Astrophotography

Newtonian Telescope

The style of this diagram is taken from "Telescope Optics", by  Harrie Rutten and Martin van Venrooij.

The Newtonian is attractive as it is virtually always cheaper than a Cassegrain.
Unfortunately, there is a major drawback in using a Newtonian telescope for astrophotography - especially for  telescopes with faster focal ratios wanting to use the larger film sizes. The Newtonian suffers from an aberration called "coma".

While a parabolic mirror happily  focuses all on-axis rays to a single small point at the focal point, it does not do so for rays that are entering at a slight angle. These rays form an image at a short distance away from the axis, but this image is not a single small point, it is a shape similar to a comet with a tail, leading to the name of "coma" for this aberration.

 

The further off-axis the incoming rays, the further away from the axis the image is formed and the more bloated is the comet image. This bloating is often significant for simple astrophotography with Newtonians.

 

There is a minimum resolution that film is capable of giving. If the bloating due to coma is always kept less than the minimum resolution of the film, - (generally taken as 0.025mm), - then it is perfectly acceptable since it cannot be seen. From the design page you will have seen that a field of 44mm is needed to fully illuminate a 35mm Negative, (36 x 24mm), and a field of 100mm to illuminate medium format, (taken to be 70 x 70mm). This relates to 22mm off axis each side for a 35mm negative and 50mm for medium format.

The diagram adjacent shows the coma from a 200mm mirror.

 

It shows two circles representing 0.025mm and 0.1mm. The smaller size circle of 0.025mm is the smallest resolution that film is capable of catching. As long as the coma spread is less than this value, you will get photographs as clear as the film allows, however it may surprise you to find that that extremely slow focal ratios of around F/15 are necessary to achieve this high standard at the edge of a 44mm field.

If you want top quality photographs right up to the edge of the film, this cannot be avoided. Note that the coma diagram for the 200mm mirror has figures of 12mm and 18mm off axis, which relate to fields of 24 and 36mm respectively, the width and length of a 35mm negative.

However, as usual, there is some compromise possible. Most of the energy in the coma spread is close to the point of the coma. The remainder is extremely faint and is unlikely to be registered by most film emulsions. The rule of thumb proposed by Rutten and van Venrooij is that the maximum length of the coma spread can be up to 0.1mm at the edge of the film, and still give adequate resolution. This means faster focal ratios such as F/8 become usable for a full 35mm negative (this is where our suggestion that you might want to stick with F/8 comes from).

The adjacent graph shows amount of coma for different focal ratios together with the two photographic film limits of 0.025 and 0.1mm and the 22mm semi field of a fully illuminated 35mm negative.

 

If you do want to use a focal ratio faster than that suggested in the adjacent table, then you still have several options:

 

You can simply accept that objects towards the edge of the field may be blurred, knowing that objects towards the centre of the field will be sharp and usable. You may find that you can live with a usable field of say 24mm or 36mm instead of 44mm and please be reminded again, if you have a 2" focuser, it will not fully illuminate the full 44mm field anyway.

 

If you already have a telescope, a 35mm camera, and a cheap means of connecting them - then please do not let this page put you off trying a few photographs with the equipment you have. You are reminded that film and developing costs are cheap and you may be perfectly happy with the results.

If you do decide you want to get under the 0.1mm limit with a mirror faster than F8, or under the 0.025mm limit with a mirror faster than F/15, and still have a large field, then another alternative is a device known as a "Coma Corrector". This is a lens arrangement that is placed in the optical path and removes or reduces coma. It can be placed close to the camera or mounted with the elliptical flat. It will affect the design and may affect the focal length of the telescope.

 

If you intend to build a telescope using one of these devices, you will have to know the effect it is going to have before you begin design. You will have to confirm that it is intended for the particular size and focal ratio of the primary mirror, what distance from the film it has to be placed, and what effect it will have on the focal length of the system.

 

One such widely advertised device is the Tele-Vue Paracorr Coma Corrector. This is said to reduce coma by six times for focal lengths between F/3.5 and F/8. This device connects directly between a 2" focuser and a 35mm camera. If you are interested in this unit, then check your 35mm camera can fit to the unit so as to keep the face of the nearest lens at 55mm from the film surface. Note we have never seen one of these units close up, but it is said to have a clear aperture of around 42mm and if so, this unit will give a fully illuminated field of around 30mm with an F/5 mirror.

 

Another commercial unit is the Lumicon Coma Corrector. This is described as useful for F/4-F/5 systems, and is said to be 48mm clear aperture. Although this is bigger than the Paracorr, this corrector needs to be sited further away at about 90.5mm from the film. It will therefore still only give a fully illuminated field of about 30mm for a F/5 system. Again you should check your focuser and camera will adapt to the unit. Note that as the focal ratio increases, the fully illuminated field increases faster than the Paracorr, but only to about 36mm at F/8.

