Building a giant aerial telescope and first observations

   

  A great amazing thing happened at the Singlet Man’s observatory the arrival of a large objective lens of 94mm in diameter plano convex lens made by Roger C. an optican and amateur astronomer from ATS.

For this I thank him very much.

Its is nice lens of very good quality crown glass 5mm thickness and with a full 94mm diameter with a useful aperture of 80 and 76mm in diameter.

Days prior of the arrival of the objective lens i have made the objective cell housing for the 94mm lens manly from cardboard typical of 17th century stile instruments but very nicely crafterd.

Its similar to the objective lens cell for an aerial telescope made by Giuseppe Campani in Kassel at the Orangerie museum Kabinet of Astronomy.

The outside decorations are from the same objective lens cell printed on cardboard.

My cell has two diaphragms of 80mm and 76mm diameter all from cardboard and its fits perfectly. 

In a total of 5 days the cell was made, it took a long time for the glue to dry and these extended the length of the construction project, the cell its very thick because the lens its thin just 5mm similar to 17th century objective lenses. The lens is made using modern amateur technique of grinding and polishing with modern methods and the polishing was done on a pitch lap not a dry paper like in the 17th century. 

The Ronchi test on Stars revealed a nice surface with no defects in the glass and perfectly straight back lines in extra and intra focal, a nice piece of optical glass, nicely made.

  The focal length its almost 9.2 meters long total of 9150mm the way i mount the giant aerial telescope its the same has with all on 4 elements, two tabels and two stools it reaches maximum height of about 4.5m and 5m with another stool this is kind of rudimentary way to mount the lens high up on those tables for me but it's ok the advantage are no costs involved.

 The maximum height of objects that i can observe in the night sky its about 30 degrades with the eyepiece in hand rested on the grass with the head on the grass and soil. 

To mount it higher-up I need a bigger ladder to climb or think about a solution to raise the ladder higher on a stand to putt 5 element's in total for objects 35 - 38 degrade up like 2 tabels and 3 stools and try to anchored and putt some weights on the stools to stabilize them in windy conditions.

The objective lens

The objective lens 2

The objective lens 3

Construction of the lens cell

The great advantage is that i have free will with this kind of mounting the entire south est, south west and south i can manage very easy with these kind of mounting setup. If i mount the lens in the south east position i can observe the hole east side and South est side + the hole west side if I turn the other way around. The south side its a problem because of the deposit constructed in the south that blocks my space and only if i cutt down the large tree i will have f visibility on south part of the sky.

 The official first light was on 1 june 2024 observing the Sun on the solar projection screen and the sun its huge at these length in comparison with the 5.5 m long objective from Tavi F the disc of the Sun by the way its brighter on the screen with the Tavi 64mm diameter lens at 5.5m length in comparison with the the 80mm diameter objective and 9.2m focal length the solar disc its less bright and the luminosity of the solar disk diminishes in comparison with the 5.5m one. Its normal because of the increase in focal length the magnifying power becomes larger/aperture, the small aperture and the very long length creates this effect, but its great because I can see many details on the solar surface without the brightness of the solar disk interfering this makes the solar granulation  easy to see also details in the penumbra of large sunspots are vizibile.

The total diameter of the sun its 10cm on the screen half of the entire diameter of the solar screen which is 20cm. This is probably the maximum I can get with the space I have, this could be the largest aerial telescope of this kind in Romania.

Now because with larger diameter lenses and longer focal length lenses I expect at large diameter and longer focal length the solar disk will continue to lower its brightness on the screen has the focal length/diameter gets larger, especially the length, this is physics its normal to expect this.

Down below are some video i took on the Sun with the 9.2m lens.

Lets talk about the night time experience, the first object seen with the lens was the double Castor perfectly positioned in the west after sunset at 25 degrades it becomes easy for me to view any object in the sky without much effort using 3 table elements.

Without eyepiece a revelation was seen because I am a myopic -4.5 and -5  i can see the image on the lens like a hologram in front of my eye with a small filed of view but for the first time I saw the airy discs of the duble Castor very easily without much effort at the focal point, no eyepiece.

