Category: Hardware

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 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 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 a 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 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 in 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 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 the 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. 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.


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 newton telescope experienced the same problem as I did. After cleaning primary mirror of my newt, BTW here is a nice tutorial how to do it, I noticed that the brighter stars have triangular shape. After some googling I found the indications that I have pinched 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