The 4 X 5 Camera

This camera has evolved over several years to meet my own needs and is now being offered to the public. In addition to providing a much larger field, this camera offers many advantages and some disadvantages when compared to the standard 35 mm, SLR. These issues are discussed in detail, at the bottom of this page.

The JSP ASTROCAMERA consists of a camera back to hold a 4 x 5 film holder at the focal plane of a telescope or astrograph. In the standard configuration, it has a 2" tube that fits into a 2" focuser. The tube is threaded at the end to accept filters, coma correctors and other accessories. Other options are available to accommodate larger focusers or interface to custom astrographs.

Top View with filmholder in place

The vacuum is applied through the quick disconnect fitting supplied with the film holder. The short latex rubber tube provides isolation from the telescope during connect and disconnect. The thumbscrews on either side hold the film holder firmly in place.

The Standard Camera

The focuser establishes the maximum field size and larger focusers can be accommodated as an option for a larger field. The camera can also be provided with no tube and a hole bored to interface to custom astrographs. An example of this application can be seen on the Astrograph page.

AstroCamera with AP 4" Field Flattener Adapter

This model is threaded at the bottom to accept the AstroPhysics 4 inch field flattener lens. The spacing to the film plane is as specified for this lens.

Focusing plate.

The Focus Plate as shipped has been pre-adjusted to the nominal film plane for the typical filmholder. Several very short test exposures at, above and below focus can be made to determine best focus. If an adjustment is needed, it can be made using the three set screws on the face of the Focus Plate. They are laid out in tripod fashion and only the single screw needs to be adjusted unless it is grossly out of focus. Usually, a half turn up or down is all that is ever needed and even this would be most unusual.

Vacuum Pump.

This pump is an aquarium air pump that has been re-engineered to provide a vacuum instead of air pressure. It includes 5 ft of latex rubber tubing and a quick disconnect that mates with the one on the film holder. The model shown is the 120 VAC version. It is also available in a battery operated model that runs on self-contained flashlight batteries.

GUIDING

Although a guidescope is the preferred approach to take full advantage of the large format, an off-axis guider can be used with the Astrocamera. For very small objects where field is not an issue, an OAG is usually a better choice. The standard tube length is cut for the Lumicon EasyGuider so that the film plane is parfocal with the guiding eyepiece.

However, as the EG was designed for use with a 35 mm camera, it puts the Astrocamera about 3" back from the "normal" focal plane. If the scope is set up for a 35 mm camera, it will work as is. If not, you will have to move the primary mirror forward far enough to reach the new focal plane. When used with a guide scope, the Astrocamera will focus in any position that an eyepiece will focus as the film plane is only 1/4" above the top of the focuser and no modification is required to the telescope.

Why a 4 X 5 Camera?

The most obvious advantage is the large format where only the telescope and discretion of the user limit the "useful" area. I put useful in quotes because even severely comatic edges of the field have some use in astrometrics and charting. When used on the 8" F5, the image scale is very nearly identical to Uranometria charts. This makes it very convenient to lay a negative on a chart page and slide and rotate it till an exact fit is found. Interlopers can be found quite readily this way in addition to making it very easy to plot using the Uranometria charts and scales.

FULL FIELD VIEW

The field is just short of 3 degrees centered on M57.

The other very nice advantage is the immediate gratification of being able to develop the negative within minutes of exposing and not have to sacrifice more than one sheet of film.

The camera is the ultimate in simplicity, with nothing to break and weighs only a fraction of what a typical 35mm camera weighs, even with the film holder in place.

The film plane is in exactly the same place as it is for normal eyepieces so no special consideration is necessary when mounting the focuser or buying the scope.

One limitation of this camera is that is has no shutter. This is no problem for deep sky photography as any large pice of cardboard can be used in front of the scope as a shutter. It does however, limit lunar photography to exposures of a second or so.

Perhaps the most serious limitation is in its use for high resolution lunar and planetary photography. The 35 mm camera wins hands down because it is normal to shoot a whole roll of film to get that good one and this obviously is not very convenient with sheet film.

