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F7 vs F9 (TS102 vs 100ED)


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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?

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I have the sw 100f9 ed it's a very well corrected scope with zero colour. It's been out for years and always got great reviews. 

It's called evostar now but was the black diamond before.  Everthings the same cept the name. B4 that it was called gold pro.

Anyway that's gonna be very hard to beat it's a doublet 53 glass and at f9 that's almost as good as the best. Its rated at 98%on the MTV chart or Abbie rating.

Mine is 6 lbs that light my takahashi 102 TSA is 15 lbs.

The ts doesnt say what it is besides a premium ed glass. So I'm not sure is that 53 glass or 51? 

Even if it was 53 glass being f7 brings it down to 93%which is still good but less if its 51 glass it's about 85% rating.

If u could find  that out that would be good. I dont see many ts scopes in cdn at all since its mainly sold over there more and here they probably got another similar name brand.

If its 53 glass then do u want bit more portable? Or best colour image u can get?. If its portability u want then do ts, if image quality then SW 100ed

However if the ts is a 51 glass I think get sw 100ed there will be a difference in 85% to 98%

Joejaguar 

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The most obvious difference will be field of view, with the f7 giving you options with a long focal length eyepiece up to say 3.6 degrees or a little more. The f9 will be limited to around 2.8 degrees. There aren't many objects that big, but it is nice for the Veil and North America Nebula for example.

There may be a slight advantage to the f9 for planetary and lunar observing and it will be a little gentler on edge correction for widefield eyepieces but f7 is still not exactly fast so the differences will be slight.

It will also depend on glass type of the ED element and also of the second element which is as important, plus the quality of the figure and polish.

The SW100 ED does have an excellent reputation I must say, and if widefield is not an issue for you then it may well be a good option.

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I believe the TS 102mm ED (non-Photoline) uses an Ohara FPL-51 glass ED element. What type of glass the mating element uses is not defined.

The F/9 100 ED uses an Ohara FPL-53 element mated with a Schott glass element.

The levels of false colour visible in the F/9 are very low indeed from what I recall when I used to own one. The TS 102 that uses FPL-51 will be a lot better in terms of colour correction than an achromat of that aperture and focal ratio but is likely to show a little more false colour than the F/9 Skywatcher.

As Stu says, the F/7 will enable a wider field of view to be delivered and the focuser is likely to be better than the Skywatcher.

Depends on your priorities I suppose ?

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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. 

Edited by grjsk
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If the objectives are figured to a similar quality (which is likely) then there won't be a lot of difference, just a touch more false colour around bright objects and possibly the F/9 might be able to handle higher powers wiht a touch more aplomb. I have a 102mm ED doublet that works at F/6.5 which I think uses an FPL-51 element but the false colour is not intrusive at all.

A well figured F/7 ED doublet is a very versatile scope being able to handle both high powers and deliver wide fields of view at low powers :smiley:

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4 minutes ago, John said:

If the objectives are figured to a similar quality (which is likely) then there won't be a lot of difference, just a touch more false colour around bright objects and possibly the F/9 might be able to handle higher powers wiht a touch more aplomb. I have a 102mm ED doublet that works at F/6.5 which I think uses an FPL-51 element but the false colour is not intrusive at all.

A well figured F/7 ED doublet is a very versatile scope being able to handle both high powers and deliver wide fields of view at low powers :smiley:

Thanks for your swift reply. Do you think both of them will be okay on a simple AZ-3? I will most likely use a simple red dot and 1,25'' diagonal and eypieces, and not of the heavy premium kind. 

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8 minutes ago, grjsk said:

Thanks for your swift reply. Do you think both of them will be okay on a simple AZ-3? I will most likely use a simple red dot and 1,25'' diagonal and eypieces, and not of the heavy premium kind. 

I have only used an AZ3 with a Coronado PST which is tiny, but based on my experience of it I would think that it would struggle with the f/9 100ED, based on a combination of the OTA weight and the tube length. 

You could give it a go to see how it worked for you, but personally I would think that something more like an AZ4 would be more suitable.  However, the AZ4 does not have the slow motion controls that the AZ3 has. 

I have used both the AZ3 mount and the OTA but never together so I am just basing the above on my gut feeling. 

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21 minutes ago, grjsk said:

Thanks for your swift reply. Do you think both of them will be okay on a simple AZ-3? I will most likely use a simple red dot and 1,25'' diagonal and eypieces, and not of the heavy premium kind. 

The F/9 will not be held steadily by an AZ-3. The F/7 would be pretty marginal as well I'm afraid especially if you try and use the sort of magnifications that these scopes are capable of.

An AZ-4 would be much better.

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I really enjoy the slow motion controls, so I don't think the AZ-4 is an option. The AZ-5 has the same weight limit as the AZ-3, but is it regarded as a more sturdy mount overall? That could be an option..

