Updated: Apr 2, 2021
The Stellarvue SVX130T is the first telescope I officially bought for Cosmos Safari, and the decision is based pretty solidly on the "buy once, cry once" philosophy. This is the type of telescope that will last you a lifetime.
Astronomy and astrophotography is not something new to me. I've been practicing observational astronomy in some capacity since 2008, and have been learning astrophotography since 2015. Over the years I have learned, most often times the hard way, that there are very few shortcuts in this hobby. I've had so many nights of frustration as a result of poorly designed equipment, incomplete and/or incompatible parts from various manufacturers.
"I've had so many nights of frustration as a result of poorly designed equipment,
incomplete and/or incompatible parts from various manufacturers."
In buying my first telescope for Cosmos Safari, and moving forward, it is my goal to provide you the best possible solutions-based approach that result in avoiding the frustrations that I've experienced over the years as a beginner. Many of these solutions took me over a decade of trial and error to learn, and I hope that by sharing my experience I will cut down on your learning curve by years. I also wanted to make sure that the telescope would be a heavy hitter when it came to ALL seasons, and all types of deep-sky objects.
"I hope that by sharing my experience I will cut down on your learning curve by years."
Which telescope should I buy?
Over the years of conducting star parties for the public, running astronomy meetings as president of my local astronomy club, and opening the doors of the public planetarium where I am director, I have been the point of contact for astronomy in my community. Many of those questions relate to "Which telescope should I buy?"
My understanding of the hobby has evolved dramatically since the early days as I have developed my understanding of the hobby. After years of recommending dobsonians to beginners, I find that more and more people are interested in astrophotography. A dobsonian in those cases is a complete waste of money.
"...more and more people are interested in astrophotography. A dobsonian in those
cases is a complete waste of money."
On the other hand, if you plan on doing observational astronomy only, then a dobsonian is 100% the best way to get started. There are some really great dobsonians available on the market today, and they just seem to keep getting better. For visual astronomy, there really is no replacement and bang for your buck than a high quality dob. So I now start my answer as a series of questions,"What do you want to do with your telescope? What kind of objects do you want to view? Do you ever want to take images through your telescope?". In time, I hope to help answer those questions through future posts on this blog.
Is this article for the beginner?
I will write a future article related to "beginner" astrophotography, and a separate article on beginner and intermediate "visual" astronomy. If you are looking for that in this article, some of this equipment is not recommended due to the price point that this telescope and the resulting gear to operate it will cost. However, I encourage you to read further as many of the points I will make below are going to hold true for beginners as well.
Intermediate Astrophotography- Getting past the "Frustration Curve"
Is this telescope for everyone? No, of course not. Telescopes come in so many different types for a reason.
Each telescope design comes with it's own inherent strengths and weaknesses, and each of those designs comes in at various sizes, price points, and levels of quality. I realize this telescope is priced at a point that is way too high for most beginners, but my point in writing this article is that at some point you will likely know for sure that you have "got the bug" and that you really want to stop wasting your money and need to find a "solution" for astrophotography that will help you get past the "frustration curve". If you are interested in astrophotography primarily, I highly recommend you start with the "refracting" telescope with lenses, as it works most like a camera lens, provides wide-field images, generally are smaller and lighter, and require less maintenance (collimation) than their "reflector" counterparts.
Throughout this article, I want to shed some light on why I chose this telescope as my main imaging telescope for astrophotography, and how it will serve Cosmos Safari for the foreseeable future, and some guidelines on how to make an educated decision on purchasing a high quality telescope for the purpose of astrophotography.
Why do I recommend an apochromatic "triplet" refractor?
Refracting telescopes are the original telescope design that was first developed by Galileo. One of the issues with refraction, the bending of light, is that single-lens refractors bend the various colors of light to different focal lengths, causing "chromatic aberration".
Achromatic "Doublet" Refractors partially correct for these aberrations, providing the Red and Green, or the Green and Blue color correction, but do not completely eliminate these effects. Apochromatic refractors correct for all three of these colors, providing spectacular color correction, a must for the discerning astrophotographer and observer.
What is the difference between an "apochromatic triplet" vs. "petzval" vs. "astrograph" ?
Apochromatic Refractors are wonderful for observational astronomy, and they provide the all-important color correction mentioned above. However, they do suffer from issues with field flatness, which basically means that the light falling onto a camera sensor will not result in sharp stars to the edge of the frame, but will be unnoticeable when conducting visible observation.
