The 4th Pier: Choosing A Galaxy Scope for My Observatory
Date: Feb 8, 2025
Revised 2-17-25
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The 4th Pier
I have been using three telescopes in my driveway for several years. I used to set them up each night and take them down again in the morning, which, as you can imagine, was a lot of work.
This motivated me to work towards having my own observatory. This involved finding a suitable new property, moving to that location, and then actually building the observatory.
I’ve told this story in other posts, which you can see HERE.
That observatory was designed to have four piers.
But I only have telescope platforms! Oh No!
What about The 4th Pier?
(My friend Rick Albrecht said “The 4th Pier” sounded like the title of a spy novel!)
Nothing is sadder than an empty, unused telescope pier!
I needed to determine what kind of scope I would put on that last pier!
To do this, let’s start by reviewing my current scopes.
My Current Scopes
My current scopes have focal lengths that are pretty evenly spaced out:
My current scopes….
My widest angle scope is the Askar FRA400 platform, which covers about 2.5 x 1.9 degrees of sky.
My longest focal length scope is the Astro-Physics 130mm, which covers roughly 1.24 x 0.83 degrees. This is about half the area of the FRA400.
My William Optics 132mm FLT has a flattener/reducer, which places it almost in the middle of the first two scopes. It covers an area of 1.8 x 1.2 degrees.
This group of scopes has worked well for me. When I choose targets, I assess their scale and assign them to the scope that is best suited for that capture.
But where do I go next? What kind of scope would be a good addition to the 4th Pier?
Strategic Options
There are several ways I could go:
Go wider! Consider getting a scope in the 200-300 mm range that will capture even larger chunks of sky!
To be honest, I have rarely wished I could go wider than 400mm. In fact, during Galaxy season, I have a hard time finding suitable targets for this scope, so I don’t feel compelled to go wider.
Duplicate! I could create a duplicate platform of one of my scopes.
This would allow me to use two scopes to go after the same target, doubling my photon capture.
Or it could allow me to choose another target with a similar image scale.
This is a real possibility!
A Future StarFront Scope! I have been interested in having a scope at a remote observatory like Starfront. Perhaps I could build a scope with a focal length of about 600mm at f/5.5, use it for a year, and then send it off to StarFront.
Thus, I'm getting ready for a 5th remote pier!
Of course, doing this means that once I send this one away, I will still need a scope for the fourth pier! So, this solution is short-term at best.
Go Visual! Set up a scope for purely visual work. Sometimes, people visit and want to see something, but I have to disappoint them by saying that all my scopes have cameras mounted on them and I don’t want to disturb them.
This is a nice idea, but I don’t have visitors often.
I don’t do much visual work.
It seems a shame to dedicate a pier to anything that does not capture photons when the capturing is good.
If I wanted a visual scope, I could store something in the corner, pull it out, and set it up when needed.
A Galaxy Scope: Get something with as long a focal length as my skies will support - running with fast optics - and be better positioned to go after small, interesting objects like galaxies and planetary nebulae.
I currently use the AP130 as my Galaxy scope. However, its focal length is not long enough, and its optics are pretty slow. Don’t get me wrong—the AP has amazing sharpness, contrast, and color—but it is a slower scope.
Getting something longer and faster would be a solid addition to the toolbox.
Choosing A Path
All things considered, I think I would prefer to go the Galaxy Scope route.
Why?
I am constantly drawn to targets that benefit from greater image scale. Despite the slow optics, the AP130 platform has produced some of my favorite images. But even here, I wish I had more focal length.
So, having said this - what would be my best options for putting together a Galaxy scope platform?
I decided to consider this from the Sky back and see what this analysis would suggest as a good path forward.
BUDGET - Another Important Consideration!
I should also mention another very significant aspect of this. And that is budget!
I will soon finish the construction of my observatory. This cost more than I expected. Much more. How much budget should I spend to fill out that 4th pier?
On one hand, I want to maximize the value of the observatory investment. I am now 67 years old. How long will I be able to use the observatory before age and general decrepitude stop me?
