RETROFIT DALL KIRKHAM 316 MM F 20 FROM Costruzioni Ottiche Zen

I’ve always been a keen Moon and planets observer, I started in 1980 with a Konus Alcor, a 114 mm Newton and then I replaced it with the glorious refractor Vixen 102M achromatic.

Then years later I bought a 150 mm f 10 oil-spaced apochromatic refractor from Zen, and a truss tube manufactured to specific requirements and then sold possibly because of a disagreement between the customer and the manufacturer who made the tube (the telescope was in Observatory). In fact the tube was really awful, when I first looked at it I nearly died. I remember that the diaphragms were cut with plumber’s tin snips, you can imagine the focuser and all the rest! Anyway, I accepted the offer knowing that I would have had to replace the truss tube but, twenty years later, I am satisfied owner of a fantastic apochromatic which has given me lots of enjoyment.

Living on a plain, far from lakes and mountains, the Seeing conditions are normally good enough for me to observe during most nights of the year and , for this reason, a couple of years ago I bought a 316 mm Dall Kirkham from Romano Zen with f 5 primary mirror and 6 mt focal length.

The tube is made of 3 mm thick calandered aluminium, whit 1350 mm length and 350 mm external diameter. Two rings, holding the breech and the secondary support, are positioned inside the tube at its two ends. The breech doesn’t have any cooling fan and there are only 15 mm between the tube and the boundary of the primary mirror.

The instrument was positioned in an outdoor shed however, because of the glass mass which could hardly achieve the thermostation and an inner turbulence, probably due to air columns inside the tube; I could only use it few nights a year.

The images supplied by this instrument are more wavering and less sharp than those of the refractor even during nights with a good Seeing. Although it is true that the bigger the telescope the more sensitive it is to the atmospheric turbulence which removes easily attainable details, the turbulence itself shouldn’t conceal details that even a smaller sized telescope would be able to achieve in similar conditions.

The questions was complicated and I didn’t know what I could do. During a web search I accidentally found the NortheK website, a company producing optics, mechanics and composite materials in the astronomic field, which I’d never heard of before in any of the various forums or astronomic magazines. After reading carefully all the technical topics discussed in their website I realized that I could have possibly found an answer to my problem. So, I called Massimo Boetto, technical advisor of the Maxproject Team, and since then we started a cooperation that became a friendship with both Massimo and his brother Mauro (high precision engineering designer).

They suggested assembling my Dall Kirkham optics with the truss tube UnitorK 35 and replacing the secondary mirror’ support (which was glued to the tube) with the model AxyS A1.

In the meantime, as I had the opportunity to work in an engineering workshop with the help of a turner friend, I decided to replace the cell and the primary baffle. In fact, the baffle was fixed directly onto the breech excluding any possibility of centering the primary mirror which was then blocked in tension whit a ring nut. The focuser, a 400 euro JMI Cryford, was screwed directly  onto the breech. Nothing extraordinary but there was a problem: among many advantages it had a weakness typical of most commercial focusers, between the body and the sleeve where the focuser would slide there was a gap, in my case, of nearly 0,5 mm! When the focuser is linked directly onto the breech the body is on an axis but the sleeve, where all the accessories are inserted, is 0,5 mm out of axis.

After making these modifications, the primary’s cell, its baffle and the focuser, have been all positioned inside a precise mechanical system and they can be individually adjusted. In this way, all the mechanical adjustments allow the preservation of the axis, the optical set can be adjusted with small movements of every element, included the focuser which is now connected to the telescope through a tilting flange.

Once the truss has been delivered with AxyS A1 support, I have assembled and adjusted all the mechanical components using a 300 mm centesimal dial gauge, added the optics and collimated the whole system. The instrument has now a total weight of 32 Kg. In assembly I could appreciate the making, the finishing and the material used for “naked” truss structure, a totally open serrurier with aluminium bars (the original project would use carbon fibre bars). The structure is 1335 mm long, 500 mm wide and 18 Kg in weight. Every component of the tube has been made with material fit for the purpose: Halo 25 alloy, stainless steel, and self-lubricant matrix technopolimers. I was pleased to notice that the strong Losmandy type bars (AW 361) ware included in the project (2).

The support for the secondary mirrors AxyS A1 is included in the UnitorK system. This is a very complex mechanical piece featuring a horizontal shifter with 0,02 mm accuracy, a double support made in Halo 25 alloy and a 3 spokes support which gives strength to the system. The optics is positioned onto a barrel featuring a rubber o-ring at the front to hold the system and some felt inside leaving the glass disc free to expand. The centring inside the barrel is done with side adjustments with a thermal expansion control system.

