Category: Hardware

Newtonian 250/1000 homemade upgrade

After 6 fruitful years with my 10” Newtonian, I started to think about an upgrade. However, I concluded that there is simply no better, reasonably priced telescope, which I can still carry. Ritchey-Chrétien telescopes are better from an optical design point of view, but they are slow F8 and with the best reducer one can speed up the scope to F6, which is still more than one f-stop slower compared to F4 Newtonian. Refractors are slow and the aperture is small compared to any Newtonian. Officina Stellare makes a very nice and fast telescope, but unfortunately out of my budget.

There is simply no other option than to keep using my Newtonian. Instead of a complete change of the telescope, I decided to upgrade the Newtonian. I noticed that the brighter stars have an ugly reflection, especially if the dual-band filter is used. The reflections can be caused by many reasons, for instance:

  • Internal reflections
  • Primary mirror holder
  • Filters

The first issue can be solved by flocking the secondary mirror, but mine is already black on the sides. Another source could be the internal surface of the tube. The simple solution is just to attach some black velour material inside. Nowadays one can get peel and stick sheets for example FLO offers a 5-meter long roll, which is nicely advertised: “Blacker than the blackest black stuff”. I had to completely disassemble the Newtonian and started to place the velour inside:

You can see how darker it gets inside even if you illuminate inside by the LED flashlight. Then, I continued and covered the whole internal surface. It was a straightforward and inexpensive upgrade.

The second upgrade was also straightforward, but more expensive – the primary mirror. I think this is the root cause of the ugly reflections/halos around the brighter stars. The problem is well described on the internet (e.g. cloudy nights) and it’s caused by the three clamps, holding the mirror. Inexpensive would be to use an aperture mask and simply cover these clamps (like suggested on cloudy nights). However, this reduces the aperture and ability to collect as much light as possible. I decided to choose another path. I noticed that Teleskop-express offers Quartz primary mirrors with thread M50 on the backside of the mirror. This is exactly what I need – a new method of clamping. I ordered the mirror with the mirror cell because the old one cannot be used. Well, I thought that this will solve all my problems with the reflections, but a surprise popped up during the unboxing. The mirror had finger imprints, but this would be the smallest problem. The main problems are the uncoated marks on the edges:

I immediately contacted Teleskop-express for an explanation. I was told that these uncoated marks have all Quartz mirrors and they are caused by the clamping during coating in a vacuum chamber and it is not a quality issue. They also claim that it should have no influence on the quality of the astrophotos. Well, we will see about that.

The uncoated regions are roughly 2 mm wide and 12 mm long. I assumed that the external edge is perfectly round and if I would know that the mirror has these defects, I would not order it. I use CatsEye collimation tools, which is a very precise method, but it requires attaching a self-adhesive triangle on the primary mirror. It’s quite a straightforward procedure. I tried to be as precise as possible and when I attached the triangle, I noticed that the original circle denoting the center of the mirror is roughly 2 mm off. Here is the picture from distance:

And here is the comparison – the new mirror (left) vs. the old mirror (right):

I had an opportunity to recapture the M99 galaxy this year. In the field of view, there is the HIP 60089 star with a magnitude of 6.5. The left picture was captured with the old primary mirror and the right one with the new Quartz primary mirror. Obviously, the new one has fewer reflections and three dark shadows in the shape of the radioactive symbol vanished. However, there are still some shadows in the halo around the star. Please note that I didn’t use any filter in the optical train, only the MaxField coma corrector. So the filter can be excluded as a root cause of this problem. There are two uncoated regions on the mirror, approximately 45° from each other, which corresponds well with the shadows. On the other hand, there is another star in the field of view, in the upper left corner. This star has a magnitude of 8.95. This one looks significantly better and definitely more circular.