20 years? Not likely!
15 years? With luck?
10 Years? I sure hope so.
But it is for a finite time.
If I wait to get the scope I want for, say, several years, I will have an even shorter period of time to reap the rewards!
So, there is an argument to get what you want now and maximize its use over time.
On the other hand, my wife probably thinks there are other things we should or could use our money for.
Yeah, I know—crazy talk!
But I still have to be fair to her. After all, she has been super supportive of both the move and the building of the observatory.
What Might A Galaxy Scope Look Like?
Most galaxies and planetary nebulae are tiny, so I need a longer focal length and a larger image scale.
I would also like fast optics, like f/5 or maybe f/6.
That sounds good, but what focal length is practical?
Any image scale beyond what my local seeing condition can support is kind of wasted. So, let's start there.
What Will My Skies Support?
My skies here are not great. On average, we get seeing of about 2.5 - 3.5 arc seconds. As I said, they are not great.
But let's assume I had a great night and was getting about 2.0 arc seconds. I would want an image scale that supports that. I may not have that opportunity often, but when I do, I would like to take advantage of it.
So, what image scale would I need to accurately sample a sky signal that has a resolution of 2.0 arc seconds?
This brings me back to my college days of learning about digital signal processing and the Nyquist-Shannon Theorem:
If a system uniformly samples an analog signal at a rate that exceeds the signal’s highest frequency by at least a factor of two, the original analog signal can be perfectly recovered from the discrete values produced by sampling.
For example:
if you were dealing with a sine wave signal of 60 hertz, you would need to sample at a frequency of at least 120 hertz. If you sample less than that, you will get distortion in the reconstructed signal called aliasing. If you sample more than this, your reconstructed signal will be good, but you have sampled more than you need to, and thus, you lose efficiency.
Nyquist’s formula suggests that the sampling rate should double the analog signal's frequency. Thus, if our seeing is about 2.0 arc seconds, our sampling rate should be about 1.0 arc seconds.
However, this may be too simplistic:
As the documentation for The Astronomy. Tools CCD Suitability Calculator tells you:
There is some debate about using this for modern CCD sensors because they use square pixels, and we want to image round stars. Using typical seeing at 4” FWHM, Nyquist’s formula would suggest each pixel has 2” resolution which would mean a star could fall on just one pixel, or it might illuminate a 2x2 array, so be captured as a square.
The solution: It is better then to image with a resolution 1/3 of the analog signal; doing this will ensure a star will always fall on multiple pixels so it remains circular.
In our case, we would want a system that can image at 2.0/3 = 0.6667 arc seconds.
How do we achieve this Image scale?
It results from the combination of the telescope and the camera, specifically the pixel size of the camera sensor and the focal length of the telescope.
Let’s make a decision right now about the size of the sensor pixel we will use.
The ZWO ASI2600MM-Pro Camera
The Camera
Let’s assume that we are using a ZWO ASI2600MM-Pro camera. Why?
It has excellent performance
The APS-C format sensor is larger than previous generation cameras
it is modestly priced compared to a full-frame sensor camera
I’ve had a lot of experience with it.
The ASI2600MM-Pro has a pixel size of 3.76 microns.
But why not look at a full-frame camera?
If I were to do that, I would consider the ZWO ASI6200MM-Pro. This is an excellent full-frame sensor camera, but the main difference is the number of pixels captured, not their size. (well, that and PRICE!)
Let’s compare them:
As you can see, they are nearly identical in performance. Since they share a pixel size of 3.76 microns, our computations would be the same in either case.
The APS-C sensor size makes sense to me for my application.
I am going after very small targets. Why do I need a full-frame sensor?
I am looking for a fast optical system. All such systems are challenged in the outer coverage areas, but the APS-C Sensor avoids that.
The smaller APS-C filter allows me to get away with using a smaller filter size. Perhaps unmounted 36mm. This greatly reduces my filter costs.
