grjsk

Thanks for your replies; it is indeed the FPL-51 variant I'm talking about here. And I'm only interested in visual btw, not AP. I'm not really sure yet if I prioritize easy access to higher magnification or wider field of view. Until I have decieded I am trying to grasp the abberation aspect of f7(FPL-51) vs f9(FPL-53), and also how the weight/lenght difference will affect the use of the AZ-3 mount.

I’m a handful of months into this hobby, and I’m considering upgrading my simply 70/500 frac to a 100mm doublet. I am struggling with what scope to get though. The two finalist are the TS 102 f7 and the SW 100ED f9. What I am trying to grasp is the subtle differences between these two. I understand the basics, and the pros and cons with, say a f5 vs f15, but how about f7 vs f9? The 100ED is 187mm longer than the TS102, but actually almost a full kg lighter (3 kg vs 4 kg). Would both be okay on a az-3 mount? The 187 extra mm on the 100ED might not be a problem because it is lighter? Does any graphs exist that explain the relationship between CA, SA, coma, astigmatism etc and focal ration? What is the drop off point in a doublet? Or is f7 and f9 close enough that only a truly experienced observer will notice the difference?

As a rookie stargazer myself, I am very intrigued by this very cunundrum: what matters most, aperture or optical quality? ST 120 vs 80ED is an excellent example, and I have them both on my "possibly upgrade one time in the future" list. Would it be correct to assume that the ST120 would show you more objects, but the 80ED would give a much clearer view on the objects it is able to see? Didn't mean to highjack the thread, but I guess the OP might be interested in this as well.

My wife and I have always been intrigued by the universe, devouring all kinds of documentaries for many years. Despite that, I have never owned a telescope. I have been looking up with my handy 8x32 bino, but it is really too small for such a purpose. After watching “The Planets” on BBC I thought it was time to finally buy something bigger. I quickly found this forum, and startet to ask around HERE. I ended up purchasing a used SW Mercury 705 set on an az-3 mount for a mere £35. On Friday the 13th it finally arrived, and here is my first evening with it: My first thought was how incredibly lightweight it all was! I could easily pick it all up with one arm and carry it around. The whole setup is quite small, so I think I might even be able to fit it in my closet, and it turned out it was a breeze to operate it out on my tiny balcony. The manual was missing, but putting everything together was fairly straight forward, so I was ready to go in a couple of minutes. The whole day it had been a clear blue sky, but of course the clouds came creeping after dark. I quickly set up, and went straight for Jupiter. The red dot worked well, and I viewed Jupiter for 0,5 seconds before a cloud came. It never left. Darn! Oh well, next target was Saturn. I had the stock 25 mm eye piece, and it worked well, but at magnification 20x Saturn was just a tiny dot. I switched to the 10 mm, and realized it was terribly dirty. Fingerprints, dirt and dust. I didn’t have proper cleaning equipment, so I didn’t dare to do anything about it, but tried it anyways. I saw rings! What a moment! Still only 50x through a dirty eye piece with quite a bit of light pollution and a somewhat cloudy night, but I saw rings! Couldn’t be happier. I spent some time there, getting to know the scope. I realized that the eye piece moved a bit when I focused (is that what is called backlash?). I tried adjusting the screws on the focuser, seemed like it became a little better. Before I realized it, a full hour had passed. It got even more cloudy, so called it a day. All in all, a great first light! Now I just have to clean that eye piece..

Thanks for all your help. I ended up with a BST 2x Barlow and the SW dielectric 90 star diagonal. Scope arrives in a week, can’t wait!

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)

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!

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?

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

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.

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.

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.

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.

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.

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!