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Optec MAXfield 0.33X Three Element Telecompressor FAQ (Frequently Asked Questions) Page

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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 is0.33x, 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 whichadapts the lens system to a particular camera. The picture below shows the three parts ofthe MAXfield telecompressor system.

In some cases the telescope mount is not needed. Since the lens system will fit into any2-inch focuser, you may be able to order the MAXfield 2"(our stock no. 17402)and save a little money.

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

The MAXfield will work fine with an f/11 C-14SCT provided the telescope has enough back-focus. Of course at 0.33x, an f/11 systembecomes f/3.6. Any SCT with an f-ratio of 10 or higher should work fine. Takahashi has anf/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 [see Optec's Image Gallery], but has not been tested on aMaksutov cassegrain. The design corrects for the coma of an SCT which tends to greaterthan a Maksutov (which is essentially coma-free).

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

Our calculations indicate that the requiredback focus is about 195mm. There is a fairly easy way to check this distance with your owntelescope. First remove the visual back and place a blank white card 195mm (about 75/8") behind the last thread of the telescope's threaded mount. If you can achievefocus 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/6.3telecompressor to get an incredible f/2 system?!?

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

How does the MAXfield attach to the back ofthe telescope?

When the standard MAXfield isspecified (our stock no. 17400) the MAXfield is shipped with a telescope mount whichthreads directly onto the rear cell of the SCT. The MAXfield lens assembly will fitdirectly into a 2" focuser such as the motorized NGF-S by JMI. In fact, owners of theNGF-S can save a few dollars by ordering the MAXfield 2" (our stock no. 17402)which does not include the telescope mount.

Explain the differencebetween the MAXfield 2" and the standard MAXfield again.

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

Right-click to view at full resolution.

How much vignetting will there be using theMAXfield?

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

Is vignetting a problem?

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

Why are different mounting plates needed fordifferent CCD cameras?

Each CCD camera has a different specificationfor important parameters such as optical distance to the CCD chip and camerafaceplate mount. Even different models from the same manufacturer may place the CCD chipat 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 theCCD?

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

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

Again, the reason is that the opticaldistance to the CCD varies with different CCD cameras. This difference in spacing canusually be accommodated for by simply re-focusing at f/10. However, at f/3.3 focusing iscritical. Moving the CCD chip as little as 1mm can degrade the edge sharpness. The imagesbelow demonstrate how critical the focusing can be. The image on the left was made at theoptimal 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 just1mm. (right)

The images above were taken by Dennis di Cicco for his review of the MAXfield in the Fall1995 issue of CCD Astronomymagazine. Reprints of this review are available by mail from Optec .

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

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

What if there isn't a mounting plate tosupport my camera?

Call us or send an email with your camera details. We're constantlystriving to add new mounts to our list of supported cameras.

How does the MAXfieldperform with the smaller pixel cameras?

The two images of M13 below were taken thesame night using the MAXfield and our 10" LX-200. The image on the left was takenwith the ST-6 and the image on the right was taken with the ST-5. Each is a 30 secondexposure and each has been scaled logarithmically. Note the difference in plate scale dueto the physical size of the CCD's used in each camera.

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M13 image taken with the MAXfield and ST-6 camera. (left) --- M13 image taken with theMAXfield and ST-5 camera.(right)

The M13 images above are shown at a 1:1 scaling. That is, each pixel from the CCD isrepresented by one pixel on your monitor. The following table outlines the differencesbetween the two cameras. As you can see, the CCD used in the ST-5 camera is only about1/3rd the size of the ST-6 CCD, yet still has nearly the same number of pixels. Of course,each pixel is correspondingly smaller.

The KAF-0400 CCD chip used in the ST-7 camera has 9 x 9 micron pixels and is capable of 2x 2 and 3 x 3 binning. Results with the MAXfield using the ST-7 would be similar to theST-5 image when not binned and similar to the ST-6 image when binned 3 x 3. Refer to theWinter 1995 issue of CCD Astronomy for an excellent discussion of pixelsize, signal-to-noise ratios, and "Optimizing a CCD Imaging System" [page 14].

So, which CCD chip size isreally better, big pixels or little pixels?

Ah, the Great Pixel Debate. This is ahotly debated topic among many user groups and mailing lists. Without going too deeplyinto the subject, we'd like to offer the images below (which are really just scaledcomparisons of the images above) to show that the MAXfield will perform equally well withsmall pixels or big pixels (though we prefer the image on the right.) You make your ownjudgement.