The smaller sensor size allows plenty of space for a large prism Off-Axis Guider. (We’ll talk about this more later!)
Why Not Go with an OSC Version of the 2600?
With an OSC, you have no filter or filterwheel costs.
Galaxies are broadband targets - so why not use OSC?
While galaxies are broadband targets, I still like to capture Ha data for them and highlight the star-forming regions. Planetary Nebulae are often not broadband targets and benefit from narrowband imaging.
Bottom Line? I am a mono-camera guy! I like the efficiency, flexibility, and results I get from Mono and prefer to work that way.
Telescope Focal Length
So, using the equation for image scale, let’s look at how some focal lengths work with this chosen pixel size:
Focal Length Image Scale
2000mm 0.39 arc seconds
1800mm 0.43 arc seconds
1600mm 0.48 arc seconds
1400mm 0.55 arc seconds
1200mm 0.65 arc seconds
1000mm 0.78 arc seconds
From this, it seems that 1200mm is about the best I could achieve with a 2600 series camera. In fact, 1175mm seems to be the sweet spot.
Astronomy.tools CCD Suitibility Caluclator results for an optimal focal lenght of 1175mm.
Assuming we have a 2” Seeing and a ZWO ASI2600MM-Pro camera, I should look for a telescope with a focal length of around 1200mm.
On the other hand, I also want a fast scope: f/5 - f/5.5 with a flat field would be ideal.
What kind of scope might fit that bill?
Looking At Telescope OTAs
I did a fairly quick search and found some OTAs that would come close to this criteria.
This was not an exhaustive search, but I did find some interesting offerings that we can compare and contrast.
Let's take a look:
Sky-Watcher Esprit 150 mm ED APO Triplet Refractor
Focal Length = 1050mm
f/ratio = f/7
Weight = 32lbs
Length = 40.5 inches
Price = $8250
Note: This is a beautiful scope. But there are a bunch of extremes here as well. The 40-inch length provides a heck of a lever arm for mount driving purposes. The scope optics are on the slow side, while the cost is on the high side. This one is very similar to my current AP 130 platform.
Agena Astro Link:
https://agenaastro.com/sky-watcher-esprit-150mm-ed-triplet-apo-refractor-s11430.html
Sky-Watcher 12" Quattro Imaging Newtonian OTA
Focal Length = 1220mm
f/ratio = f/4
Weight = 57 lbs
Length = 42 inches
Price = $1815 + $440 for coma corrector
Note: This one was a surpise to me. The focal length is in the sweet spot and you can’t beat the f/4 optics for light gathering. But it does weigh in at 57 lbs and is 42 inches long. You would need a very heavy duty mount to drive this one.
Agena Astro Link:
https://agenaastro.com/sky-watcher-12-quattro-imaging-newtonian-telescope-s11230.html
Sharpstar SCA260 v2 f/5 Aspherical Cassegrain with Integrated Field Flattener
Focal Length = 1300mm
f/ratio = f/5
Weight = 32 lbs
Length = 28 inches
Price = $4195
Notes: This is a very interesting new offering! 1300mm @f/5. From what I have heard and seen, it seems like a very solid offering.
Agena Astro Link:
https://agenaastro.com/sharpstar-sca260-aspherical-cassegrain-telescope.html
GSO 10" f/8 Ritchey-Chretien Astrograph Telescope
Focal Length = 2000mm
= 1600mm w/ 0.8x Reducer
f/ratio = f/8
Weight = 36 lbs
Length = 24 inches
Price: $2999 + $499 Teleskop-Service RC 0.8x Reducer
Total Price = $3499
Note: This is a very interesting option. With the reducer, you have a decent focal length and a fairly fast f/6 system! But it is oversampling a bit. I have seen some amazing images resulting from this system. However, to get there, you will need to do some modifications and become experienced with aligning an RC system - which can be tricky.