Because of its large focal length, this instrument is ideal for visual observations and big magnification imaging. The 32% obstruction is quite a high ratio but we shoudn’t forget that the diameter and the baffle length of the two mirrors have been designed to remove the side light which could get through the eyepiece field and therefore degrading the instrument performance (certainly more than what a lower ratio obstruction would have done).

In order to remove the reflection which generates diffuse light, the primary’s baffle has been made with seven especially designed cone light diaphragms, while secondary’s baffle has been made with a diaphragm that takes the light cone to 79 mm. In order to remove any faults on the primary’s raised edge; a turned aluminium ring has been positioned on it reducing the aperture to 310 mm.

One of the picture published below shows an unusual link between the  focuser and the binocular turret: it is a highly precise mechanical piece specifically designed to keep any accessory on axis and, inside it, there is a light cone diaphragm. This is an alternative proposal to the traditional 50.8 mm link systems which, more often than not, are made with excessive tolerances leaving accessories and eyepieces regularly out of axis.

Another very useful mechanical piece is the micrometric system positioned between the clamp and the Losmandy slide (AW 361). This piece helps the balance of the telescope in declination (as this system has only recently being designed it still needs some aesthetic improvements). How many of you would be able to balance the axes in declination on a German equatorial mount holding a 32 Kg telescope? Believe me, this accessory allows one person only to do the job easily and safety!


Star test.

The star test has been carried out observing a star high in the sky at 1150X during a good Seeing night, using a mirror diagonal with a Pentax XL 5.2 mm eyepiece. The mechanics allow me to collimate the optical train in a simple and precise way even with unbelievable magnification! The intra and extra-focal diffraction images are almost perfectly identical; no zonal error or residual spherical aberration were detected. Incredibly the atmospheric turbulence has disappeared and the star images are in focus and quite sharp so the aim has been achieved brilliantly, not only, during my observation night I never noticed the warm air column and heat wave distortion effects that I was sadly used to.


Although the conic primary mirror doesn’t normally have an even differential of thermal stabilization because of its thick centre and thin edges, the solution suggest by the NortheK technicians to leave the optics exposed to the air has been proved right.

In order to be able to use the Mark 5 binocular, I adjusted the back focus by simply turning the knob of the AxyS A1 horizontal shifter with 0,02 mm accuracy and I could observe Saturn with a couple of 18 mm Ortho Genuine eyepieces at 340 X.

To make sure that the mechanics could keep the axes in parallel and therefore preserve the collimation, I focused on the planet perfectly on the meridian side and I tried to overturn the telescope on the other side: incredibly Saturn was still perfectly in focus! The generous diameter of this instrument made it particularly good for moon observations. I used the Mark 5, which I can’t do without for my visual observations anymore, with 12.5 and 7 mm eyepieces Ortho Genuine (480X – 670X – 860X) and I could experience the tridimentional sensation that this accessory offers but, most of all, I noticed the total absence of diffused light. The instrument can now offer very detailed and contrasted images showing fine details such as Rilles, mountains, ridges and lots of small craters “invisible” before have now become visible….this time the challenge between the apochromatic and the reflector, unsurprisingly, has been won by the reflector!

See you soon!

The telescope on the top of the self-made German equatorial mount.

Link on the UnitorK 35 system. Please note the precision and the accuracy of the construction.

The AxyS A1 support seen from the secondary mirror, please note the absence of any reflection inside the baffle.

The micrometric system positioned between the clamps and the slide AW 361. Very useful to balance the axes in declination.

Unusual link between the focuser and the prismatic diagonal Zeiss.

The new baffle: please note the 8 mm diameter rubber o-ring holding the primary mirror.

Optical characteristics Technical Specifications
Optical scheme Dall Kirkham
Nominal optical opening 310 mm
Thickness of primary mirror glass 51 mm
Secondary mirror diameter 85 mm
Thickness of secondary mirror glass 15 mm
Glass of primary and secondary mirror Pyrex
Primary mirror focal lenght 1500 mm
Focal ratio f 5
Secondary mirror magnification factor 4x
Equivalent focal lenght 6000 mm
Diameter of the 100% illuminated field 79 mm
Fully illuminated field 18 mm
Back focus 320 mm
Actual obstruction including the mechanic 32%
Primary’s baffle diaphragms 7+1
Secondary mirror holden NortheK AxyS A1
Truss tube NortheK UnitorK 35 (alloy version)
Maximum diameter of the structure 500 mm
Maximum lenght of the structure, without accessories 1335 mm
Swallow-tailed/plate connection NortheK AW 361
Standard searcher 8×50
Focuser Crayford JMI EV1c
Focuser support Axial adjustable and tilting
Cell Axes and tilting adjustments
Primary mirror baffle Axes and tilting adjustments
Primary mirror weight 5 kg
Primary mirror cell weight including baffle 5,5 kg
Unitork 35 weight including + 2 plates AW 361 18 kg
AxyS A1 weight including secondary mirror 1.9 kg
Focuser EW1 weight including flanged link 0.9 kg
Optical tube weight without accessories 32 kg


