Buying my first scope: a few technicalities

[grjsk]

Hi there,

I am in the process of buying my first scope. I'm quite certain I'll end up buying a used (but from a supplier) Sky-Watcher Mercury-705 AZ3 (70 mm f7,1) just to find out if this hobby is something I enjoy. It comes with a 45 deg. erect diagonal, so I can always use it for terrestrial viewing.

 

I have a few technical questions though;

 

1) It is my understanding that a high f-ratio will give me a low FOV, while a low f-ratio gives a high FOV. But since I will be using a fairly cheap scope, I wont be using magnification above say x100 or x140. Would it be correct to say that a having a high f-ratio wont restrict my FOV as much on low magnification? Would most Messier objects fit nicely in my FOV with the scope I am going for? (disregard problems with low aperture etc for the moment). 

 

2) Chromatic aberration increases with  a low f-ratio AND with a larger aperture. So f7 might not be too bad in regards to CA on a 70 mm scope, but it will be worse on a 100mm?

 

I can get my hands on a Sky-Watcher Startravel-80 AZ3  (80 mm f5) instead, for a bit more money. I am trying to wrap my head around how the increase in aperture and decrease in f-ratio will affect my experience.

 

Thanks!

[vlaiv]

Hi, and welcome to SGL.

Actual FOV depends on couple of factors and is in principle related to F/ratio. Maybe easiest way to think about it is via magnification and apparent FOV of eyepiece.

Eyepieces have apparent FOV (AFOV) and most likely you will get eyepieces supplied with your scope that give you about 52 degrees of AFOV. Eyepieces have focal length which is used to determine the magnification that they give with any particular scope.

To obtain magnification - divide focal length of scope with focal length of eyepiece. To get true field of view - divide AFOV with magnification.

Let's do an example - let's compare 25mm 52 degree eyepiece in two scopes that you mentioned 70mm F/7.1 scope (500mm FL) and 80mm F/5 scope, again 500mm FL.

25mm EP will give same magnification in both scopes - because their focal length is the same and it will be 500mm / 25mm = x20.

You have 52 degrees of AFOV, and divided with magnification that gives - 2.6 degrees of true field visible in both scopes. Although scopes are of different "speed" (f/ratio) they will give same field of view with the same eyepiece because their focal length is the same and hence obtained magnification.

This goes to show that F/ratio alone can't be used to determine true field of view. F/ratio can tell you something though - slower the scope - more likely it is to have longer focal length, and longer focal length means higher magnification with given eyepiece - which in turn means smaller field of view. However this all also depends on aperture (there are two things that go into F/ratio - aperture and focal length). For this reason, while in principle F/ratio is related to true FOV, you need to know what you are comparing (like same aperture scopes with different F/ratio, or different F/ratio but same focal length, or in general case - everything is different - so you need to use above calculation to determine TFOV).

Here is another good thing to know - in general, maximum useful magnification of a telescope is about twice that of aperture in mm. So 70mm scope will have max magnification about x140. 80mm will have max magnification of x160. This rule applies for "perfect" scopes. If scope has some issues - like chromatic aberration or less then perfect figure - you will not benefit from such magnification. On the other hand magnification is also limited by stability of atmosphere. In general average seeing conditions (seeing means how "wobbly/blurred" image is due to atmosphere) you will be limited to x100-150. In better seeing conditions you can go to max x200. Only in excellent seeing conditions it's worth going over x200.

Magnification is not everything. It is useful for planets and the Moon and some types of observing like splitting close double stars. Aperture is more important in the scope.

If your preferences are sharp views of the planets - get your self first scope 70mm F/7.1. If your preferences are deep sky observing - get the scope with 80mm aperture - second one.

In answer to your question relating to levels of chromatic aberration in refractor - there is nice little chart that sums it all, let me see if I can find that for you:

According to this chart, 80mm F/5 scope is in "yellow zone" - CA (short for chromatic aberration) will be visible and obvious, and in principle you can reduce it with appropriate filter. 70mm F/7.1 will also show it, but it will not be as obvious (only on higher magnifications) and you will be able to filter it pretty well with appropriate filter.

I have 102mm F/10, which according to this chart will show about the same level of CA as 70mm F/7.1.