Agena Astro Link:
https://agenaastro.com/gso-10in-f8-rc-astrograph-ota-truss-tube.html
Celestron 11" EdgeHD 1100 Schmidt-Cassegrain Telescope
Focal Length = 2800mm, 1960mm w/ reducer
f/ratio = f/10, f/7 with reducer
Weight = 28 lbs
Length = 24 inches
Price = $4399 + 0.7 Reducer 839 Total = $5238
Note: This offering has a significant focal length, so using a reducer is a must. However, while many people use this combination to create wonderful images, the quality of the stars is often an issue. This is tied to the use of the reducer. Also, while the scope does have mirror brakes, most samples still will have some level of mirror tilt to deal with.
Agena Astro Link:
https://agenaastro.com/celestron-edgehd-1100-ota-91050-xlt.html
Discussion of Options
Refractor
All of my current scopes are refractors of one kind or another. I really like refractors. Let’s look at some Pros and Cons:
Pros
With good glass, they can produce excellent images
No central obstruction means higher contrast
APO designs mean minimal chromatic aberrations
They offer a sealed tube by their nature, and they are easy to keep clean.
They rarely, if ever - need alignment
Some designs with flatteners or astrographic designs have very flat fields
Cons
Larger diameter objectives can substantially increase cost, weight, and physical size.
Longer focal lengths tend not to be as fast as shorter focal lengths.
Refractors only get so big - reflectors can have larger apertures which impact SNR and Resolution.
As I looked for a refractor solution, the best I could find was f/7, but they were extremely expensive. Their length and weight would require a very high-end drive. As much as I love refractors, I am not sure this is the best solution for me.
So this suggests that some form of reflector might be a good option. I found offerings representing Newtonian, Aspherical Cassegrains, Ritchey-Chretien, and Schmidt-Cassegrain designs. Let’s take a look.
Newtonian
On one level, this looks like a very interesting option! A twelve-inch aperture with a 1220mm focal length running at f/4 at a low price? What is not to love here.?
Well, a few things:
The size and weight of one would make it a beast to mount. You would need a very heavy-duty mount and a lot of space in the observatory.
You would also need a coma corrector, which will add to the cost.
I also don't like the way you would have to mount the camera here. Balancing it seems challenging.
I have knee and back issues, so the heavy tube is not great for me to deal with. Also my pier is taller than I would want for a scope like this, and I prefer not to change it at this point. So, the Newtonian is out for me.
Schmidt-Cassegrain
This is a super popular scope with excellent optics. As I said, many astrophotographers have created amazing images with it, so we know it is capable. It certainly has focal length to spare, but this is also somewhat of a downside. To use it for our purposes, a reducer is necessary; as I noted, these reducers can cause issues with star quality. I am also not a fan of image shifts.
Ritchey-Chretien and Aspherical Cassegrain
That leaves us with two very interesting options: the new Sharpstar SCA260 V2 and the GSO 10” f/8 Ritchey-Chretien.
The 10” RC would need a reducer. But with one, this offers two very capable optical platforms that - in some ways - are difficult to choose between. I have seen some amazing images done with the RC. The SCA260 is very new, but early images seem equally impressive to me.
Feature SCA260 GS0 10” RC
Optical Design Corrected Cassegrain Ritchey-Chrétien (RC)
Aperture 260mm (10.2”) 254mm (10”)
Focal Length 1300mm 1600mm (with 0.8x reducer)
Focal Ratio f/5 f/6.4 (with reducer)
Collimation Difficulty Easier – spherical secondary? more difficult?
Field Corrector Built-in 3-element corrector Requires separate field flattener/reducer
Backfocus Sensitivity Moderate (see note below) Highly sensitive to back focus spacing
Field of View (FOV) Wider Sightly narrower
Image Circle 80mm 44mm (with reducer)
Cooling Fans Yes Yes
Weight ~11kg (24 lbs) ~13kg (29 lbs)
Length 24” 24”
Tube Material Carbon Fiber Carbon Fiber (Truss version available)
Price ~$4,195 $3499
Note: The SC260 has a three-element corrector as part of its imaging chain. They say there is between 70 and 102mm of backspace available. Sharpstar says that as long as you can bring the camera to focus, backspacing just isn’t important.