Conclusions:

Well, I have mixed feelings about the upgrade of my Newtonian. It was easy and inexpensive to attach the velour material inside the tube. The change of the mirror and its fixation was also simple, but quite expensive and it didn’t solve the problem fully. It only reduced the reflection around the +6 magnitude starts, but there are still some reflections, most probably caused by uncoated regions on the edge of the mirror. If you already have a Newtonian and if you are considering purchasing TS-Optics Quartz Newtonian Primary Mirror, probably the aperture mask would be a more effective and significantly cheaper solution.


iOptron SkyGuider Pro, Askar FMA180 f4.5 review

I made a big step towards ultra-portable astrophotography. On Kythira I first time placed my workhorse Canon EOS 6Da on mini mount Baader Nanotracker and made quite stunning pictures of the Milky Way. I was so excited and started to think about more serious “pocket-size” astrophotography. The requirements were the following: no external batteries, telescope, and camera should have max. 1.5 kg weight, focal length around 200 mm. The camera it’s simple, I simply keep using my modified Canon 6D.

The telescope is quite trickier, but I found Askar FMA, which has a 220 mm focal length, which is shortened by included a full-frame reducer to 180 mm. Aperture 40 mm yields for this focal length to speed F 4.5. The scope weighs only 400 g (700 g with the rings and EOS adapter). There is an M48 thread at the front of the telescope, which is very useful to attach the 2” mounted filters, particularly if you live in a light-polluted area.

There are several travel mounts on the market. Probably the most famous is Skywatcher Star Adventurer. I was about to pull the trigger on this mount, but I found another one, which will fulfill my requirements even better – iOptron SkyGuider Pro. iOptron has the same payload (5 kg with counterweight, 1.5 kg without) and has many interesting features:

  • Integrated illuminated polar scope
  • The torque from the motor to the worm gear is transmitted by a belt (this should minimize the backlash),
  • Integrated battery, which can be changed by USB cable
  • ST-4 socket for autoguiding

iOptron is slightly lighter than Skywatcher, so the decision was made. And how does it perform? I must say: VERY WELL! I got a chance to use it at home, so I screwed the IDAS NB1 filter in front of the Askar and captured the center region of the Orion and California nebula. The mount is tracking very accurately even without any counterweight and the number of bad photos caused by poor tracking was zero.

The telescope surprised me very positively as well. The connection to the camera is done by T-thread (M42), so I expected significant vignetting on the full-frame sensor, because the diameter of the thread is 42 mm, whereas the diagonal of the sensor is 43 mm. Surprisingly, it vignettes very little, and even flat frames are not necessary. The darker corners can be corrected by dynamic background extraction in PixInsight. The stars are a little bit oval at the bottom corners, but we are here looking at a full-frame sensor. In the end, I am very happy with the price/weight/performance ratio of Askar.

Let’s have a look at how heavy is the whole rig:

Tripod2.3 kg
iOptron SkyGuider1.5 kg
Canon EOS 6D0.8 kg
Askar0.7 kg
Total5.3 kg

This can fit into any backpack.

Here are the pictures:

Technical details:

LensAskar FMA180 F4.5
CameraCanon EOS 6Da
MountiOptron Skyguider Pro
Exposure60x60s, ISO 1600
Date2021-02-18

Technical details:

LensAskar FMA180 F4.5
CameraCanon EOS 6Da
MountiOptron Skyguider Pro
Exposure44x120s, ISO 1600
Date2021-02-18

Samyang 24 mm f 1.4 review

I have been searching for a wide lens for my recently astro-modified, second-hand Canon 6D. This means the lens should be suitable for a full-frame sensor 36 x 24 mm. The requirements on lenses are very tough for astrophotography because you photograph the stars – pinpoint sources of light. The design/manufacturing flaws of the lenses are revealed on every astrophoto and optical aberrations spoil the good shot. Astrophotography of the Milky Way needs a lot of effort. Specifically, you have to travel to reach the dark sky and if you do so, you want to make nice pictures. In my opinion, the lens is the most important piece of equipment for astrophotography, because nowadays you can buy second-hand Canon 6D, which is still very good and relatively inexpensive.

All the lenses are very sharp and aberration-free in the center of the picture, but the more you go off the axis, the aberrations start to pop up. There are many kinds of optical aberrations. A very nice article about the most common aberrations is on Lonely Speck.