In that scope - CA is there, you can see it on high magnifications, on very high magnifications it is obvious, while on medium you need to search for it. This is of course provided that target you are looking at is bright enough - like the Moon, or planets, or very bright stars. Using appropriate filter almost completely removes it.

I used to have 102mm F/5 scope. That is a bit worse than 80mm F/5 scope and it was all but usable to view planets on anything than low power views. Such telescope is wide field deep sky scope - good for fainter stars / star clusters / wide nebulae and galaxies all viewed on low to medium power.

 

[domstar]

Hi and welcome,

I started with a 70mm f10 scope. I think you'll find most (apart from the full extent of the Andromeda Galaxy in the dark ) Messier objects will fit nicely. The shorter the focal length, the easier it is have a low magnification, the more space you'll see with a low power eyepiece and the easier it will be to star hop. I didn't mind the chromatic aberration on my scope but I did want more aperture. I found that terrestrial observation became boring after about 5 minutes (unless you have a specific reason to use the telescope during the day).

Good luck with the stargazing but be prepared- even a skinflint like me wanted to upgrade after a year.

 

[Carbon Brush]

Hi and welcome to SGL.

Another one to throw into the information overload if I may?

The ST80 is a shorter tube. This makes it easier to get a stable image for the same magnificatuion, and the same mount.
It might also help you in transport and storage.

The ST80 will give good enough views to keep you happy for some time.
I used to own the bigger brother. The ST102.
I thought that was good value for money.
Of course, when I did a side by side against another scope costing 3x the price......

By buying used, you won't lose much money should you decide to sell on and buy something else.
You said you were looking to buy from a supplier. I assume you mean an astronomy equipment retailer, not some other retailer.
 

Enjoy the journey.

David.

[grjsk]

Thank you for all your replies, really helpful. 

vlaiv: excellent explanation, and a very handy table. I'll keep that for later. What kind of filter would you recommend to deal with the CA if I go for the 80mm?

 

domstar: thanks for the Messier info, really nice to know. In the beginning I'm guessing the moon, planets and the Messier objects is what Im going to focus on.

 

Carbon Brush: Retailer (not supplier) is the correct word indeed. They are a dedicated bino/scope shop yes. I've looked at both the ST102 and the ST120, but I think: 1) it is a bit too much to spend before I really know if this is something I would like to spend time on, and: 2) since I have very little experience I don't really know what I prefer to look at. If I really enjoy this I don't mind spending quite a bit more than what a Mercury 705 will cost me, but atleast then I know what kind of scope I really need/want. That is the logic I'm trying to follow anyway.

 

 

[Adam J]

20 minutes ago, grjsk said:

Thank you for all your replies, really helpful. 

vlaiv: excellent explanation, and a very handy table. I'll keep that for later. What kind of filter would you recommend to deal with the CA if I go for the 80mm?

 

domstar: thanks for the Messier info, really nice to know. In the beginning I'm guessing the moon, planets and the Messier objects is what Im going to focus on.

 

Carbon Brush: Retailer (not supplier) is the correct word indeed. They are a dedicated bino/scope shop yes. I've looked at both the ST102 and the ST120, but I think: 1) it is a bit too much to spend before I really know if this is something I would like to spend time on, and: 2) since I have very little experience I don't really know what I prefer to look at. If I really enjoy this I don't mind spending quite a bit more than what a Mercury 705 will cost me, but atleast then I know what kind of scope I really need/want. That is the logic I'm trying to follow anyway.

 

 

I am going to be putting a SW ED80 DS-Pro up for sale tonight, its got a tiny scratch about the size of a eye lash on the lens so ill be selling it at good price £190. The reality is that its performance is compleatly uneffected by this and it will vastly outperform any of the scopes you mention here in terms of colour correction and general build quality. I just thought I would point it out to you as these are normally outside the price range your considering and so it might be an option for you.

Adam 

 

 

Edited September 2, 2019 by Adam J

[grjsk]

45 minutes ago, Adam J said:

I am going to be putting a SW ED80 DS-Pro up for sale tonight, its got a tiny scratch about the size of a eye lash on the lens so ill be selling it at good price £190. The reality is that its performance is compleatly uneffected by this and it will vastly outperform any of the scopes you mention here in terms of colour correction and general build quality. I just thought I would point it out to you as these are normally outside the price range your considering and so it might be an option for you.