On the other hand, the RC requires a Reducer. When you add a reducer/flattener to an RC system (or any other optical system), you must get the backspace between the back of the reducer and the camera sensor just right, or you will have distortion (coma, vignetting, etc.).
Often, this backspacing is 55mm - but other systems may have different targets. The thing is that the target backspace is just that - a target. Each optic is different and you need to do a series of tests to find the best position to achieve the best optical results. This can involve adding or subtracting thin spacers or adjusting a threaded barrel. But it can be nit-picky work and the SCA260 does not require this,
So where does that leave us? Frankly - it leaves me with a tough call.
The RC is proven and capable and seems a little cheaper. However, I have heard of many people feeling they need to swap out focusers and such, so the price may not be that different in the end.
The SCA260 is new, faster, and maybe more expensive.
I have read that the SCA260's spherical secondary makes it easier to collimate. However, I don't know how true that is or how significant an advantage it is.
Personally, I am leaning towards the Sharpstar. I like:
The faster optical system
The built-in corrector - which prevents a lot of back-focus issues.
the slight weight advantage
A solid tube framework ( I like the idea of not dealing with shrouds and being able to seal up the tube.)
Update 2-4-25: I have now leaned all the way over. I just placed an order for the SCA260 V2, beating out any price increases that may be driven by the Tariff nonsense going on these days!
Given this, the rest of my analysis will be somewhat based on this decision, but the information shared should hold true for just about any “galaxy” scope.
My new Sharpstar SCA260 V2!
Lookin down the business end!
The view of back - note that I used a William Optics Rotolock eyepeice holder (in gold) with the included Cheshire eyepiece)
Extending the focuser…
The Mount
The Scopes that seem to fit my needs are at least in the 24-29 lbs range by themselves. But of course we will not just be mounting the scope all alone. On no, no! We are adding all kinds of goodies on it as well:
ZWO ASI2600MM-Pro Camera -> 1.54 lbs
ZWO EFW 7-slot 36mm -> 1.1 lbs
OAG Guider -> 0.1 lbs
Guide camera -> 0.3 lbs
EAF Focuser -> 0.61 lbs
Pegasus Astro System Pockt Power Advanced -> 0.3 LBS
So that adds about 3.9 lbs - let’s call it 4 lbs.
We want the mount to handle a load of about 28-32 lbs. Since we want fine detail, we want the mount to be able to handle this load and track it with precision.
Top-Tier Offerings?
One way to accomplish this is to get a premium, well-engineered mount. When I say this, I think of names like Astro-Physics, Software Bisque, 10 Micron, and Planetwave. These mounts are extremely high-quality and have best-in-class performance. You cannot go wrong with them.
However, you pay for this excellence.
But - I just put a lot of money into building my observatory, and frankly, those price points are out of reach for me right now.
So, I think I need to go with something more mid-range.
WaveStrain Technology Mounts?
How about one of those new WaveStrain technology mounts? I think this technology is great, and I own and greatly value my ZWO AM5 mount. While there are WaveStrain mounts out there that claim to handle these kinds of weights, I am not sure. But there are people out there using these mounts for telescopes in the SCA260 class!
Electronic Encoder Mounts (EC)?
EC can be a big help, but again, this greatly increases the cost, and not all implementations are worth the money. So, a non-EC mount would probably be best for me.
At this point, all you can do is get a good mount with a solid reputation and be prepared to tune its performance:
PHD2 runs well and can enhance the performance of a decent mount. More about Guiding later.
Do a good 3D Balancing
Good cable management
What Are Others Using?
I then went to Astrobin.com and searched for images taken with the SharpStar SCA260. This was very interesting. I saw some wonderful images, and every time I found one that impressed me, I looked to see what mount was used for that image.
I found a collection of high-end mounts.
I also saw a group of IOptron CEM70s and StarWatcher EQ6s.