Petzval telescope designs are based on a portrait camera lens and have been designed for the purpose of projecting light onto a flat surface rather than an eyepiece. They also have slight issues with field flatness and as a result still require a corrector/flattener.
Many times are built for the purpose of imaging primarily and often do not include a visual back to view with your eyes. You may be able to purchase the visual back as an al-la-carte option in some cases, but not in others. Definitely check with your manufacturer or distributor prior to purchase!
"Astrographs" are meant to be imaging telescopes primarily. Although astrograph telescope designs are the best solution for astrophotography, I enjoy the ability to conduct visual observations with my gear as well.
So, for me, apochromatic refractors are my first choice. Apochromatic refractors can be outfitted with additional elements called "field flatteners" and "field flattener-reducers", discussed below, that will eliminate elongated stars (coma) at the edge of the sensor and provide the flexibility for visual use if you desire.
What is FPL-53 glass, and why does it matter?
One of the most important aspects of astrophotography is the quality of the optical glass. Optical quality starts with the quality of the original glass used to make the lens. The center element of the Stellarvue SVX130T is composed of Ohara S-FPL-53 super low-dispersion glass. The purpose of this glass is to more accurately focus the various colors of light to produce improved color correction, and higher contrast. The rear element of the primary lens system of the SVX130T is a Lanthanum rear element made to further improve the image of these refractors.
How can I be sure that my telescope will perform as advertised?
In short, not all telescopes are produced to the same level of precision. Even the "high end" manufacturers are allowing telescopes to leave their manufacturing facilities without knowing the specs for that telescope.
The reality is, many/most telescopes are mass-produced in factories. That industrial process has served the hobby well as a whole, driving down costs and improving overall quality. This is a wonderful thing and has provided access of astronomy to millions of people that otherwise would not have access to such instruments, and for that I am truly thankful. The reality is, checking the optics and hand-figuring the lenses on each telescope would be extremely time-consuming, would require an enormous team of experts, and would drive up costs dramatically.
If you want to get over the frustration curve, that last 10-20% of quality matters, and you need to start to test the optics throughout the lens-production process, as well as test the quality of the lens before they leave the manufacturer.
My particular telescope came with a specs sheet using Stellarvue's Zygo Interferometer. This device passes a laser through the telescope's optical train and allows for the optics to be studied at a level of precision not possible using any other technique.
The interferometer is not only used in the final report of the optics (shown below). The Zygo Interferometer is used throughout the process of the production of these high quality lenses to identify minor defects in the automated grinding and polishing process. The lenses then go through a hand-figuring process where a master optician is able to make corrections to the optics. This process can take up to 6 months from start to finish. The "Final Report Card" shown below is simply a testament to the level of dedication by Stellarvue to provide one of the highest quality lenses on the market.
Lens Topography and the "Oblique Plot"
The "Oblique plot" (middle top) is the lens topography and is represented in both a color scale as well as a vertically exaggerated scale to indicate variations in the lens as a function of their delta +/- values with respect to the "ideal" perfect lens. I want to stress here that the graphic is using vertical exaggeration as a way to display data. The performance of this lens is absolutely top notch.
Airy Disk and Strehl Rating
The "airy disk" (middle bottom) shows that this telescope does not have any "trefoil" or
"on-axis astigmatism" issues. Below is the airy disk image is a the rating from my telescope, indicating a Strehl rating of 0.995!
Remember, that means that 99.5% of the light is making it to the correct place on the sensor / your eye, producing extremely sharp and high contrast images / views.
What factors are involved in choosing the correct focal length telescope?
When looking at a telescope one of the first things people look at is the "magnification" factor, which is a function of focal length. The reality is, many objects in the night sky are actually very large in scale. For example, the Andromeda galaxy, the most distant naked-eye object in the night sky at 2.537 million light years away, is actually VERY large in the sky. It is approximately 6 times as wide and twice the height of the full moon, representing 12x the surface area in the sky. Other objects like distant galaxies, planets, and asteroids can be very small and require significant magnification. The Stellarvue SVX130T, at 910mm f/7 fits nicely in the middle. It's a great first scope as it is somewhat of a "jack of all trades" focal length. As a result, it provides the best results for an "all around" telescope for astrophotography, but is not necessarily a solution if you really want to go "deep".