Adam 

 

 

That is indeed very interesting. I am clearly out of my depth here, but the prize seems good. Does it come any any diagonals, finders, okulars, case, etc? Without a mount I assume.

[Adam J]

1 hour ago, grjsk said:

That is indeed very interesting. I am clearly out of my depth here, but the prize seems good. Does it come any any diagonals, finders, okulars, case, etc? Without a mount I assume.

See my pm mate. 

[vlaiv]

Ah, I need to correct myself first - that is the second silly thing I've written on SGL today, it is indeed time for a holiday ....

80mm F/5 will provide a bit wider fields than 70mm F/7.1 - simply because it has 400mm focal length and not 500mm like I written in my first post - 80x5 =400 and not 500 like I written for some unknown reason

Everything else is ok in my first post apart from that.

As for filtering, there are number of filters that you can use, some cheap and some more expensive.

In cheap range, simple yellow wratten #8 (light yellow) will give good results on reducing blue/purple halo that is most noticeable in CA. It does impact a slight yellow cast on the image.

On expensive side, there are different "semi apo" and "fringe killer" filters. I've got Baader Contrast Booster filter and it removes much of purple halo on my 4" F/10 scope.

There is another thing that you can do to reduce chromatic aberration of your achromat refractor - you can actually "slow" it down. If you for example have 80mm F/5 scope, you can easily turn it into 50mm F/8 scope that will have color index about 4 in above table and have minimal levels of CA. You do this by use of something called aperture mask - it is simple mask that you can do yourself out of cardboard or piece of plastic - with a central hole of wanted diameter (smaller than diameter of the lens).

This will make image a bit more dim and reduce maximum usable magnification (again it will be x2 the aperture, so for 50mm aperture mask it will be x100) but it will clean up CA nicely.

Have a look here for example:

https://10minuteastronomy.wordpress.com/2017/02/11/why-and-how-to-make-a-sub-aperture-mask-for-a-refractor/

On the other hand if you purchase something like ED80 above - you won't need to bother with any filters and aperture masks. ED80 will have much lower levels of CA (you will be hard pressed to see it even on highest magnifications) than you can get with either filters or aperture masks. Do take into account that it will be quite more expensive route than mentioned ST80 or 70mm Mercury even if diagonal and EPs are included, although it is offered at very low price for such scope, as you will have to purchase an astronomical mount for it as well.

 

 

[cletrac1922]

12 hours ago, grjsk said:

That is indeed very interesting. I am clearly out of my depth here, but the prize seems good. Does it come any any diagonals, finders, okulars, case, etc? Without a mount I assume.

I have the ED80 on a EQ5 pro mount

You can now also get a WiFi adapter for SW mounts

Takes guess work out of SynScan controller, as the app loads GPS, date/time from your mobile device

My Solarmax11 also goes on same mount

 

[Carbon Brush]

An ED80 would keep you happy for much longer than the ST80.
They are a good scope and should you wish to part with it, it won't take long to sell.

David.

[Adam J]

2 hours ago, Carbon Brush said:

An ED80 would keep you happy for much longer than the ST80.
They are a good scope and should you wish to part with it, it won't take long to sell.

David.

Unfortunately the OP is in Norway so difficult to get it out to him, costs quite a bit to post insured. 

[grjsk]

1 hour ago, Adam J said:

Unfortunately the OP is in Norway so difficult to get it out to him, costs quite a bit to post insured. 

It would be problematic indeed. I'm quite sure I'll go for the budget option Mercury-705. I'm able to get it for 30 % less than the ST80, and I am guessing I'll be consentrating on planets, the moon and double stars in the beginning, so the 70mm f7,1 seems a decent fit. 

An 80ED seems like a really good allrounder, and it certainly seems like a good upgrade if I decide to stick with this hobby. Thanks alot for all your help.

[vlaiv]

What is your budget btw?

Maybe check out this little guy instead?

https://www.firstlightoptics.com/evostar/sky-watcher-mercury-707-az-telescope.html

It is F/10 version of 70mm SW scope

[JOC]

For around the same budget you could get a bit more aperture and possibly an easier life, pointing it at things wise, with a Dobsonian mount with something like this:  https://www.firstlightoptics.com/dobsonians/skywatcher-heritage-130p-flextube.html

[grjsk]

12 minutes ago, vlaiv said:

What is your budget btw?

