the telescope mount. By lining up a small hole on the flange of the MAXfield lens mount with this pin, you can easily register the two images between filter changes.

Sorry, but changing filters still sounds like a pain. Isn't there an easier way?

O.K., O.K.. Because of the great demand for filter wheel compatibility we've developed the Intelligent Filter Wheel. This is a five-filter wheel system which uses a highly repeatable stepper motor for accurate registration. The IFW uses 2" (well, actually 50mm) filters and is fully compatible with the SBIG CFW-8 filter wheel. The 5-position filter wheel can be quickly and easily removed and replaced with a second or third filter wheel. The neat thing about this setup is that you don't have to remove the camera or telecompressor to change filter wheels. Swapping out filter wheels takes about 30 seconds and doesn't even require re-focusing (unless you use filters of different thicknesses). Check out the IFW.

Which filters are recommended?

We recommend that a pale yellow (#12) filter be used to remove an undesirable chromatic difference. The #12 Yellow cuts off all light below about 480nm or so to completely eliminate this problem. Remember that the MAXfield design was optimized for the peak spectral response of the CCD sensor - from about 550nm to 850nm with additional correction into the near infrared. The trade-off was the color correction below 500nm.

What exactly does this chromatic difference look like?

The image of NGC 6888 below left was taken without a filter and shows the chromatic difference in the edge stars. The image at right was the same exposure length with a #12 Yellow filter in place. Look carefully at the stars near the corners of each image to get a feel for the differences between the filtered and unfiltered images. Remember that the chromatic difference is more apparent in hot, blue stars.

Unfiltered image of NGC 6888.(left) --- The same image with #12 Yellow filter in place.(right)

Each image above was 30 second exposure using an ST-6 camera mounted on a 10" Meade SCT. Optical design software shows the chromatic difference in the series of images below. The leftmost image shows the blur spot of a star at infinity directly on-axis. The middle image shows the same star at about 70% (4mm off-axis) of the full field and the rightmost image shows the star at the edge of field (5.5mm off-axis). Note the scale is in millimeters (right-click for full resolution). As you can see in the off-axis images, most of the blue light (435 nm) is focused farther from the axis than the rest of the star's light. This blue light shows up in the actual unfiltered image above, but not in the image filtered with a #12 Yellow.

A screen shot is shown below comparing the two images with a close up of the stars in the lower right corner.

You've talked about "blur-spots". What the heck is a blur spot anyway?: Blur spot analysis is a modeling technique which uses optical ray tracing to determine the size and shape of the real image of an infinitesimally small point of light after passing through a lens system. Blur spot analysis allows an optical engineer to analyze a given optical system at a variety of wavelengths. The blur spot images shown below were generated with the SCT/MAXfield optical system.

Blur spots of the MAXfield telecompressor system (left to right: On-axis, 70% field, edge-of-field.) In these blur spot diagrams, blue represents blue light at 436nm, cyan represents 480nm, green represents green light at 546nm, red represents red light at 656nm, and magenta represents infrared light at about 852nm.

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