Roberto Milan ®


Our customer Daniele Cipollina, who is a very experienced amateur astronomer, contacted us regarding some issues with his photographic instrument which never performed. After several unsuccessful attempts to resolve the problem, and considering the cost of the optical tube, our customer has decided to submit the matter to us.

In the trial image above (please disregard the light haloes which are not significant because it is a trial image), a consistent blur is noticeable on the lower right hand side. Moreover, although our espert customer has performed a RIGOROUS focusing with the Bathinov mask, the star puntiformity still remains inadequate for this class of telescope.

Despite the accurate collimation, the CCD Inspector picture immediately shows a serious distortion. with this condition it is practically impossible to get punctiform images: According to the supplier the cause of the problem is the seeing (!).


The image magnification shows a noticeable distortion on the right hand side of the field edge which results in a poor star punctiformity; this is particularly bad when compared to the telescope price.

The chromatic dominance is due to the meniscus (and to the turbolence) which was found significantly out of axis from its correct geometrical position.

Although less deformed, the quality at the center of the field is still poor as it is on the left hand side edge where an even worse chromatic dominance is noticeable.


Having dismantled  the telescope we could report to the customer the instrument’s conditions as shown below:

Please observe carefully the clamp holding the meniscus. The one marked with the number 1 is fixed and cannot be moved but the other two (positioned 120°) are free to move when they are touched and can be unscrewed. Here is the first clue: the tube edge is not orthogonal.


The meniscus is kept in position by a simple scissor cut cardboard.

Althoug the aluminium tube looks solid from the outside, from the inside it is a simple calandered metal sheet that has been and riveted toghether and the rivets on the outer surface have been ground down and covered with enamel. Two rings positioned at the tube’s ends hold the meniscus and the breech. Obviously the tube is not orthogonal and nor are the rings.

Here you can appreciate the overhelming mechanical approximation. The optical axes are endemically warped.


We judged that the main problem was caused by the axis not being orthogonal. We suggested  to the customer to start with a retrofit to achieve effective results avoiding too higher costs. For this reason the breech, the focuser and the corrector have not been changed but the Maxproject Team designed some modifications to be implemented. In this way the cost – still important – was kept under control using effective but not too sophisticated solutions.

The first step  has been to replace the calendered aluminium tube with a carbon tube. The reasons for doing this have been the following:

a) The customer wanted to keep the weight under 9 kg (final result 9,7 kg);

b) The use of carbon fiber avoids any thermal expansion of the tube and this is quite important for a                                photographic telescope;

c) It is possible to fix the setting of the rear cells and to use Epox adesive with 4 hours solidification time           directly on the bench;

d) The use of carbon fiber avoids any axial and radial deformation which otherwise will be very difficult to eliminate in such a large tube obtained from a thick extruded metal.

To further avoid any deflexion problem, the carbon tube has been over-dimensioned with a 265 mm internal diameter and 269 mm external diameter, more or less the same as the 10 mm thick aluminium solid tube. The outer surface has then been painted to give it an even more pleasant look. A 0,5 mm tolerance has been used to cut the tube but it is brought back to size when the cells and counter-cells have been fixed to it with epoxy resin/metal.

The picture above shows the fixed counter-cell on the right and side. This one is attached to the carbon tube with epoxy resin adhesive (you can see the holes that are use for bonding; other confidential details have been omitted): At the top of the ring it is possible to see the three supports for the meniscus and also the holes where the refrigerating air (then filtered) passes through (again, some details have been omitted). On the left and side there is the meniscus cell – with some omitted details – with aereation holes alla precision engineered to hold the meniscus accurately. At the top the picture there is a simple 2 mm thick pierced stainless steel cover also used to keep the meniscus in position.

This is a cell put in place before being anodized. During the assembly all the tolerances have been checked considering the anodization process. All the components have been produced using numerically controlled machinery with exclusion of the cover which – for cost and actual effectiveness – has been laser cut. A collimation system is also implemented but the pictures are omitted.