Maybe check out this little guy instead?

https://www.firstlightoptics.com/evostar/sky-watcher-mercury-707-az-telescope.html

It is F/10 version of 70mm SW scope

I don't really have a precise budget; I am quite flexible. What I want to avoid is buying something fairly expensive that just are going to collect dust in the basement. I think I prefer the 705 over the 707. The mount is better, so I may actually use it even if I upgrade the scope. f7 seems like a nice compromise between a fast and a slow scope. Even though I assume I will be spending the most time on planets and double stars, I would at least attempt a few nebulas and galaxies. I am assuming that will be easier with the 705 than the 707? (but I am fully aware it probably will be hard with either one of them!). I fear the 707 is more of a one-trick-pony? Please feel free to correct me if you disagree with my logic.

I simply want to buy something "balanced" that may give me a good idea about what this hobby is about. The 705 seems like nice fit.

 

[grjsk]

18 minutes ago, JOC said:

For around the same budget you could get a bit more aperture and possibly an easier life, pointing it at things wise, with a Dobsonian mount with something like this:  https://www.firstlightoptics.com/dobsonians/skywatcher-heritage-130p-flextube.html

I am fairly set on starting with a refractor with a AZ mount. The mount is more flexible in regards to location, I can use it for terrestrial viewing as well, and no collamination. 

[vlaiv]

10 minutes ago, grjsk said:

I don't really have a precise budget; I am quite flexible. What I want to avoid is buying something fairly expensive that just are going to collect dust in the basement. I think I prefer the 705 over the 707. The mount is better, so I may actually use it even if I upgrade the scope. f7 seems like a nice compromise between a fast and a slow scope. Even though I assume I will be spending the most time on planets and double stars, I would at least attempt a few nebulas and galaxies. I am assuming that will be easier with the 705 than the 707? (but I am fully aware it probably will be hard with either one of them!). I fear the 707 is more of a one-trick-pony? Please feel free to correct me if you disagree with my logic.

I simply want to buy something "balanced" that may give me a good idea about what this hobby is about. The 705 seems like nice fit.

 

Old saying goes: "Best scope is the one you use most often", so you are right in saying that you don't want something that is going to end up collecting dust because it is too complicated/heavy to use regularly, or does not provide enough enjoyment.

There will be no difference in 705 vs 707 with regards of what they can show you on deep sky objects. 707 will have slightly narrower field of view when using same eyepieces, but 700mm focal length is not very large focal length, in fact as far as visual goes it is shorter focal length. I use visual scopes that have 1 meter of more of focal length and never felt "boxed in" with "small" field of view. There are just a few objects out there that can't fit in such FOV - Andromeda galaxy and couple of large nebulae (North American nebula for example) which are couple of degrees in diameter.

If you want to compare the two scopes on FOV that you will have with 25mm stock eyepiece - visit this page:

https://astronomy.tools/calculators/field_of_view/

For example, here is comparison of two scopes on Orion Nebula (something that you will definitively enjoy observing - although probably won't be able to see as much of it with 70mm aperture, unless you have very dark skies).

A side note, I've seen Mercury 705 come with AZ3 mount. I had that mount and was not particularly happy with it - sold it. It was stable enough, but my primary objections were - It has to be rewound every so often to be able to use slow motion controls. It's not very smooth - at least mine was not, and it is very hard to observe near zenith - and that is the place you most often want to observe deep sky objects as there is least of atmosphere and light pollution is often of the least intensity.

I've found a very interesting package that you might want to have a look at:

https://www.firstlightoptics.com/sky-watcher-az-gti-wifi/sky-watcher-mercury-707-az-gte.html

Some people prefer manual scopes to start with, some like go-to ability. For planets and double stars it would be good to have tracking. But this is not main reason I'm telling you to take a look at that package. In the case you decide that observing is not quite your thing, I'm fairly certain that you will sell more easily AzGti mount than any of the starter mounts.

Unfortunately az-eq avant mount is not sold with Mercury line of scopes, neither as stand alone mount, although I feel it would be probably best starter mount for such scope - it can work in both EQ and AZ mode, and can be easily fitted with tracking motor (and rather affordable one - it's like 50 euros or so).