 

If you are considering using a Coma Corrector, - You can use the Design page and spreadsheet for focuser field of view to calculate the approximate field of view available. On the same page, you will see that these typical figures of 30-32mm aperture roughly correspond with the illumination level at the edge of a 44mm field reducing to about 70%, so they are still very usable with a 35mm camera.

 

Oldham Optical can supply the optics for larger coma correctors, but the minimum arrangement is always at least two lenses to combat most colour aberration. This means four surfaces to figure. Although physically small by primary mirror standards, - the corrector will be expensive to produce. It can cost more than the main Newtonian optics themselves, - and less than choosing a Cassegrain telescope to start with.

 

For film, the film grain provides the limiting factor of resolution at about 0.025mm (or 0.1mm depending on your interpretation.) For digital cameras it is the size of the individual pixels in the CCD array. A 6 mega pixel camera typically has pixels of about 0.008mm, which is already a lot smaller than the resolution of film and about equivalent to the Airy disc diameter of a F/6 telescope.

 

So for a performance on axis that matches the resolution of the CCD, you need a telescope of F/6 or faster. But as you go off-axis, then increasing coma means you really want a slower telescope! To get equivalent performance at the edge of a 28mm diagonal CCD sensor theoretically needs a telescope of about F/16, - which is obviously unrealistic.

 

Not enough experience has been gained yet to assume that the Rutten and van Venrooij rule of thumb of  four times increase from 0.025 to 0.1mm for film is still appropriate for CCD's. This would give 0.032mm and about F/12.

 

Perhaps the real limit is determined by what size you intend the final CCD images to be viewed at? A normal 6MP image would happily print at about A3 size without seeing the pixels, so if you are expecting a bit of blurring at the edges, you could reduce to A4 size - which is still BIG!

 

You will have gathered by now that obtaining a wide enough field for CCD & 35mm film might be difficult with a Newtonian telescope. If you were thinking of medium format, then you must use a specially made and very expensive coma corrector! You might perhaps think about a Cassegrain telescope as a suitable alternative?

Cassegrain Telescope

After reading the Newtonian section, you will be happy to learn that if you have a Cassegrain telescope with Ritchey-Chretien figuring, then coma is effectively removed as a problem. This means the field of a Cassegrain telescope can be much bigger before other aberrations begin to intervene. Achieving a 44mm field for a 35mm negative is easy in comparison and 100mm for medium format becomes achievable.

 

As the field size increases, the next aberration most likely to affect Cassegrains is curvature of the field. The radius of field curvature for Cassegrains is one of the figures output by the spreadsheet available on the design page. Since normal camera film is flat, the effect of the curvature is to de-focus the edges of the picture when the middle is in good focus.

 

This can be partially compensated for by adjusting the position of good focus to half way between the middle and the edge of the picture. As long as the de-focussing effect is still less than the film resolution, - (0.025mm or 0.1mm limit as appropriate) - then the photographs will be perfectly acceptable.

We do produce 20" F/8.2 Cassegrain optics designed to give a 100mm fully illuminated field for medium format film photography without any other correctors or flatteners, but It does require to be focused as described above.

 

 

For our 20" F/8.2, the actual figure for the blur is about 0.066mm without any compensation, reducing to about 0.033mm when focus is shifted to compensate. This is slightly over the 0.025mm limit at the edge of the field, but well within the 0.1mm limit.

 

The optics of this telescope obviously make a superb telescope for 35mm film cameras, where the uncompensated spot size is about 0.012mm at the edge of the field.

 

We also can also produce a 20" F/12 Cassegrain set which has greater magnification and a lower obstruction ratio while still retaining a field wide enough for 35mm or CCD photography.

However if you are using a DLSR with the pixel size down to or even less than the Airy Disc, then even this field curvature may be a problem. First - it is perfectly possible to design a Cassegrain system with no field curvature (better described as an infinite radius of curvature). Consult the design page for details, but the two drawbacks are the increased length of the telescope and higher obstruction ratio of around 50%. It usually is not worth it!

 

More usefully - Like the "Coma Corrector" suggested for a Newtonian, it is also possible to add a device to the optical path of a Cassegrain called a "Field Flattener". If this is a film camera and it can can be placed close to the film surface, then often only one lens is needed, but if a DSLR camera or similar is being used the Field Flattener must be set at some distance from the sensor. Then at least two lenses are required to correct for colour.

 

Oldham Optical can supply an optical system comprising the two Cassegrain mirrors and a field flattener system. We are calling it a "Ritchey Chretien Flat Field".

 

However it is not really a Ritchey Chretien! That name means a particular curve is used on the mirrors. To work correctly with the field flattener the curves are slightly modified. However there is not really anything else sensible to call the combination of Ritchey Chretien type coma removal that also offers a flat field. Hence its "Ritchey Chretien Flat Field" until some better name is developed.  

 

The field flattener has a small effect on the focal length of the Cassegrain system. But not sufficient that the Cassegrain page in the design spreadsheet cannot be used to fix sizes. The Field Flattener lens system is  fitted close to the primary mirror. It may be behind the mirror (as illustrated), or it can be mounted in front of the primary on the baffle found on a Cassegrain primary.