Do I need a field flattener & field flattener reducer?
One of the best purchases that can be made with an apochromatic refracting telescope for conducting astrophotography is a field flattener and a field flattener reducer. Field curvature is one of the first issues that beginner-intermediate astrophotographers run into. Field curvature is easily solved with a field flattener.
If you don't factor this into your budget and realize what is occurring too late, you are going to be sorely disappointed with your results, and you are going to waste tons of clear nights taking images that, in the end, will not result in satisfactory data. As mentioned above, field flatteners correct for issues with field flatness in the refracting telescope design. Unfortunately, it is just the underlying physics of the optical system, so we need to eliminate the problem.
Stellarvue makes the SFF4-130-35SV field flattener specifically designed for the SVX130T, and the SFFRR.72-130-48 field flattener reducer, also designed for this optical system. The ability to "mate" these to the telescope by purchasing them as a package is extremely important. The main difference between a field flattener, and a field flattener-reducer is that the reducer does two main things.
1) The main optical focal length is reduced from 910mm by x 0.72 = ~655mm producing
a wider field-of-view for larger deep sky objects or groups of objects.
2) The main optical focal ratio, f/7 is also reduced. f/7 x 0.72 = ~f/5. This makes the
telescope "faster", allowing it to collect light more quickly and permitting brighter
images in the same amount of time or shorter exposures.
Like any relationship, the focal reducer is a give/take relationship. As a result of the wider field and shorter focal length, the telescope now sacrifices its theoretical resolution. However, the reality of this is that most times atmospheric conditions are the main controlling factor and the differences will likely be negligible.
I was able to have these field flatteners collimated with the main optical train in Stellarvue's optical shop prior to shipment, providing significantly more control over the process than having bought them separately. I cannot stress the importance of this step in the purchase. You really need to buy field flattners and field flattener-reducers together as a package with your telescope to get the highest quality results. (P.S. Since posting the blog I have had a few people message me upset that they didn't do this step in their purchase of a telescope. Although it is best to purchase them as a package, purchasing them separately is always still better than no field flattener at all. Just make sure you buy the RIGHT field flattener for your scope. Consider contacting OPT for help for your specific telescope design.)
If this is not possible, try to buy the best possible field flattener or field flattener reducer you can afford. Remember, the optics are always going to operate at the level of the lowest quality optic in the optical train. If you try to go cheap on this part, you are wasting your money on the rest of the optical system.
How do thermal affects change the way in which my telescope performs throughout the night?
Thermal effects due to the expansion rate of the glass vs. the telescope plague astrophotographers. It often times is the culprit that shows up unwanted to the party. Late in the evening, after you are in bed and your imaging run is continuing on, your focus starts to creep. You wake up in the morning excited for the results, only to find out that after temperatures dropped your focus was so far off that your data was unusable. We'll talk more about this on an upcoming post related to computer controlled motorized focus, and why it is a must if you want to get past the frustration curve.
However, another less-known culprit related to thermal expansion/contraction is due to the "cell" that holds the telescope's lens. If the thermal properties of the telescope's cell is significantly different from those of the glass, an issue known as "pinched optics" can occur. In pinched optics, your lens is being distorted by the cell's compression upon it. This is of course a very bad thing for optical performance. Pinched optics result in noticeable flares that cause stars to appear triangular or appear to "flare" in one particular direction. Unfortunately, there are very few ways to adequately correct for this effect without sending it back to the manufacturer. Even then, the original material / design of the optical cell are can make eliminating this issue challenging to resolve.
The closer the cell's expansion/contraction rate is to that of the glass, the higher the performance of the telescope's optical train as temperature varies. The cell in my Stellarvue SVX130T is made of steel, which is heavier and more expensive than other options, but the result is a more thermally stable lens cell with significantly lower chances of resulting in "pinched optics".
Need help getting past the frustration curve?
One of the most upsetting things to me is seeing a fellow astronomer in a situation where they have already purchased a telescope, and are dealing with issues that cannot be fixed or corrected for. A new telescope is sometimes the only solution. If you are interested in buying a telescope, and have yet to make the purchase, or are interested in getting help to get past the "frustration curve", than consider subscribing to CosmosSafari.com's various social media outlets (links above), and use the contact form to reach out to me. I would be more than happy to help you make the right decisions on your telescope equipment purchases.
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