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The DG Filter series is so easy to install and use, when used on supported cameras and configurations, the only thing necessary is to explain a few general things that relate to photographing spectrums using a DG Filter. If you don't find the help you need here or on our FAQ page then go to our contact page and email us your question.

Q1: What is a Supported Camera and Configuration:

A: It is one listed on our products page. For cameras like the SBIG and Starlight Express there are specific configurations that are supported. These are shown on our CCD page and our FAQ Page.

Q2: Do I need to do anything Special to use a DG Filter?

A: 1. You need to make sure it fits in your camera without hitting anything it shouldn't. See Below.

2. You need to adjust the orientation of the DG Filter to match the pixel alignment for your camera. See Below.

3. You need to Find your First Spectrum. See Below.

4. You need to Focus on the Spectrum not the Star. See Below.

Q3: How do I Install a DG Filter on my Camera?

A: First, make sure it fits into your camera without hitting anything like optical windows, etc. If it hits something do not force it (except as noted for the Type A units in the Type A User Manual).

If its a Type A DG Filter you just put the grating end down into the empty lens well of your LPI camera. Make sure that the filter doesn't fall out if the camera is held upside down (with the lens well facing downward). If it does then you will need to add some more friction between it and the lens well wall. This can be accomplished by putting a piece of a small wooden toothpick between it and the lens well wall. The Colored dot on a Type A - DG Filter is on the opposite end to where the grating is located. The end with the Colored dot (the dot in this picture is Yellow - indicating it is a Compact Spectrum Filter) should be sticking slightly out of the LPI camera if it is inserted correctly.

If you have a Type B DG Filter then it should screw onto your camera. The Type B - DG Filter screws onto your camera's standard 1.25" filter threads. The long nose, with the grating on the end, goes down inside the camera's empty lens well. Refer to the picture on the Right.

If you have a Type D DG Filter it should fit in your filter tray/wheel without doing anything else. It doesn't matter which way the Type D DG Filter is facing when installed.

The camera in the last picture is Celestron's NexImage CCD camera. It uses a Type D - DG Filter. To Install a Type D - DG Filter on a NexImage camera you need to first take off the empty lens housing.

Q4: How do I Adjust the DG Filter to my camera?

A: If you have a Compact Grating unit (A1, B1-L, B1-S, or D1) there will be a Yellow dot somewhere on the unit depending on which unit you have. You need to position the DG Filter on your camera so that the Yellow dot is in a position that is perpendicular to the longest axis of your CCD chip. If you have a High Resolution unit (A2, B2-L, B2-S, or D2) there will be a Red dot somewhere on the unit depending on which unit you have. You need to position this DG Filter using the same procedure as set out above for the Yellow dot.

Here, the Colored dot (the dot is Yellow in this picture) is seen to be facing the top of the camera. This is because the horizontal axis of the LPI' CCD chip lies across the camera from side to side, so when the Colored dot is facing the top of the camera it is perpendicular to the LPI' CCD chip's horizontal axis. Refer to the picture on the Left. A White dot was painted on the top of the lens housing after the chip orientation was determined. This way it can be easily seen at night when attaching and orientating the DG Filter to the camera.

The Type A DG Filters are generally just twisted with the hand until its orientated with the Yellow dot in an upright position with respect to the LPI's body.

On Type B DG Filters you can do this by loosening the retaining ring a slight bit and twisting the filter slightly to get the proper alignment, then retighten the retaining ring. You can use a pair of very small tipped needle nose pliers to fit into the two ring slots. The retaining ring has a standard right-hand thread so it unscrews in the counter-clockwise direction. Be sure to tighten the ring after you are finished or your adjustment will be lost.

In the picture to the Left, the Colored dot (the dot is Yellow in this picture) is seen to be facing the side of the camera. This is because the horizontal axis of the DSI' CCD chip lies across the camera from top to bottom, so when the Colored dot is facing either side of the camera it is perpendicular to the DSI' CCD chip's horizontal axis. A White dot was painted on the side of the camera's case after the chip orientation was determined. This way it can be easily seen at night when attaching and orientating the DG Filter to the camera.

If you have a Type D DG Filter then you should be able to easily reposition/turn it in the filter tray/wheel to get the proper orientation.

The last picture on the Left shows a Type D1 - DG Filter mounted on a Celestron NexImage CCD camera. The NexImage camera has the Type D Filter mounted right above the CCD chip. The empty lens housing needs to be removed to screw the Type D Filter into place, then the lens housing is screwed onto the top of the DG Filter. The Colored dot is seen to be facing the top of the camera. This is because the horizontal axis of the NexImage' CCD chip lies across the camera from side to side, so when the Colored dot is facing the top of the camera it is perpendicular to the CCD chip's horizontal axis.

Q5: How do I Find the Spectrum?

A1: Finding the Spectrum the First time with a Compact Grating Unit (A1, B1-, D1):

If you have installed the DG Filter on a supported camera in a supported configuration, and have the filter adjusted for that camera, when you have a bright star on the monitor move the scope's drive to position the star very close to one side of the monitor and you should see its spectrum spreading out like a long horizontal line pointing toward the other side of the monitor.

If you are using a color camera like Meade's LPI camera you will see a vivid colorful spectrum (if the star is not too dim - for the first time use a bright star until you know your Yellow dot alignment is correct). You will have to over-expose the star to see its spectrum as the spectrum is a lot dimmer than the star's central image.