To my surprise, I also saw several images taken using the ZWO AM5 Wavestrain mount!
I would not have contemplated putting a scope of this size on one. But - its specs say that it can handle 33 lbs without a counterweight and 44 lbs with a counterweight. While I might have doubted this combination, the images I saw from it looked amazing.
Looking at Mid-Range Mounts
Let’s look at some good mid-range mounts.
Sky-Watcher EQ8-R
Payload: 110 lbs
Price: $5300
Note: This is a solid, observatory-grade mount that would be excellent to use. I read a lot of good things about this offering! The price however, may put it out of reach for me.
Agena Astro Link:
Sky-Watcher CQ 350
Payload: 77 lbs
Price: $3500
Note: This seems to be a solid mount supporting a payload of 77 lbs for the modest price of $3500. This would be a solid choice that would work very well for this application. A serious contender in my mind!
Agena Astro Link:
https://agenaastro.com/sky-watcher-cq350-pro-head-with-counterweights-no-tripod-s30820.html
IOptron CEM 70
Payload: 70 lbs
Price: $3228
Note: I currently own two IOptron CEM60 mounts, and I have nothing but good things to say about them. So I am predisposed to really like this mount. But I sense that these mounts can be spotty. Some are outstanding, and some are not so. This version does come with the iPolar system, which I tend to like very much. Between this one and the Sky-Watcher CQ350, I am personally leaning in the Sky-Watcher direction.
Agena Astro Link:
IOptron CEM120
Payload: 115 lbs
Price: $5128
Note: This guy is a beast! A big heavy mount that seems to get high marks from those who have it. A local friend is using this with a large cluster of telescopes mounted on it, and he is getting superb results using a 10” RC. This may be outside my current price point, but it still is a solid offering!
Agena Astro Link:
https://agenaastro.com/ioptron-cem120-center-balanced-goto-eq-mount-head-7300.html
ZWO AM5N
Payload: w/o counter weight 33 lbs
with counter weight 44 lbs
Price: $1999
Note: While this mount will technically handle the load of a 30 lbs scope - especially if a counterweight is used - I had not thought that this was a likely option for a galaxy scope. However, I found more than a few people doing great images with this mount, so perhaps I should reconsider!
Agena Astro Link:
https://agenaastro.com/zwo-am5n-strain-wave-equatorial-mount-head-new-am5.html
Sky-Watcher 150i
Payload: w/o counter weight 33 lbs
with counter weight 55 lbs
Price: $2195
NOTE: Given that some people are using the AM5 mount, I thought I should include this one as well.
It can handle more payload when using the counterweight.
It can also allow the axes to be disengaged so that more accurate balancing can be done - something the AM5 cannot do!
Finally, the wiring goes through the mount, and the plugs are on a section that does not move!
Agena Astro Link:
https://agenaastro.com/sky-watcher-wave-150i-strain-wave-drive-mount-s30905.html
At this point, I am torn between going with the Skay-Watcher CQ350 or going with a WaveStrain. If I went Wavestrain, I think the Sky-Watcher 150i might be a possibility!
Guiding
Until now, I have guided using a separate guide scope, which has worked well for me.
But with this new galaxy scope, I want to move to an Off-Axis-Guider or OAG arrangement. The OAG mounts between the telescope and the camera and uses a small prism to peel light off the edge of the field and redirect it to a guide camera.
An example ZWO OAG set up.
The problem with using a separate guide scope is that your guide camera does not see the same exact thing as your main camera. Differential flex can cause this difference to change dynamically when the mount tracks across the sky. You might see a great guide plot but still end up with stars from your main image that are no longer around.
Shorter focal lengths can minimize these differences, and the separate guide scope approach can be quite successful. However, higher focal lengths are less forgiving. In this case, OAG can really help.
As with anything, there are Pros and Cons to this approach.
Pros
The guide camera sees what the man camera sees.
You will guide camera is now working at the same focal length as your main camera.
OAG can reduce the weight of your OTA.