 

[grjsk]

45 minutes ago, vlaiv said:

Old saying goes: "Best scope is the one you use most often", so you are right in saying that you don't want something that is going to end up collecting dust because it is too complicated/heavy to use regularly, or does not provide enough enjoyment.

There will be no difference in 705 vs 707 with regards of what they can show you on deep sky objects. 707 will have slightly narrower field of view when using same eyepieces, but 700mm focal length is not very large focal length, in fact as far as visual goes it is shorter focal length. I use visual scopes that have 1 meter of more of focal length and never felt "boxed in" with "small" field of view. There are just a few objects out there that can't fit in such FOV - Andromeda galaxy and couple of large nebulae (North American nebula for example) which are couple of degrees in diameter.

If you want to compare the two scopes on FOV that you will have with 25mm stock eyepiece - visit this page:

https://astronomy.tools/calculators/field_of_view/

For example, here is comparison of two scopes on Orion Nebula (something that you will definitively enjoy observing - although probably won't be able to see as much of it with 70mm aperture, unless you have very dark skies).

A side note, I've seen Mercury 705 come with AZ3 mount. I had that mount and was not particularly happy with it - sold it. It was stable enough, but my primary objections were - It has to be rewound every so often to be able to use slow motion controls. It's not very smooth - at least mine was not, and it is very hard to observe near zenith - and that is the place you most often want to observe deep sky objects as there is least of atmosphere and light pollution is often of the least intensity.

I've found a very interesting package that you might want to have a look at:

https://www.firstlightoptics.com/sky-watcher-az-gti-wifi/sky-watcher-mercury-707-az-gte.html

Some people prefer manual scopes to start with, some like go-to ability. For planets and double stars it would be good to have tracking. But this is not main reason I'm telling you to take a look at that package. In the case you decide that observing is not quite your thing, I'm fairly certain that you will sell more easily AzGti mount than any of the starter mounts.

Unfortunately az-eq avant mount is not sold with Mercury line of scopes, neither as stand alone mount, although I feel it would be probably best starter mount for such scope - it can work in both EQ and AZ mode, and can be easily fitted with tracking motor (and rather affordable one - it's like 50 euros or so).

 

Hmm, I havent realized you could get a EQ-AZ hybrid. Are they any good? Could something like this be an option:

 

https://www.firstlightoptics.com/sky-watcher-az-eq-avant/sky-watcher-skymax-102-az-eq-avant.html

Edited September 3, 2019 by grjsk

[vlaiv]

9 minutes ago, grjsk said:

Hmm, I havent realized you could get a EQ-AZ hybrid. Are they any good? Could something like this be an option:

 

https://www.firstlightoptics.com/sky-watcher-az-eq-avant/sky-watcher-skymax-102-az-eq-avant.html

Well, I was thinking about mentioning that scope, but decided not to because you mentioned previously that:

1 hour ago, grjsk said:

I am fairly set on starting with a refractor with a AZ mount. The mount is more flexible in regards to location, I can use it for terrestrial viewing as well, and no collamination. 

As far as AZ-EQ avant goes, I have no idea how good it is. I like the idea of such mount, and I think that it is probably one of better light weight grab & go beginner mounts. It looks versatile, and only down side that I can see is fairly small capacity of only 3kg. Then again, one can't expect grab and go small mount to handle 10kg of load

Here is fairly nice review video of this mount:

https://www.youtube.com/watch?v=wkVergOxt_E&t=706s

I already fast forwarded the video to important part (102mm Mak and AZ-EQ avant section as video overviews whole avant range, but you have "jump list" in description to go to particular segment of review).

Here is quick list of differences between Mak102 and small refractor that you need to be aware of before deciding:

- Mak will have significantly higher magnification and smaller FOV with same eyepiece because it has about twice focal length of refractors (more than twice compared to 705 and just a bit less than 707). It can give you about 1.2-1.3 degrees field of view with 32mm eyepiece - still wide enough for general observing, but it simply can't provide you with wide field of view - like 2-3 degrees that is possible with short focal length refractor (or reflector). You can use tool that I linked previously to see field of view with different eyepieces.