Last year I purchased Samyang 14 mm f2.8, which is a great lens for the money, but the corners are not perfect and the stars are strongly deformed in every corner. I assumed that almost twice more expensive Samyang 24 mm f1.4 will perform much better and I also assumed that it’s easier to make a 24 mm lens compared to 14 mm. Moreover, f1.4 is a brilliant convincing argument. On the other hand, Samyang 24 mm doesn’t communicate with the camera, which means no EXIF of aperture and manual focus. This makes the lens a one-trick pony, suitable mainly for astrophotography and not that practical for regular photography.

Before I take this lens to the dark site, I decided to test it from my light-polluted home. The equipment: Canon 6Da, Baader Nanotracker, and of course, Samyang 24 mm f1.4. The main aim was to find the best aperture/sharpness ratio. Most of the lenses get sharper if slowed down. So I kept the exposure time 20 s, ISO 800, and was systematically changing the aperture from 1.4 to 2.8.

Here are the results of uncropped and uncorrected (no flats, no bias, no darks, and no noise reduction) pictures:

F1.4

F2.0

F2.4

F2.8

And the winner is…. obviously, the largest aperture (the smallest F number) collects the most of the light, but it vignettes strongly and honestly, the stars are ugly even in the center – this is totally unusable for serious astrophotography. The situation is not much improved by slowing the lens down to F2.0. At F2.4 the situation is significantly improved but at F2.8 the star roundness is acceptable almost everywhere, except the left corners.

Let’s have a look to the upper left corner – there the stars are the worst.

F1.4

F2.0

F2.4

F2.8

Conclusions

Samyang 24 mm F1.4 should be slowed down to at least to F2.4, to offer decent quality of the stars on a full-frame sensor. At F2.8 the quality is even slightly better, but at the top-left corners are the stars still elongated by astigmatic aberration. I expected better star quality, but in the end it’s not so dramatic, because the right side is not perfect, but acceptable. At least I know which side of the camera I should turn towards the ground if making a portrait picture of the Milky Way.


SharpStar 150 mm f/2.8 Hyperbolic Astrograph review

I think that nearly all enthusiastic astrophotographers must have noticed the news about the hottest, portable, fast, affordable telescope from SharpStar. The rumors were spreading in 2019 and when I saw the specifications, this telescope landed on my “must-have” list. The specifications were incredible: F-stop 2.8, carbon tube, solid spider wanes, primary hyperbolic mirror 150 mm in diameter, robust 2.5-inch rack, and pinion focuser, and mainly the corrector, which should be able to correct the coma on the whole full-frame sensor (36×24 mm), WOW! I already had some trouble making the F4 Newtonian work on F2.8 by using the ASA corrector/reducer. This was quite painful, but SharpStar’s corrector is much bigger in diameter, so theoretically it should work.

Immediately as soon it was available, I ordered one at http://www.teleskop-express.de/ but their own rebranded version called TS-Optics hyperbolic astrograph. How does it perform? Let’s have a look.

I somehow expected that reaching the focus will be quite a challenge, therefore I ordered ZWO electronic focuser and easily attached it to SharpStar.

The secondary mirror has very nice spider vanes. This was my dream to get very stiff and perpendicular vanes. I have two Newtonians from TS and it was a bit struggle to get them perfectly perpendicular. On the other hand, the secondary mirror is not flocked/blackened. Moreover, there are many shiny screws. All this can cause some reflections, but we will see.

Secondary
Secondary mirror

The primary mirror’s edge is covered by the ring. I really like this feature and all Newtonians should have it. A cheap solution is to clamp the primary by L-clamps, which cause ugly reflections/flares around the bright stars.

Primary mirror
Primary mirror

The corrector has an M48 male thread, so any 2″ extension can be attached. Included is only an M48 to 1 1/4″ extension, but I already have a couple of 2″ at home and these are not expensive.

M48 to 2" extension and Howie Glatter 2" laser collimator
M48 to 2″ extension and Howie Glatter 2″ laser collimator

The secondary mirror can be collimated even with the corrector attached.

Secondary collimated

But to collimate the primary mirror by a Barlow laser, the corrector must be unscrewed. The tool to do that was included and it was easy to unscrew it.

