NGC 2683

I am not really seeing the UFO look of spiral galaxy NGC 2683 but none the less it was nicknamed the UFO Galaxy by the Astronaut Memorial Planetarium and Observatory.  Maybe I need a bigger telescope.  It was discovered by William Herschel in 1788 and is located between 16 and 25 million light-years away in the constellation of Lynx.   Although not seen in the image, astronomers believe it to be a barred spiral galaxy based on research.

So this was a stretch for my set up in that most of the really good images of this on Astrobin were done with telescopes much larger telescopes. I decided on this because it was available after I was done with my main target but it was still clear and I was awake.  It is also the second object without the focal reducer.  I collected some Ha and luminosity data but after reviewing the Ha data I decided it would not add anything and may detract from it.  The luminosity was collected on another questionable night and was not as sharp so I went with straight RGB.  I do like the so of the intricate structure in the central portion but this object is better done with a much larger telescope.

I captured Luminosity after originally post this so there are two images now.

With Luminosity & Gary Imm Deblotcher

With Luminosity

Without Luminosity

NGC 2683 - UFO Galaxy
Home Monroe, CT
Date: 3-7-20, 3-13-20, 3-17-20, 3-21-20, 3/27/20
Camera: ZWO ASI1600MM-Pro
Telescope: Astro-Tech AT115EDT 115mm Refractor Telescope
Barlow: None
Focal Length: 805mm
f/7
Focal Reducer: HoTech Self-Guiding Field Flattener for Refractor Telescopes
Mount: Orion Sirius
Filter Wheel: ZWO EFW 8 x 1.25"
Filter: ZWO L, R, G, B
Focuser: ZWO EAF
Autoguiding: ASI120 Mini attached to an Agena 50mm Guide Scope with Helical Focuser
Exposure: L 110 x 60s, R 38 x 90s, G 35 x 90s, B 27 x 90s
Gain: 139
Offset 21
Temp: 3 C
Processing: APT, PixInsight, Photoshop.
https://kurtzeppetello.smugmug.com/
http://astroquest1.blogspot.com/
http://youtube.com/c/AstroQuest1

NGC 2359 - Thor's Helmet (2020) - HOO

It has been three years since I last imaged the beautiful emission nebula NGC 2359 or Thor's Helmet.  Located in Canis Major Thor's Helmet is approximately 12,000 light-years away and is about 30 light-years across.  The central star is a Wolf-Rayet star (hot star in a pre-supernova stage) which is creating the bubble structure as the fast moving solar winds interact with the surrounding molecular cloud.  This has a strong oxygen signal which produces the blue color although there is some hydrogen.  There is a lot of faint nebulosity surrounding the main bright areas which was one reason why I did not crop it any further.

I was not at all planning on imaging this this year, however, weather and available objects in my field of view made this an 'ideal' target.  Ideal is questionable as it sits low in the south sky where there is heavy light pollution and my FOV only allows a maximum of less than 2 hrs on it.  Oddly enough the Ha data was not great as I had trouble with getting good focus.  I had to toss the first night of data.  The transparency was not good and given the normal light pollution to the south made good focus challenging.  I had slightly better focus subsequent nights as transparency was better and decided to go with the data.  Conversely, the OIII data was very good, usually that is worse than the Ha but this time.  This turned out much better than I expected after seeing the initial data.  This image, not surprisingly, much better than my not so good image from 3 years ago - before PixInsight, smaller telescope, and DSLR.

Finally, this is my first image taken with the HoTech Field Flattener as I will not be using the Focal Reducer/Field Flatter combo for galaxy season.  Although I will be concentrating on galaxies for a couple of months I wanted to see how it worked on a nebula since - the other reason I went with Thor.  I wrote with extensive help from Gary Imm a webpost on How Focal Reducers Work and attached the link here.  I also created a video How Focal Reducers Work as well.

Webpost: http://astroquest1.blogspot.com/2020/03/how-do-focal-reducers-work.html
YouTube: https://youtu.be/4ins4hXIzLM

NGC 2359 - Thor's Helmet
Home Monroe, CT
Date: 3-7-20, 3-8-20, 3-13-20, 3-15-20, 3-17-20
Camera: ZWO ASI1600MM-Pro
Telescope: Astro-Tech AT115EDT 115mm Refractor Telescope
Barlow: None
Focal Length: 805mm
f/7
Focal Reducer: HoTech Self-Guiding 0.8x Field Flattener for Refractor Telescopes
Mount: Orion Sirius
Filter Wheel: ZWO EFW 8 x 1.25"
Filter: ZWO Ha, OIII
Focuser: ZWO EAF
Autoguiding: ASI120 Mini attached to an Agena 50mm Guide Scope with Helical Focuser
Exposure: Ha 63 x 180, OIII 69 x 180s
Gain: 139
Offset 21
Temp: 3 C
Processing: APT, PixInsight, Photoshop, Lightroom.
https://kurtzeppetello.smugmug.com/
http://astroquest1.blogspot.com/
http://youtube.com/c/AstroQuest1

How Do Focal Reducers Work!