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Scaled and cropped ST-6 image using MAXfield. (left) --- Comparison ST-5 image takenwith the MAXfield.(right)

The images above are slightly out of rotational alignment (with respect to each other) andthe logarithmic scaling may not have been identical. To be fair to the large pixel cameraswe really ought to provide another comparison set of, say, a faint nebula using the sameexposure time and identical processing. Theory says the ST-6 would provide a better imagein that case. (But heck, we still like the image on the right.)

Are there any cameras that cannot be usedwith the MAXfield?

Cameras using CCD chips larger than 11mm(diagonal measurement) would certainly be vignetted. The CompuScope (ISIS, Inc.) camerasinclude a built-in filter wheel that doesn't allow enough back-focus for the MAXfield.We're presently working on additional camera mounts and always appreciate any customerfeedback.

Speaking of filters, can filter wheels beused with the MAXfield?

Unfortunately, with the limited back focusthere 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 suppliesfilters for tri-color work, but any 48mm photographic or deepsky/LPR filter should fitinto the front of the MAXfield.

Exactly how are the 2" filters usedwith the MAXfield?

After attaching a mounting plate to yourcamera, the lens mount is secured to the camera mounting plate with setscrews. (Thesesetscrews 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-Sfocuser) and is held in place with thumbscrews. By holding the camera and loosening thethumbscrews, you can withdraw the camera and telecompressor to change filters. Thread anew filter onto the front of the lens mount and re-insert the whole assembly back into thetelescope mount. The whole process takes about 30 seconds.

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

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

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

O.K., O.K.. Because of the great demand forfilter wheel compatibility we've developed the MAXfilter 2". This is a threefilter position system which uses a highly repeatable stepper motor for accurateregistration. The MAXfilter 2" uses 2" (well, actually 50mm) filters andis fully compatible with the SBIG CFW-8 filter wheel, except that it has only threefilters. To get around this limitation the 3-position filter slider can be quickly andeasily removed and replaced with a second or third filter slider. The neat thing aboutthis setup is that you don't have to remove the camera or telecompressor to change filterbars (or sliders as we like to call them). Swapping out filter sliders takes about 30seconds and doesn't even require re-focusing (unless you use filters of differentthicknesses). Check out the MAXfilter 2" page at http://www.optecinc.com/astronomy/products/maxfilter.html.

Which filters are recommended?

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

What exactly does thischromatic difference look like?

The image of NGC 6888 below left was takenwithout a filter and shows the chromatic difference in the edge stars. The image at rightwas the same exposure length with a #12 Yellow filter in place. Look carefully at thestars near the corners of each image to get a feel for the differences between thefiltered and unfiltered images. Remember that the chromatic difference is more apparent inhot, blue stars.

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Unfiltered image of NGC 6888.(left) --- The same image with #12 Yellow filter inplace (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 imagesbelow. The leftmost image shows the blur spot of a star at infinity directly on-axis. Themiddle image shows the same star at about 70% (4mm off-axis) of the full field and therightmost image shows the star at the edge of field (5.5mm off-axis). Note the scale is inmillimeters (right-click for full resolution). As you can see in the off-axis images, mostof the blue light (435 nm) is focused farther from the axis than the rest of the star'slight. This blue light shows up in the actual unfiltered image above, but not in the imagefiltered with a #12 Yellow.

These images were all processed using SkyPro (now called CCDSoft) by Software Bisque. A SkyPro screenshot is shown above comparing the two images above with a close up of the stars in thelower right corner.

You've talked about "blur-spots".What the heck is a blur spot anyway?

Blur spot analysis is a modeling techniquewhich uses optical ray tracing to determine the size and shape of the real image of aninfinitesimally small point of light after passing through a lens system. Blur spotanalysis allows an optical engineer to analyze a given optical system at a variety ofwavelengths. The blur spot images shown below were generated with the SCT/MAXfield opticalsystem.

 
Blur spots of theMAXfield telecompressor system (left to right: On-axis, 70% field, edge-of-field.) Rightclick with most viewers to see full resolution.

In these blur spot diagrams, blue represents blue light at 436nm, cyanrepresents 480nm, green represents green light at 546nm, red represents redlight at 656nm, and magenta represents infrared light at about 852nm.

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