The blue color its nice very intense the airy discs are small but visibile, no diffraction rings, i can see the back space between them easy, larger then hair split.

With a 110mm Kepler eyepiece the view its very close with the image and the magnifying power that I saw without eyepiece, but the field was larger the two stars airy discs visibile and with a good separation and nice colors, with the handheld eyepiece.

To record the image I saw without eyepiece I used my camera phone on PRO MODE at ISO 50 and out of focus mode the challenge of filming a star through the lens situated 9.2m away its very hard, without the camera just with my eyes its very easy also with a Kepler 110mm focal length eyepiece its child’s play, but with the camera its very hard to do this, I have the chance to record a couple of seconds before I lost it, and its hard to center on the lens in the dark, I have switched the ISO from 50 to 6400 to actually see the star in the background sky before center it in the objective and at a 6x zoom and a very narrow filed of view and huge magnification it’s the hardest think to do.

On planets the same deal, but it takes practice, not all time you will have luck to spot it and record it steady, its impossible to mount the camera on a tripod because it will drift away I in a couple of seconds, the moon its much easy to do record through the lens than stars.

The eyepiece image is exceptional when simply holding the eyepiece in my hand the method was used by Cassini to make observations of planets with very long focal length lenses, and I am pretty sure that these guys where experts in these method of observations because there are so many drawings of planets through aerial telescopes at the Paris observatory dated to the end of the 17th century.

And by observing stars and other objects with these kind of devices you are put into very interesting positions its like a dance with the eyepiece in my hand trying to find the focal point of the lens and observe the image of the star this was its quite hard at first but in time you will get used to it and becomes easy.

The chromatic aberrations with this new lens its very small similar to a F.12 or F. 12.5 achromatic refractor the unique thing with these long focal length lenses is they can reach huge magnifications with modern day eyepieces and it’s an exceptional instrument for double stars in comparison with a achromatic refractor witch for high power you need very short focal length eyepieces and probably a Barlow lens to observe the airy discs of stars, but with a long focal length singlet the stars airy discs are vizibile even at low power a very nice advantage.

The disadvantage is that with these aerial system you can observe only bright stars vizibile to the naked eye and you must plan every observations before because its impossible in my case to observe the full patch of sky at these focal length only objects that are 30 degrades up in the sky using 2 tabels and 2 chairs + the Dobsonian mount.

The airy discs of stars small in fact can bee seen through the objective lens without eyepiece with my myopic 4.5 and -5 eyes this is quite amazing, 

Down below some videos i made of Castor double star.

Castor double stars through the objective lens

Castor double stars through the objective lens

I have tested on stars with 2.5 magnitude and I can see a very small bright airy disc with some diffraction rings around it. Lower magnitude stars the airy discs and diffraction rings are easy to spot because the stars are not so bright.

For solar observing its an perfect instrument to do systematic solar observations, the sun has 10cm in diameter on the projection screen and its very clear. However the luminosity of the disc its much dimmer than with the Tavi F 60mm diameter 5.5 m lens.The details of the surface can bee easily seen with the 9.2m focal length lens because of this dimming in light including the solar granulation, details in large sunspots, and faculae at the edge of the Sun.

I am assuming with even longer focal length lenses the Sun brightness in projection will drop has it becomes larger do to the increase in focal length, also chromatic aberrations will be vizibile at larger focal lengths. The solar granulation is very easy to see when you move the sun slow up and down or in any direction on the solar screen.

The moon.

The most amazing object to view through the 76mm diameter aerial telescope is the Moon and I have quite a few surprises. When I view the moon without eyepiece through the objective lens in the far distance it I can see the image of the moon at high power like a projected hologram in the distance two times larger then the objective lens cell + cardboard disc 15cm in diameter. The magnification its about 102x with my left eye and the image super clear its like a Newtonian view with a smaller filed of view.