Introduction
I have been a big of Focal Reducers and have been using one for over four years on every image I have taken with my telescope.  When I first purchased the reducer all I knew was they increased the Field of View and the they lowered the Focal Ratio making them 'faster'.  I did not know how they did this but believed some sites which erroneously said they do this by letting in more light.  I also believed the benefit of more light/less exposure outweighed the loss of resolution.

Galaxy season was rapidly approaching so I wanted to get closer in on these objects without getting a bigger telescope so I did some research and opted for removing the reducer.  This 'research' was very confusing because there was a lot of contradictory statements on the various discussion boards and forums I was reading.

I am no expert and this article is meant as a brief conceptual explanation of what is going on.  I created some diagrams to aid in the explanations as a picture is worth 1000 words.  Please feel free to add something if I missed it or stated something incorrectly or if you feel like you should.

YouTube Video

I have included links to more comprehensive articles by experts:
https://www.allaboutastro.com/the-focal-ratio-myth.html
http://www.stanmooreastro.com/f_ratio_myth.htm

http://www.clarkvision.com/photoinfo/f-ratio_myth/
https://www.davemorrowphotography.com/camera-sensor-size-guide#Camera_Sensor_Image_Quality_Basics

Discussion
The basic function of a Focal Reducer (or telecompressor) for a telescope is to increase the field of view of an optical system.  It does this by decreasing the Focal length of the telescope which has the added effect of decreasing the Focal Ratio.  So what is the confusion and why not leave it on and use it all the time?  This is the question and where a lot of controversy is - the so called Focal-Ratio Myth, that is "the faster the scope is, the less time it takes to acquire an image".  Jay Ballauer from allaboutastro.com presents a great discussion on this subject.

Inner Workings - My Telescope & Focal Reducer
Focal reducers provide an good way to increase your FOV without purchasing a new telescope and also flatten the image in some cases and they do indeed lower the Focal Ratio.  Figure 1 shows is a conceptual light ray diagram of my AT115 refractor.  As shown on the diagram the Focal Length is determined from the aperture, refractors use a lens for the aperture and reflectors use a mirror, to the focal plane, where focus is achieved.  The top image is the of the telescope with no Focal Reducer.

Figure 1

The bottom image shows what happens when a focal reducer is used.  I use the AT 0.8 Field Flattener/Focal Reducer combo designed for the telescope.  Taking the Focal Length (FL) and dividing by the aperture gives you the Focal Ratio (F-ratio).  In my case, 805mm/115mm = 7.0 without the FR and 644mm/115mm = 5.6 with the FR.  Notice the aperture is constant.  This is important as the aperture alone determines how much light or photons the telescope setup captures.

Note: Changing the F-ratio of a telescope with a FR is not equivalent to changing the F-stop on a 
          camera lens since changing the F-stop involves changing the aperture. 

The bottom diagram shows how the FR concentrates the light sooner by reducing the FL resulting in a larger FOV.  The new Focal Length is from the Focal Point to what I call the Virtual Objective Lens.  It is at the position where the Objective Lens would be if the telescope were really F5.6  and the Aperture were 115mm.

Inner Workings - With a Sensor
Now we come to the confusing part - the sensor and how it relates to the system.  The sensor is a rectangular grid containing millions of tiny square pixels.  Pixels are the meet & bones of the sensor.  Digital photographers Roger Clark and Dave  Morrow both have extensive information on sensors and pixels.  Each pixel is composed of a semiconductor material which absorbs photons and frees electrons (think Photoelectric Effect - Einstein).  Those electrons are stored in potential wells.  Roger Clark (Clarkvision.com) has a good analogy, think of a pixel as a "bucket of water holding rain drops, and the photons are the rain drops falling on the buckets" (see Figure 2).  When the bucket (potential well) is filled, it is saturated and can't absorb any more.  What happens next is the signal from the pixel is sent to an amplifier and then an analog-to-digital (A to D) converter so an image can then be produced.

Figure 2

What the F-ratio (Focal Length & aperture), sensor size, and pixel size determine is how many photons can be absorbed.