If you have a SBIG camera or other CCD camera that only takes Black and White images then you will only see a white streak with no color. In order to see any color you will need to take three different pictures using three special color filters and then process the image before you will see any color. An exception to this is Meade's DSI camera. It is a color camera but for exposures longer than 0.5 seconds it washes out the colors in a spectrum. For the DSI camera and exposures over 0.5 seconds you need to take B/W pictures. VSpec will only work with B/W images anyway. You do not need color spectrum images to find and identify spectral lines or do Red Shift measurements.

The picture on the Left is a Compact Spectrum taken with a Color CCD camera, while the picture on the Right is a Compact Spectrum taken with a B/W CCD camera. For more detail read the related FAQ question.

A2: Finding the Spectrum the First time with a High Resolution Grating Unit (A2, B2-, D2):

The only difference between the Compact unit and the High Resolution unit is the width of the spectrum produced. Most everything mentioned above applies to the High Resolution units.

Adjusting the orientation (the Red dot position) is extremely critical when using the High Resolution unit. You need to be within 10 or so degrees of getting the Red dot aligned with the longest axis of the CCD chip. You can't spend too much time getting this correct.

Attach the DG Filter to your camera (after you have it horizontally adjusted) and find a bright star <1 magnitude. Center this star on your monitor. Then increase the exposure until the star's image is way over exposed then slowly move the scope's drive till the star slowly moves to one side of the monitor (it doesn't matter which side of the monitor you choose).

As the star's over-exposed image gets close to one side the Blue end of its spectrum should be just visible on the other side of the monitor, very near the edge/side of the monitor. Continue to move the drive forcing the star off of the monitor and you should see more of the spectrum coming into view on the other side of the monitor. If you have a B/W camera then you will see a white streak, if you have a color camera you will see a colorful streak.

For stars that are so dim you can't see their spectrum on the monitor this is a problem. Generally spectrums for stars too dim to show their spectrums on the monitor can be brought out during image processing as long as you don't go more than one or two magnitudes dimmer than can be seen on the monitor.

The picture on the Left is a High Resolution Spectrum taken with a Color CCD camera, while the picture on the Right is a High Resolution Spectrum taken with a B/W CCD camera.

Q6: How do I Focus the Spectrum?

A1: For a Reflector focus on the spectrum. You want the spectrum as sharp as possible. This will cause the star's image to blur out as will the over-exposure condition but don't worry about that. The important thing is to get the spectrum as straight and thin as you can. This way the image contains the most information.

For a Refractor you also need to focus on the spectrum. But unlike a reflector the spectrum produced will not be a thin line (unless you are using a perfectly corrected small telescope with a long F/D ratio). To this end, with a Refractor, focus near the Blue end of the spectrum (the blue end is the end that is closest to the star's image). For more detail and a picture read the related FAQ question. This way most of your spectrum will be as close to a thin line as you can get with your scope. If your scope is poorly corrected you can buy a Baader "UV & IR Block" filter and an IR stop filter. This will cut the bandwidth down to just the visible (400 nm to 700 nm) which will help a great deal in making the spectrum a thin line.

A2: Refractors versus Reflectors Spectrums:

A Reflector (SCTs are a Reflector) produces a very thin straight spectrum line.

A Refractor (even a very good APO) will produce a fat pointed spectrum. This is due the extended CCD sensitivity in the UV and IR, where our eyes are not very sensitive. Look at Mike's image on our Web Site User's Page. He used a VERY Expensive APO to capture the spectrum of Vega. This image is a Really Great spectrum image. This type of image won't process well in VSpec but there is enough of it immediately visible to do Red Shift measurements and spectral line identification without using VSpec.

The picture on the Left was taken with a B/W CCD camera through an APO Refractor where the image was focused on the IR portion of the spectrum (the IR is to the right - away from the image of the star). The picture on the Right was taken with a B/W CCD camera through a SCT Reflector where the image was focused on the whole spectrum.

Q7: How can I use my DG Filter on more than One Camera?

A: A Type B DG Filter can generally only be adjusted for use on a single camera. While it can easily be used with different cameras the threads on the various cameras will start at a different location on each camera, even identical cameras like two LPI cameras. This is a function of the manufacturing process where the camera is made, it is not a short coming of the DG Filter but rather one from the camera manufacturer.

If you adjust your Type B DG Filter to your camera so that when its screwed tight onto your camera you get a straight horizontal spectrum, then you take the DG Filter off and screw it tight onto one of your other cameras, even an identical model, in most cases you will find that the spectrum produced will not be horizontal in the second camera. You need to match the DG Filter to one particular camera to get the best spectral images.

The picture on the Left was taken with an LPI camera and a Type B1-S DG Filter, where the DG Filter's Yellow dot was aligned to the horizontal chip orientation of the camera. The picture on the Right was taken with a different LPI camera (an identical camera from Meade) using the very same DG Filter used to take the picture on the Left (only here, the DG Filter alignment was not changed from what it was in the first picture). The difference is about 1/8 of a turn difference between where the thread starts in one of the LPI camera's versus the other LPI camera.

This is not an issue for the Type A DG Filters as they are positioned by friction and can be rotated very easily.

This is not an issue for most uses of a Type D DG Filter as they sit in a filter tray/wheel and can generally be rotated easily.