I can see details in large craters 100km diameter, like for example Langrenus and Petavius, details on the floor of the crater, the central mountain, terraces at the egde of the crater in much greater detail than with the 5.5m meter focal length lens the image its still perfectly clear with no chromatic aberration. Through the 40mm Plossl eyepiece the view its amazing with a magnification of 228x also with other larger focal length eyepiece has well 45mm 203x and 100mm 91x. The contrast it’s a bit lower when the moon its at waxing gibbous phase has it gets larger and brighter in the first quarter the contrast will be better.

This focal length its at the limit of recording the moon with a smarthphone at 6x zoom and ISO 60 or 120 because of the very high magnification the image it gets darker. You have to process the video captures to eliminate the background noise in the image and to adjust the brightness and contrast and enhanced the details in the recording to obtain a finer result. Down below are some videos i took through the objective lens.

But this is at the limit of recording, longer the focal length darker the image will be also the contrast its going down considerably and the clarity of the image.

For visual observations the limit will be in this case depending on my diopters eye lets say 22m long focal length and 120mm lens reaching a magnification with my eyes at the upper limit of 244x two times bigger than the diameter of the lens.

The 9.2m its at the middle of good resolution an clarity without eyepiece with my eyes at my dioptry of -4.5 and -5. The liquid in my eyes can bee seen in the image projection without eyepiece and does not affect the finer details of the image.

At a focal length of 13.2m / 93mm objective delivers a 146x magnification with my left eye. 

At 22m and 120mm aperture the image it’s at the maximum magnification of 244x and I can say that this is the limit with my eyes but it will be difficult to observe without eyepiece because the liquid in my eyes will start to spoil the details and the brightness of the image gets lower at such high magnification and small aperture.

The limit for recording a video through the objective lens with my smartphone its at 9.2m with 6x zoom even at this length it gets difficult to obtain a clear image because of the increase of magnification. All off these combined gets a magnification higher than 200x with the extra zoom of the camera + the very long focal length of the lens. At 13m focal length using this method of recording the image will be unclear because of the very high magnification and small aperture also much darker and you will need to set the ISO up higher to see something at this length even at the moon. This will degraded the image quality and the recording will need to be process and eliminate the background noise and enhance details. So at best in the future a 13.2m long focal length lens will be at the maximum for visually observations of the moon without eyepiece using my setup and space. This by the way its just theoretical assumption i don't know what is the limit for naked eye observation through the objective lens and at what optical parameters the image will be affected or the maximum resolution and magnification reached when the image starts to degrade itself, this is just based on optics and observations and the limitations of my eye. I know now that the limit for recording the view through the objective lens is at 9,2m focal length because of the high zoom factor and bigger lengths the camera optical zoom must remain at lower levels to obtain a clear image instead the field of view will be very narrow. But for now the 9.2m works amazing and the image its clear all across the lens field with no distortions or rays, no filed curvature distortion or defects in the glass are visible, it’s a clear image.

Lets remain in the domain of the 9.2m aerial and let’s discuss the details in the image.

I have observed that with my myopic eye of -4.5 and -5 that the image is very clear even at this length and the magnification around 100x on big craters like Petavius, Langrenus i can see huge amount off details like the terraces on the rim of the crater very easy and also floor details on the crater Petavius ( 177km) and Langrenus ( 144km) the mountain peak details of structure, the image is apochromatic visually and I have seen even some little mountain peaks on the floor off Langrenus. Small craters 40km in diameter the mountain pecks are visibille without eyepiece and the terraces on larger 50km craters. It’s like observing through a very high quality Newtonian with very narrow filed of view but the details are at the maximum amount and smaller features are visibile. The mountains peaks of large craters are far more clear seen  and complex features appear in the mountain structure thinks not visible with the 5.5m lens.

The crater Langrenus was half the field of view and Petavius more than half the field of view.

Rimae Petavius was visible with the naked eye on spot very easy and some craterlets on the crtaer floor, details on the mountain peak of Petavius. Mare Crisium edge mountains was visible in finer details a great sight.

Details of the crater Geminius 86km was visible the crater terraces where remarcabile to see at the terminator shadow made a good contrast to observe the edge of the crater.