Inner Workings - With My Sensor

I have the ZWO ASI1600 which has a sensor size of 17.7 mm x 13.4 mm (Diagonal: 21.9 mm), pixel array of 4656 x 3520, and pixel size 3.8 um.

Figure 3

The following is not specific to my sensor, thus can be applied to any sensor.  Figure 4 shows a highly idealized diagram of what is happening at the sensor with and without the Focal Reducer.  As a result of the FR the image FOV is concentrated (see Figure 1) and the light is collected by fewer photons.  The Medusa nebula takes up 5 pixels on the on the right model with the FR while it takes up 9 pixels on the left model without the FR.  Because of this, the photons will fill the pixels with the FR sensor faster.  However, it's not always a good thing to have all the photons hitting fewer pixels (something called "undersampling" the image) as the additional photons will be wasted.

Figure 4

The aperture and the amount of light per unit time does not change.  As Jay Ballauer puts it, "Changing f-ratio with a constant aperture puts the same # of photons on fewer pixels…that's all.". 

Other Aspects
The quality of the reducer is important - cheaper reducers are really going to impact the optics, so the trade off for FOV has to be a strong desire.  Also, adding the reducer will affect the collimation, so redoing the collimation is an added factor to consider.

I did NOT mention or discuss the Signal to Noise Ratio (SNR), Read Noise, Total Noise (Total Noise = sqrt [signal + (read noise)^2]), Quantum Efficiency, Oversampling, or Pixel Scale (Pixel Scale = Pixel Size*206.3/Focal Length) as I wanted to keep things simple and I don't have enough experience discussing these issues.  Ideally you should also take into account these aspects.  However, some of those factors will be just as affected by sky conditions, light pollution, and weather.  If you are good with the math, by all means do all the calculations to find out if using a FR will or will not produce a better image.  However, I also recommend you do your own experimentation - try using one and then not using one.  I suspect in many cases it won't matter.

For a more in depth discussion of the above use the following Links:

https://www.allaboutastro.com/the-focal-ratio-myth.html

http://www.clarkvision.com/photoinfo/f-ratio_myth/
https://www.davemorrowphotography.com/camera-sensor-size-guide#Camera_Sensor_Image_Quality_Basics

Example

The following comparison image was kindly provided by Gary Imm.  This comparison was made the  following telescopes: Takahashi 130, Celestron C11R (with reducer), and Celestron C11 (without reducer).  Doing comparisons such as this are extremely difficult as the sky conditions are never exactly the same.  For example the seeing is not the same, the quality of focus may be slightly different, the moonlight is different, and etc.  Although this is not a true apples to apples comparison (maybe apples to pears), it is very useful information.

From his own words:

"I have concluded that the only way to discern this for a given system is to carefully make a comparison. Attached is my image comparison between my Tak130, my C11R (with reducer), and my C11 (without reducer)., with everything made equal the best I could. I did not have any prior agenda here. The results are close, but I like the native C11 a bit better."

I also like the C11 results better.  Although it may hard to see in this web post, the C11R image is a bit brighter but under close observation it is not as detailed and a bit noisier.

Figure 5

Conclusions

What is the reason for a Focal Reducer?  The correct answer that everyone agrees to is that it increases the FOV.  If you were buying for the lower F-ratio to increase the speed so less exposure time is needed?  Nothing is free in this world.  So why are objects captured with a FR brighter and appear to need less exposure time?  Resolution was the price!

If you use a focal reducer on very small objects and then crop the image, the decreased resolution may be more apparent then if you did not use the focal reducer in the first place.

My personal current take on this whole thing is for large DSO's such as nebula which takes up the majority of the FOV, use a Focal Reducer and for small objects such as galaxies, don't use one.  What about mid sized objects?  I plan to go with how I happen to have the telescope configured or what I plan to image in the future.  This is not the most scientific or technical method but it is the most practical.

Additional Acknowledgements:
In addition to the websites and individuals all ready mentioned, I also want to thank Gary Imm for his fruitful suggestions.  Gary's input made this article possible.
(https://www.astrobin.com/users/GaryI/collections/)

Other Related YouTube Videos:
AstroBackyard
https://www.youtube.com/watch?v=46WoTDhuAZc

Slymin
https://www.youtube.com/watch?v=sFYD4aR8WQs

AstroForum
https://www.youtube.com/watch?v=k4VRaqT759E&t=73s

SH2-261 - Lower's Nebula

This colorful object is known as Lower's Nebula or SH2-261.  The nebula is a hydrogen alpha (Ha) region about 3,200 light-years away in Orion, specifically where the hand meets the club.  The Orion constellation has so many neat deep sky objects in addition to the most commonly imaged objects.  There is only one page of images of beautiful nebula currently on Astrobin, however, images are increasing quickly as more people discover it.  It is named after amateur astronomer Harold Lower and his son Charles who discovered this nebula in 1939 in their home of San Diego.