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It's not the weight so much but the length of the scopes that stresses the mount. The AZ-5 might be OK with the F/7. I have doubts as to how suitable it would be with the F/9 but it might be OK. The stock tripod of the AZ-5 is not too stable so a tripod upgrade should be strongly considered.

 

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tsapo805-spot-1000.jpg

56 minutes ago, grjsk said:

Until I have decieded I am trying to grasp the abberation aspect of f7(FPL-51) vs f9(FPL-53)

Look up spot diagrams like this one (from an 80mm f/6.25 triplet) for various telescopes. They show the shape and size of star images across the field; the standard black circle is 10 microns across but sometimes it's another size so beware of that when you compare diagrams.

The first line shows the spot size and shape at the center of the field for each color from violet to deep red. The second line does the same 0.25° from the center, and the last one is 0.5° away from center. The   AIRY DIAMS : 6.7-10.86        REFERENCE : MIDDLE   line probably means Airy disks vary from 6.7µ to 10.86µ across in green light (at the middle of the diagram and of the spectrum), as green light is the standard reference.

You should also view spot diagrams for eyepieces and reflector telescopes to grasp the whole concept.

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weird i made 1 more post from my cell phone but now i see it not here

ok i said this

i have 2 more things to add to this

1- i think get the SW 100ed f/9 that way u get the best corrected/image quailty from a mass produced scope in this price range u can get. If you need a wider view you can always buy the focal reducer (not cheap) but least its an option. That way IF you need/want to make it an f/7 which matches that other scope. BUt its still a much better glass scope with 53 glass. it just will still be abit longer thats all.

so bascally what iam saying u can make the SWf/9 a F/7 BUT if u buy an f7 51 glass scope u cant make it f/9 and better corrected/better image quailty later.

2- or if you have a 2" diagonal and like me a meade #4000 56mm 2"ep then thats only 16x power and 3.25 degree fov which is big. theres only a few things that may be abit bigger and if you go this way u dont need the focal reducer then.

joejaguar

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17 hours ago, John said:

It's not the weight so much but the length of the scopes that stresses the mount. The AZ-5 might be OK with the F/7. I have doubts as to how suitable it would be with the F/9 but it might be OK. The stock tripod of the AZ-5 is not too stable so a tripod upgrade should be strongly considered.

 

Hmm, that's a shame.  You don't happen to have any experience with any of these:

https://www.teleskop-express.de/shop/product_info.php/info/p8069_TS-Optics-Altazimuth-Mount-with-Fine-Adjustment-and-Quick-Release.html

 

https://www.teleskop-express.de/shop/product_info.php/info/p1753_TS-Optics-Altazimuth-Mount-GSAZ-with-fine-adjustment-and-tripod.html

 

The first seems to be the same as the VersaGo III/Twilight 1, and the last one the GSO Skyview. Both seems like a step up from Az-3/Az-5 in regards to stability, without a giant leap in price.

 

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1 hour ago, grjsk said:

Hmm, that's a shame.  You don't happen to have any experience with any of these:

https://www.teleskop-express.de/shop/product_info.php/info/p8069_TS-Optics-Altazimuth-Mount-with-Fine-Adjustment-and-Quick-Release.html

 

https://www.teleskop-express.de/shop/product_info.php/info/p1753_TS-Optics-Altazimuth-Mount-GSAZ-with-fine-adjustment-and-tripod.html

 

The first seems to be the same as the VersaGo III/Twilight 1, and the last one the GSO Skyview. Both seems like a step up from Az-3/Az-5 in regards to stability, without a giant leap in price.

 

I used to have the second mount you've linked to in the form of an Astro-Tech Voyager and used it quite happily with a 102/500 refractor OTA or a 150/750 Newtonian OTA, but I sold it a while ago I'm afraid. 

I really liked the mount a lot but ultimately I wanted something a bit more compact and travel friendly so opted for a Tele-Optik Ercole Mini which is a very capable little mount but again does not have the slow mo controls that you are looking for. 

@JohnSadlerAstro purchased my A-T Voyager and was using it with a 90mm/800 OTA for a while I believe so something a bit closer to the 100ED in dimensions but a bit lighter I suspect. 

Hopefully John see the ping above and be along shortly to give his thoughts on that combo.

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1 hour ago, grjsk said:

Hmm, that's a shame.  You don't happen to have any experience with any of these:

https://www.teleskop-express.de/shop/product_info.php/info/p8069_TS-Optics-Altazimuth-Mount-with-Fine-Adjustment-and-Quick-Release.html

 

https://www.teleskop-express.de/shop/product_info.php/info/p1753_TS-Optics-Altazimuth-Mount-GSAZ-with-fine-adjustment-and-tripod.html

 

The first seems to be the same as the VersaGo III/Twilight 1, and the last one the GSO Skyview. Both seems like a step up from Az-3/Az-5 in regards to stability, without a giant leap in price.