This makes the telescope more streamlined and easier to balance
Often delivers a significant reduction in guiding RMS
Cons
OAGs can take up critical backspacing
Because they have a narrow field of view, a guide camera with a sensor size and sensitivity that match the size of the prism should be used.
Net costs can be higher due to more expensive guide cameras and the cost of the guider.
The prism can cast a dark shadow on your imaging if not adjusted right.
OAGs come in many configurations:
Prism size can be from small to large
Some have differing levels of adjustment
Some have helical focusers, while others just have a set screw used to lock to eyepiece into the proper focus position.
Some support different methods of attaching the OAG to the imaging chain.
My initial working assumption is that I needed one with a large prism.
The SCA260 has an image circle of 80mm, and I plan to use an APS-C sensor. This should have plenty of room for the larger prism, which will pull more light into the guide camera and offer more stars for guiding.
As I searched, I settled on two options that looked attractive to me.
ZWO OAG-L Large Off-Axis Guider with 1.25" Holder
Prism size: 12x12mm
The effective light-passing
aperture is 11.7mmx8.3mm
Thickness: 17.5mm thick
Focusing: Helical Focuser
Price: $199.99
Note: Can be bolted directly to ZWO EFW
Agena Astro Link:
Askar Off-Axis Guider with 1.25" Holder and M42, M48 & M54 Threads
Prism Size: 10mm x 10mm
Thickness: 19.5mm thick
Focusing: Helical focuser
Price: $179
Agena Astro Link:
https://agenaastro.com/askar-off-axis-guider-with-1-25-holder-and-m42-m48-m54-threads.html
The ZWO unit is thinner, has a larger prism, and can attach to the supported ZWO EFW.
The Askar unit is thicker and has a smaller prism, but it is also $20 cheaper.
I think, at this point, my first choice would be the ZWO unit!
Guide Camera
All of my guide scopes sport ZWO ASI290 Mini cameras. Should I use the same?
The ASI290MM-Mini Camera.
The ASI174MM-Mini Camera.
One of the biggest differences between these guide cameras is the size of the sensor. The 290 sports a smaller, high-resolution sensor. The ASI174 Mini, on the other hand, sports a much larger sensor with larger pixels. This larger and more sensitive sensor works well with the larger prism size to bring in more stars for guiding.
A spec comarison of the two guide cameras discussed.
Comparing Guide Camera sensor sizes.
For OAG with a larger prism, I think the ASI174MM-Mini is a solid way to go!
So Why not just Buy the ZWO Duo Camera?
It has the same main sensor that I am currently using, and it also has its very own guide sensor built in!
In many ways, this is a very cool package. It simplifies your imaging chain and eliminates the adjustments you must make when setting up OAG hardware while running a second camera. It is very cost-effective. So why not go that way?
The ZWO 2600 series Duo camera.
As I said, I use mono cameras exclusively.
With the traditional OAG arrangement, the prism pulls off the light before the filter wheel. This means the guide camera sees unfiltered light and the brightest possible image. With the Duo arrangement, the guide sensor would see all light through the filters. All filters attenuate light - so there is less light for the guide sensor to work with. When using narrowband filters, a lot of light is lost. This will make choosing guide stars more difficult. Maybe even impossible in some cases.
So, I chose to use a separate light path and camera to guide me. While some have successfully used the Duo arrangement with narrowband filters, this does not seem like an optimal solution to me.
So Where Does That Leave Us?
The Scope: Already ordered. I have placed my bet on the Sharpstar SCA260 V2!
The Camera: This is likely to be a ZWO ASI2600MM-Pro.
The OAG: I will choose the ZWO OAG-L.
The Guide Camera: The ASI174MM-Mini.
The Mount: I am kind of split between the Sky-Watcher CQ350 and the Sky-Watcher 150i. The budget may drive this final decision!
Stay Tuned…
My new telescope will be delivered next Monday! I will be sharing new posts as my Galaxy Camera comes together during the year - so stay tuned!