It was hard to find information on this object although Jens Zippel (https://www.astrobin.com/322218/0/), who took a great image of this a couple years ago, talked about the interesting location of the nebula.  Apparently it on the outermost edge of the Milkyway, on the border of the galactic region between the Orion and Perseus arm. 

One of my favorite parts is the lower portion where the gas seems to be concentrated most and it has the sharpest detail.  I cropped the image to remove the two largest stars as I tried to keep them in the image but they ended up being more of a nuisance than anything else.  I did not crop any further as I wanted to keep the faint nebulosity off the top and bottom of the main portion.

YouTube: https://youtu.be/RXhwTIy3R5Y

SH2-261 - Lower's Nebula
Home Monroe, CT
Date: 2-23-20, 2-29-20, 3-1-20
Camera: ZWO ASI1600MM-Pro
Telescope: Astro-Tech AT115EDT 115mm Refractor Telescope
Barlow: None
Focal Length: 805mm
f/7
Focal Reducer: Astro-Tech 0.8x Focal Reducer/Field Flattener for Refractor Telescopes
Mount: Orion Sirius
Filter Wheel: ZWO EFW 8 x 1.25"
Filter: ZWO Ha, R, G, B
Focuser: ZWO EAF
Autoguiding: ASI120 Mini attached to an Agena 50mm Guide Scope with Helical Focuser
Exposure: Ha 83 x 180, R 60 x 60s, G 60 x 60s, B 75 x 60s
Gain: 139
Offset 21
Temp: -4 C
Processing: APT, PixInsight, Photoshop.
https://kurtzeppetello.smugmug.com/
http://astroquest1.blogspot.com/
http://youtube.com/c/AstroQuest1

SH2-274 - Medusa Nebula (HOO - LRGB stars)

This neat looking object is known as the Medusa Nebula as it bears a resemblance to the snake-like hair of Medusa in Greek myths.  It is also known as SH2-274 of the Sharpless catalog and Abell 21.  It is located approximately 1500 ly away in the constellation of Gemini and may be my first Abell object!  Originally this was thought to have been a supernova remnant, however, research done by Soviet astronomers concluded it was a planetary nebula.

A planetary nebula represents the final phase of a sun-like star, first transforming to a red giant, then to a hot white dwarf while releasing its outer layers. The Medusa is estimated to be 4 light-years across, roughly the same distance to the nearest star to our own sun (Proxima Centauri).  The reddish colors are due to the large amounts of hydrogen gas while blue-green is from ionized oxygen.  There is a least one small galaxy visible in the upper right background but there may be more.

I was going to do a straight LRGB version as I thought would be too small to see any detail but I changed my mind - moon and weather issues.  Doing the Ha-OIII brought out the faint dusty gas area in the lower left.  I really like the colorful star field so merged it with the HOO nebula using a star mask in PI.  After several bending iterations I determined that red - Ha (100%), green Ha (30%) + OIII(70%), and blue (100%) gave the optimal detail and color variation.  The image was cropped and probably could be cropped further, in fact many people would, but I like the star field it sits in so I wound up with this compromise.

SH2-274 (Abell 21) - Medusa Nebula
Home Monroe, CT
Date: 2-3-20, 2-14-20, 2-15-20, 2-19-20, 2-20-20, 2-21-20, 2-22-20
Camera: ZWO ASI1600MM-Pro
Telescope: Astro-Tech AT115EDT 115mm Refractor Telescope
Barlow: None
Focal Length: 805mm
f/7
Focal Reducer: Astro-Tech 0.8x Focal Reducer/Field Flattener for Refractor Telescopes
Mount: Orion Sirius
Filter Wheel: ZWO EFW 8 x 1.25"
Filter: ZWO Ha, OIII, L, R, G, B
Focuser: ZWO EAF
Autoguiding: ASI120 Mini attached to an Agena 50mm Guide Scope with Helical Focuser
Exposure: Ha 107 x 180, OIII 80 x 180, L 75 x 90s, R 37 x 90s, G 42 x 90s, B 45 x 90s
Gain: 139
Offset 21
Temp: -4 C
Processing: APT, PixInsight, Photoshop, Lightroom.
https://kurtzeppetello.smugmug.com/
http://astroquest1.blogspot.com/
http://youtube.com/c/AstroQuest1