- It is better suited for viewing Moon, planets and double stars than those small refractors

- It won't have any chromatic aberration and will give sharper image

- there will be a bit of "cool down" time when you want to use it at higher magnifications - about 10-20 minutes or so for image to stabilize. There is front glass element and main mirror in this scope design - larger pieces of glass need more time to come to ambient temperature. This is more pronounced in winter if you keep your scope at room temperature (like in house / apartment) and take them out to observe.

- you can still use it for terrestrial observations - with 90 degrees astronomical diagonal it will have left/right reversed so you need amici prism to get proper image orientation. However it will give you quite a bit magnification - about x4 or more over standard binoculars (depending on eyepieces used)

- there is a very small chance that you will have to collimate it. It is very slim chance, as these scopes hold collimation very well and are sold already collimated, but unlike small refractors (that can also get out of collimation, although similarly unlikely) there is a way to collimate it yourself. Small cheap refractors usually have lens cell that won't allow for collimation - lens are fixed in place at factory. With Mak you can adjust primary mirror, but it is hardly unlikely that you will have to do it - it will come in good collimation and stay that way for years. In fact - there is a section in user manual that describes this:

https://smallscopefocus.files.wordpress.com/2016/06/makcasscollim1.pdf

and to quote it:

Quote

Collimating is the process of aligning a telescope’s optics.Your StarMax or Apex’s primary mirror was aligned at the factory and should not need adjustment unless the telescope is handled roughly. This manual contains information on howto test the collimation of your telescope and instructions for proper alignment should that be needed.

This is from Orion's manual - but those are same scopes under different brand.

[grjsk]

The 707 package look really interesting. Too bad it’s the Gte, not the Gti. The Gti could be a keeper for many years to come. I assume the capabilites of Freedom Find is really neat to have?

[vlaiv]

32 minutes ago, grjsk said:

The 707 package look really interesting. Too bad it’s the Gte, not the Gti. The Gti could be a keeper for many years to come. I assume the capabilites of Freedom Find is really neat to have?

Well spotted - I did not see it is in fact different mount. I guess freedom find is indeed handy - you can point the scope by hand and it will know where it is pointing and continue tracking.

It sort of defeats the point in getting this package - resale value won't be as good as full Gti version.

[grjsk]

In the end the decision was made for me: I stumbled across a (slightly) used Mercury 705. Got the mount/tripod, eye pieces, finder and diagonal as well. I got it for £35. Couldn’t say no to that. Thank you for all your help!

Edited September 4, 2019 by grjsk

[grjsk]

Follow-up question: I certainly will try out the equipment that comes with the scope (even though I understand that the 10mm has a bit of a bad reputation around here), and I see no reason to upgrade anything just yet. However, I do lack a barlow and a 90 diagonal (the scope comes with a erect 45). Since the 10mm eyepiece "only" gives me 50x, I feel like buying a barlow right away, and I think my back will thank me for buying a 90 diagonal. 

Planning a bit ahead, I would like to buy something that wouldn't need to be upgraded even If I end up buying a better scope down the line. have any of you have any experince with:

Celestron X-Cel Barlow (https://www.firstlightoptics.com/barlows/celestron-x-cel-barlow.html)

versus the much cheaper:

Astro Essentials (https://www.firstlightoptics.com/barlows/astro-essentials-125-2x-barlow-with-t-thread.html) 

and the: 

Sky-Watcher Di-Electric Star Diagonal (https://www.firstlightoptics.com/diagonals/SWdiagonal-20967.html)

versus the much cheaper: 

Astro Essentials 90 (https://www.firstlightoptics.com/diagonals/astro-essentials-90-erecting-prism-diagonal.html)

[ScouseSpaceCadet]

Keeping with the cheap as chips trend, this was my first 90 degree diagonal and I've no complaints for the price:

http://www.opticstar.com/Run/Astronomy/Astro-Accessories-Telescopes-Opticstar.asp?p=0_10_5_1_8_201

I also have the 2" version of the Sky-Watcher dielectric, and again no complaints. Especially as it was £70 on special offer.

Tbh if I bought a small aperture refractor for £35 I'd be tempted to stick with the cheaper end of diagonal. You can always sell the the lot later should you really enjoy the hobby and want something more substantial.

 

 

Edited September 6, 2019 by ScouseSpaceCadet