Above is the plan view of the rear cell. It has been machined from a solid 60 mm thick piece of Anticorodal. The holes are for the positioning of the diaphragms structure which had to be made for the purpose as the existing one was attached to the original aluminium tube and also over-size.


Another problem, pretty common in this kind of commercial telescope, is the use of slightly thermally unstable grease which should ease the slide of the primary mirror inside its sleeve as well as acting as a baffle. Unfortunately, with use, the grease tends to accumulate at the bottom of the sleeve consequently the focusing movement can jerk and, being no longer smooth, it doesn’t help with the search of the best punctiformity. Unless external focusers are used, this grease should be replace periodically after cleaning.


The anti-reflex diaphragms structure is applied to the rear (please note the thickness) and kept separate from all the other optical and  mechanical elements in order to avoid affecting the dimensional stability during thermal variations.


Above, the tube assembly ready to be mechanically and optically tested.


We’ve also provided the rings holding the tube to the mount. They were obtained from a thick section and then precision engineered. For weight reason the metal had to be very thin, so we put a 5 mm thick nylon band inside each ring to avoid any deformation. Once the rings have been assembled and before being anodized we tried to fit the original Losmandy type bar and the bar on which the customer would normally fix the guide telescope etc. Despite the original constructor being quite famous and the telescope rather expensive we noticed with regret that all the components used were made with absolute approximation. The hole centres do not correspond with those set with numeric control (sometimes the discrepancy is about 0.5 mm) nothing appears to be orthogonal and even the bare eye can see the rings are not straight. The upper bar seems even worse. Not only the holes are not orthogonal, but the threaded holes which have been drilled on it are not in line with 1 mm or more  variation between each other. This piece couldn’t be used at all and had to be made  from scratch; in the case of the Losmandy we tried to drill again knowing that, if worse came to worse, we could have replaced it with a modified one from NortheK.

We believe that is possible to judge a wholesaler/constructor from the small detail, as a matter of fact, they might have beautiful pictures in their websites but still struggle to drill the holes straight.


Above is the final result. Luckily we could use the original Losmandy bar but the upper one was so much out of axis that the couldn’t  mend it and we had to make a new one (in the picture it still has to be anodized). We worked meticulously and all our holes fit perfectly  and keep the ring orthogonal (the previous manufacturer tried to achieve that by re-drilling the holes, because they were obviously uneven).



Good evening Massimo,

It is a very windy night, the seeing is worse than horrible, the stars are twinkling and everything is twitching. However l’ve been able  to take a few shots, just 5 images of 30 sec each in simple JPEG format at 1600 ISO, they have been done WITHOUT GUIDE and composed with Maxim DL. My first and quick impressions that it is fine: I tried to contrast the image and the eccentric, vignetting that I could see before has now disappeared (there is still a bit on the corners but it is symmetric) and the stars over the whole field are even, regular and punctiform. If you enlarge the picture you can notice that the star discs are slightly elongated but this is due to the strong wind and not to the telescope. Therefore, after a first visual inspection, I dare say that the problem has been solved in an excellent way. Congratulations!

I attach this awful picture so you can take all the measurements you want and as soon as I will have done something remarkable I will certainly send it to you.



Above are the three enlargements (left, center, right). The deformation is  symmetric due to strong turbulence and because the guide has not been used. The halo too is certainly due to the altitude winds. The image at the right hand side has been taken at the far edge (nearly cut) and therefore in a worse conditions than the one on the left and side.

Please note that the CCD Inspector reports an insufficient number of stars to make the measurements. A wider star field should be considered to have a more reliable information.

In this kind of telescope a field nearly without curvature seems to confirm the hypothesis that the star number is indeed too limited to give a good image exploration, considering also that the lack of flatness of the image on the left. With more stars the situation should improve even more.

The second image is especially significant because it partly takes into consideration the collimation. We have now a field almost perfectly flat, with the right part to be improved with the collimation and it will certainly be done as soon as the improved seeing will allow it. As you can see, the difference is huge and once properly set this instrument will perform at a very high standard (high quality optics and mechanics used by an expert and reputed customer).

Whit this project we are proud of being able to satisfy an experienced amateur astronomer and to have upgraded a prestigious optical train to perform as per the original design. Beyon the monetary aspect, which should be left to personal judgment, retrofitting a prestigious instrument, like this Intes MK 200 f 6 with excellent optics but poor mechanics, has been a good investment for the owner and a big achievement for the NortheK Team!

A further adjustment has given much better results:

I’ve quickly collimated the “mak” and done four exposures at 800 ISO of 10 minutes each: I can state that there are all the makings of excellence, the star disc images have definitely improved and become clearer and smaller.

Daniele Cipollina