THE BOTTOM LINE

So what does it cost?

4 X 5 ASTROCAMERA..................................... $155
AP Field Flattener Adapter .......................................90

Vacuum Film Holder.................................................. 65 (with camera purchase, $100 seperately)
Focusing Plate ........................................................... 85
Vacuum Pump, 110 VAC........................................... 50
Vacuum Pump, Battery Operated.................................75

Notes:

You can use standard filmholders without vacuum for experimentation
but results will be random.

If you have a vacuum pump or want to canabalize an aquarium pump yourself,
you can save some money also.

For those who have been discouraged from using a guidescope, I offer the following words of wisdom.

GUIDE SCOPE GUIDELINES

1. The most fundamental mistake is to attempt to mount the guide scope directly to the primary tube. Unless your scope is built like the Palomar Schmidt, it is doomed to failure. A guide scope is just too heavy and will bend the main tube in all sorts of random ways when in use.

The most convenient way to mount the guide scope is with a set of adjustable rings. These must however, be attached to the mounting in some direct way that will not influence or be influenced by the primary telescope.

2. Unfortunately, one of the most popular methods of attaching a scope to the saddle mount is by the use of straps of one sort or another. I would nominate this for the number two position of barriers to successful guide scope use. They are not only an unsatisfactory method of securing the primary but provide no stable platform for the guide scope. The only way I know of to deal with these is to replace them with solid cast aluminum saddle rings. These are available from several suppliers or can be built by the more intrepid ATMer.

The rings made by Parallax provide the same flat surface on both sides of the ring for tapped holes. One side mounts to the saddle and the opposite side is where the guide scope rings are mounted. The spacing between the rings is also important. Most commercial scopes I see advertised, have them too close together to provide a stable base. I would suggest the minimum spacing be about 1/3 the length of the primary tube. Just keep the length of the guide scope in mind also.

3. A stable focuser is next on the list of absolute requirements. The one on the guide scope is no less important than the one on the primary. This is particularly true if you intend to use an autoguider. The additional weight will attempt to move the focuser and will stress the tube if it is not very rigid. They do not have to be expensive but they must not move after setup is complete. The inexpensive rack and pinion one offered by Univ Optics is quite satisfactory for use in the guide scope. It's a bit crude for visual use but it is very tight and well suited for this application.

4. The tube is the connection between the focusers and the primary and if they are not rigid and well designed, nothing else will help no matter what method of guiding is used. I highly recommend the use of aluminum tubing for scopes and irrigation pipe provides a high quality, low cost material for scope building or re-building. My experience is limited to aluminum tubes for the guide scopes and my 8" has a fiberglass tube from the 60's and the new 10" has an aluminum tube. I have my doubts about paper tubes and if all else has been tried, this could be a cause of grief.

5. A great many words have been written about the evils of mirror flopping in reflectors that preclude their use in guide scopes but I think it is just an excuse for not solving the other problems. If one could not solve the problem in the guidescope, we would still have to deal with it in the primary. I have never seen this problem in my systems but the primary in the guide scope is silicone glued in place and this may be the best way to totally eliminate the movement of the mirror.

The primary in my 8" is also glued to its 3 pt cell but my 10" is free to flop around all it wants and I see no difference in the end result.

Finally, the guide scope itself is probably the least critical element in the system as long as all the above rules have been followed. The most bang for the buck is a small Newtonian and I have been using a 4 1/4" F 7 with excellent results. If you don't have one, they are cheap to buy or would make a nice weekend project to build. If you have never made a mirror, here is a chance to join the club the easy way. If it is to be a dedicated guide scope, it doesn't even have to be parabolized. You would have a hard time making one that would produce images as bad as the best off-axis guider.

My first one was an F13 and I only had random luck with it and concluded that bigger is not necessarily better. It was just too long to be stable and so I refigured it to F7 (another weekend project) and have never had a problem since.

The problems with refractors are that one big enough to be useful would be very expensive and small ones are not rigid enough and the dark images are difficult to guide on.

It goes without saying that a good stable equitorial mount with dual axis correction is a must for serious astrophotography.

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