 Astrophotography with Newtonian or Cassegrains is limited to fairly narrow angles of say up to 2 degrees. What can you do if you want to take really wide angle pictures? You may consider a Schmidt Camera. 

 

Normally we would only recommend one if you already have a fairly large telescope or have some specific need to take detailed wide angle pictures. While virtually any telescope can be used to take pictures, a Schmidt camera cannot be used as a telescope, it's only use is for taking wide angle pictures. It uses the same design rule as for the other telescopes - the angle of view is directly related to the focal length of the system and the size of the film used.

 

However spherical mirrors faster than F/2 are easy to make, - we can manage F/1 at 300mm without any great problems although the curved field at this focal length may be a problem for the film carrier, - this would give about 8 degrees with 35mm sized film plates. We should point out that a 300mm lens on a 35mm Camera would give the same view, (albeit with smaller aperture), for a lot less money.

Companar Camera

We have made sets of these optics since early 2005. The manufacture of the second corrector plate is special.

 

This camera is a development of a Maksutov. It uses the two spherical mirrors and the meniscus lens corrector from a Maksutov, but differs in having a second corrector lens in advance of the Maksutov meniscus.

 

The benefit of the extra corrector is an extremely large and virtually flat field allowing near perfect pictures of up to say 6" diameter to be taken with a telescope of aperture only about 12". Unlike the Schmidt camera where the film is internal to the tube and must be bent to fit the curved field - this camera produces a flat field outside the camera where the film can be easily accessed.

 

It can theoretically be used as a telescope, but the fast focal ratio is not ideal for viewing with standard eyepieces and it is really optimised as a wide field camera. But the important point to note is that unlike the Schmidt, this camera can be aimed either using an eyepiece or a standard SLR camera can be attached. Although if its a SLR, its really only worth considering a medium format type - You would be able to obtain excellent results with a 35mm SLR from a Ritchey Chretien for less money!

 

In an ordinary Maksutov, the single meniscus lens introduces aberrations which become significant with wide fields. The second corrector is used to cancel out most of the colour aberration (effectively forming an achromatic corrector). Selection of the right glasses, spacing and curvatures gives the freedom to vary the radius of field curvature of the pair of correctors while still removing spherical aberration.

 

It only remains to match and balance out the remaining radius of curvature by adjusting the radii and separation of the two mirrors. The result is a superb camera with a potentially massive field.

Warning - A large field does result in some vignetting.

 

If you can afford it, this type of wide field camera may be the only one worth having. The big trick is making the aspheric second corrector and this is definitely not cheap or easy to do. Sorry - We aren't going to tell you how we make them!

Note that the field flattener part will theoretically introduce colour aberration. However its so low down that it cannot be noticed. If you can obtain a Cassegrain telescope that has no coma and a reasonably flat field then the next aberration that will become dominant is astigmatism, but by this time it will be so far down that it will not cause an issue for normal photography.

Schmidt Camera

If you are using 35mm film with a 2" focuser, then go to the Design page and spreadsheet for details of how to calculate the maximum available field from the focuser dimensions. You should also note the section on the same page discussing illumination levels caused by the restriction.

Prior to 2004, not many amateurs would have seriously considered digital photography, but with the introduction around then of relatively cheap digital SLR cameras (DSLR's), this area has suddenly become extremely attractive.

From 2004 a lot of these new DSLR's standardised on a CCD sensor of about 23mm x 15mm. This gives a diagonal or field size of about 28mm, and this is a very good fit for 100% illumination with a 2" focuser. Also at 2004 these cameras were typically offering 6 - 8 mega pixels in their sensor area. This could have been said to be about a practical limit for Astrophotography as the pixel size is already at about the Airy Disc size for an F/5 telescope.
However reality is that a camera offering a greater number of pixels will sell more cameras. By 2008 cameras with the same sensor area were offering 15M pixels. This equating to the Airy Disc of a F/3.5 telescope. 

 

Also, by 2007, some Digital SLR's with larger sensors up to the size of a 35mm film camera were becoming more affordable, and of course they have increasing numbers of pixels and are more affordable each year. As of 2013 this trend has continued.
 

If you are considering buying a DLSR for Astrophotography, then make sure you know the sensor area and understand what it means in terms of the angle of view you will get and the level of illumination given by the telescope focuser you want to use it with.
 

If you are thinking of buying a camera to try digital Astrophotography and you already have a telescope with a 2" focuser, then perhaps a digital SLR with 6-8 Mega pixels in the 23mm x 15mm format might be a good first investment? It should be fairly affordable to the average budget. It will get 100% illumination over its entire field from the 2" focuser - and it will take very good "ordinary" photographs as well!
 

The next thing to decide is what angle of view you want to capture on film with your camera. This is decided purely by two things, - the focal length of your system and the size of the film you are going to use. A benchmark that is useful in illustrating the angle is the view of the moon, which subtends an angle of 0.5 degrees.