 

I have seen both above mounts and used the top one briefly but I've no long term experience with them.

From what I've seen I would agree that they are both a significant step up from the AZ-3 and probably the AZ-5 as well. I would be more attracted to the 1st one that you link to because of it's steel legged tripod. Heavier but a lot more stable than an aluminum tripod and it will resist the twist force that a longer tubed scope creates much better which = less vibrations and more stability.

The Skytee II mount is also available from that vendor and that is a very steady mount even with an ED120 F/7.5 on board. Another step up the ladder in terms of solidity and capacity. And cost of course:

https://www.teleskop-express.de/shop/product_info.php/info/p4537_TS-Optics-AZ5-Azimutale-Montierung-mit-Stativ-und-Feinverstellung.html

 

 

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Yeah, seems like the cost is steadily rising :D Suddenly I'm up to 1100 euro for scope, mount + tripod and travel case. And I would have to get some dovetail clamps as well it seems. I was considering a 80mm doublet in the beginning (something like a 80ED, 435 euro from FLO) but I figured the jump up from 70mm to 80mm wouldn't be worth it. Might have to reconsider that now, since at least it would be more viable on the AZ-3.

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15 minutes ago, grjsk said:

Yeah, seems like the cost is steadily rising :D Suddenly I'm up to 1100 euro for scope, mount + tripod and travel case. And I would have to get some dovetail clamps as well it seems. I was considering a 80mm doublet in the beginning (something like a 80ED, 435 euro from FLO) but I figured the jump up from 70mm to 80mm wouldn't be worth it. Might have to reconsider that now, since at least it would be more viable on the AZ-3.

This is unfortunately a sad fact with the hobby. The basic alt az mounts supplied with entry level OTAs are just about adequate for the OTA they are supplied with if you don't push their abilities too much. 

However, on a more positive note, once you have found a mount that works for you and your chosen setup it may well out last several OTA upgrades. 

The Skytee that John mentions above definitely falls into that category, it is certainly capable of handling larger OTAs than you are looking at.  I had one for a long time and only sold it on because I decided that I wanted something motorised with goto.....a decision I later regretted. 

With regards to costs, if you look on the used market you can frequently pick up bargains if you're willing to wait. 

I now have both a used ED100 and a used ED120 for less than the cost of a new ED120 which I could not justify new.  Astronomers tend to look after their equipment very well. 

Ade

 

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Go for the Telescope Express TS-AZ5 over the Skytee II. Looks the same but costs more but worth it as you get a properly assembled and adjusted mount. Extra quality control does cost a bit extra. 

Edited by johninderby
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1 hour ago, AdeKing said:

Astronomers tend to look after their equipment very well. 

True. My sixteen-years-old 16x70 Fujinon is like new; I cleaned the objectives today (wondering how they got these stains by the way, do gremlins go out at night and mess with my equipment when I'm not looking?) and they got like new, shiny like jewels. The warranty will last another nine years, so total peace of mind. 

Not everything has that 25-years warranty, but as johninderby says, it's worth buying solid equipment without necessarily going for the top-class and top-price gear. It's more stable and trouble-free, and the resale value is good while still moderate enough that the new buyer makes a bargain.

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26 minutes ago, Ben the Ignorant said:

Look at that, slashed prices on minimal-power, maximal-field eyepieces that give the right exit pupil for scopes around f/7 or f/9!

https://www.bresser.de/Sale/EXPLORE-SCIENTIFIC-Maxvision-68-Okular-40mm.html

https://www.bresser.de/Sale/EXPLORE-SCIENTIFIC-68-Ar-Okular-34mm-2.html

 

Hanging around on this forum gets more costly by the minute!

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I found the AZ5 acceptable for my 100mm F7.4 and it did carry the 100mm F9 reasonably well too, although it was at its limit. The SW AZ5 uses worm gears and not a tangent arm as with the AZ3. Also, the AZ5 allows easy observation from the horizon to the zenith without overbalancing or the clutches slipping.

However, the SW AZ4 is better built and much stronger despite its lack of slow motion controls. I've used an AZ4 now for many years and love its simplicity and solidity. Like many mounts at the cheaper end of the market something's got to give, and with all of these mounts its the tripod, unless you opt for the steel version.

My own AZ4 is mounted on an old but very solid Vixen aluminium tripod, and with the clutches properly set, I can observe at very high powers without fighting with the mount. Below is a pic of my 100mm F7.4 and my friends 100mm F9 both happily mounted on SW AZ4 mounts. ☺

1775199243_2019-03-1017_21_35.thumb.png.aace884e38fca5b16ccf1e7f2d9343db.png

Edited by mikeDnight
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