Primary collimated

First light

I was so excited to test the telescope outside. The spring season provides many deep space objects. All you need is just to point the telescope into the constellation Virgo or Leo and you will find many galaxies anywhere you look. In fact, this was the main reason I purchased this telescope – to have a focal length of around 400 mm and a fully illuminated 44 mm image circle. I don’t have a full-frame cooled camera, so I did the first light with an APS-C sensor size (ZWO ASI 071). The first target was the galaxy M105. After the post-processing, I was very disappointed. The stars in the corners are oval and they have a color shift. The blue channel is shifted towards the center and the red one outwards. See for yourself:

I pushed the saturation and the vibrance to magnify the aberrations. Here is the detailed picture of the corners vs the center:

I think we all agree this is not OK at all. If I see such ugly stars on a cheap telescope, I would not complain, but this one was not a cheap telescope. This test was done on an APS-C sensor, which means a diagonal 29 mm and not advertised 44 mm. I immediately contacted TS and asked what the hell is going on. I was told that probably it could be the back focal distance (BFD). It’s the distance from the corrector to the sensor. Normally, it’s 55 mm, but a bit shorter or longer could improve the performance. Fortunately, I have many M48 adapters and distance rings, which could cover the BFD from 58 to 51 mm, so I started the experiment. This time, no stacking, no image calibration by darks or flats. Just a single picture and cropped corner:

I don’t see any significant differences between BFD 53 – 55 mm, subjectively. The stars are still oval and if I would stack many images together, even the color aberration would pop up. Let’s put a subjective assessment aside and let’s use a deterministic method to analyze the data – CCD inspector. Here is the 55 mm, which is the recommended BFD.

The corners are obviously much worst than the center, but again, it’s an APS-C sensor and not a full-frame. I decided to plot the FWHM as a function of the BFD. I evaluated the center and each corner separately. If the telescope would be perfect, I would get the same FWHM value anywhere on the picture. Based on this diagram one can easily find the sweet spot of the back focal distance, which is in this case 55 mm. Things don’t get significantly worst if the BFD is reduced to 54 mm, or even 53 mm. Unfortunately, I didn’t have a 1 mm washer to test 56 mm so this distance remains a mystery.

Conclusions

When I showed the graph to TS they agreed that there is something wrong with this telescope and I have full right to return it. And this is what I exactly did. They didn’t offer me a replacement telescope (I can only guess that the product from the same batch would have the same optical quality). The sample I tested was a good-looking telescope with very poor optical quality. If you are not a pixel peeper, you better get a cheap Newtonian 150/750 mm f4 and you get the same outcome in the corners, maybe even better with a decent coma corrector. I really don’t understand how this can be advertised as a telescope for a full-frame sensor. I asked TS if some customer managed to get this “astrograph” working with a full-frame sensor. They replied that so far non of their customer used this telescope with such a large sensor (APS-C). Ha ha ha. This was a good one, but really this is what they told me. I would really appreciate it if other users have a similar experience as me. Please write me a comment below.


Worldwide coma correction exercise

Introduction

What is coma and why needs to be corrected. Coma is a special type of an optical aberration. It’s mainly visible in the astrophotography – the stars are deformed into comet-like shapes:

Fast Newtonian telescopes suffer from comatic aberration, mainly in the corners of the photos. These telescopes are nowadays very popular between amateur astrophotographers just like me, because they offer the best price/aperture ratio. There is easy way how to suppress the comatic aberration – coma correctors. There are many coma correctors on the market, but there are just a few published reviews or comparisons between them. Therefore I decided to perform the comparison myself. Quite big collection of coma correctors has assembled in my drawer ready to be tested:


Triangular stars

Maybe someone with a Newtonian telescope experienced the same problem as I did. After cleaning the primary mirror of my newt, BTW here is a nice tutorial on how to do it, I noticed that the brighter stars have a triangular shape. After some googling, I found the indications that I have pinched the primary mirror. Fortunately, it was just elastically deformed by the rubber clamps. When I placed the mirror back I overtightened the screws, therefore the mirror was not reflecting brighter stars properly.

Triangles

By loosening the screws everything went back to normal, so the solution is quite simple.

IMG_5801
IMG_5808