A friend has a question

[northwalesparry]

Which is better for visual use , planets ,DSOs and clusters .

Skywatcher ST120 ... Cheapest

           Or 

Skywatcher 80ed pro .

Any advice I can pass on would be greatfull .

 

[DirkSteele]

Honestly, and this is coming from a hardcore refractor guy, I would not choose either of them as my primary visual instrument for planets, DSOs and clusters.  While you will need to consider mount choice, I would suggest Skywatcher 200p newtonian.  It will have better reach and performance on DSOs thanks to larger aperture, and will also beat both on planets.  The only drawback is the diffraction spikes could be a distraction but they do not bother me that much.  It is about the same price I believe as the ST120.

https://www.firstlightoptics.com/reflectors/skywatcher-explorer-200p-ota.html

 

If your friend is dead set on one of those two instruments, I would go with the 120 but he must be aware that in will show some substantial chromatic aberration on planets and bright stars which is compromising its performance slightly.  But on DSOs, the larger aperture will win out, even if the absolute optical quality is below the 80ed.

[vlaiv]

I second above for 8" dob being the best by far in that price range. Only consideration is the bulk/weight of it, both for transportation and storage and in use.

If size is major factor, how about about having two scopes? ST120 will be very good on DSOs and wide field, and adding something like Mak102 will fill planetary role better than both of the listed scopes. I think that ST120+Mak102 will still be in 80ED price range. Only difference would be on mounting consideration - if your friend plans to use both scopes at the same time, then mount such as SkyTee or Giro Ercole (mini) that has mountings on both sides is a solution. If not, any mount that is usually used will support the both scopes (like AZ4/AZ5 for example).

[grjsk]

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. 

[PEMS]

Entirely personal but I would go with the better quality 80 ED and sacrifice the aperture. An 80mm is also often considered the best all round aperture refractor to "do anything" , at least within reason.

It can go on a basic mount, or a small goto, do astrophotography, add a solar filter and observe the sun. Moon and most planets will show a fair detail. Althogh as I have read there are not really that many observable planets. Jupiter and Saturn are OK but Mars seems to cause problems.

I would say that abything the 80ED struggles with the 120 ST would struggle with. So the better quality is my choice.

[vlaiv]

27 minutes ago, grjsk said:

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. 

Depends on intended targets. ST120 has rather good optical quality for its intended purpose - viewing DSOs at relatively low magnification. It will show more, and go deeper than 80ED as it will gather x2.25 more light. At low magnifications, problems with CA will not show. You will see purple halos only when looking at the brightest stars. Other than that, objects of deep sky will not look worse in ST120, on the contrary, you will be able to see deeper.

On planets, story is much different. ST120 will give much poorer results than ED80. Although larger aperture will resolve more, CA blur will simply ruin this and ED80 will show more details and views will be much more pleasing.

[Knighty2112]

If the choice is between the two options listed only then the 80ed will give sharper views due to the ED glass, so that would be my choice between the two. The ST120 is OK if your budget is tight.

Edited September 26, 2019 by Knighty2112

[northwalesparry]

Ok thank you all I will pass on your comments . Just a thought but what about the 100ed I believe this now comes with a 0.85 focal reducer included in the price at FLO at just under £800 its more money .. worth it ? Mmm

[vlaiv]

1 hour ago, northwalesparry said:

Ok thank you all I will pass on your comments . Just a thought but what about the 100ed I believe this now comes with a 0.85 focal reducer included in the price at FLO at just under £800 its more money .. worth it ? Mmm

It would be the best of both worlds. It won't have that much aperture disadvantage over ST120, and a bit smaller field (not sure if focal reducer will be useful visually - it is intended for photographic applications), but it will show planets better than each of original options. 8" dob will out perform it on all counts (Except perhaps slightly wider field from ED100).

[northwalesparry]

I'm not sure if the focal reducer can be use visually or not either or is it for AP only . 

So will the star diagonal connect to the focal reducer? . Sorry if that's a dumb question , tried searching the web can't find anything.

[vlaiv]

Depends on focal reducer and star diagonal.

Photographic accessories often use T2 thread for attachment, sometimes M48 (2" filter thread) but often other larger threads. Star diagonal is either 2" or 1.25". There are models with T2 attachment as well.

There is one more important consideration - focal reducers that also act as field flatteners work on specific distance to the sensor (or rather in visual case - focal plane of telescope). This means that you need to place it at exact distance to eyepiece for it to operate to specs. Problem with this is that eyepieces are not par focal - some need a bit more in focus travel, some need a bit more out focus travel. Ideally, eyepiece should have its focal point at the "shoulder", but not all eyepieces do.

Once you know the working distance and type of thread of field flattener / focal reducer, you need to make sure it is mounted at proper distance to eyepiece for it to work as intended. If you miss the distance, you will get different aberrations (astigmatism for example). This is why visual coma correctors often have tuning mechanism, to allow you to alter distance to eyepiece so you can find proper distance for each eyepiece that you use (CCs are also sensitive to proper distance to focal plane).

For that particular reducer, thread size is M48, so it should screw in 2" diagonal (one that has 2"/M48 filter thread), however, optimal working distance is only 55mm - that is enough for AP applications, but not nearly enough for visual as most 2" diagonals are 100+mm of optical path. Only way I see it being used is in straight thru configuration (not sure if anyone would like to use long refractor in straight thru configuration), with suitable 2" extender and 2" eyepiece.

[northwalesparry]

10 minutes ago, vlaiv said:

Depends on focal reducer and star diagonal.

Photographic accessories often use T2 thread for attachment, sometimes M48 (2" filter thread) but often other larger threads. Star diagonal is either 2" or 1.25". There are models with T2 attachment as well.

There is one more important consideration - focal reducers that also act as field flatteners work on specific distance to the sensor (or rather in visual case - focal plane of telescope). This means that you need to place it at exact distance to eyepiece for it to operate to specs. Problem with this is that eyepieces are not par focal - some need a bit more in focus travel, some need a bit more out focus travel. Ideally, eyepiece should have its focal point at the "shoulder", but not all eyepieces do.

Once you know the working distance and type of thread of field flattener / focal reducer, you need to make sure it is mounted at proper distance to eyepiece for it to work as intended. If you miss the distance, you will get different aberrations (astigmatism for example). This is why visual coma correctors often have tuning mechanism, to allow you to alter distance to eyepiece so you can find proper distance for each eyepiece that you use (CCs are also sensitive to proper distance to focal plane).

For that particular reducer, thread size is M48, so it should screw in 2" diagonal (one that has 2"/M48 filter thread), however, optimal working distance is only 55mm - that is enough for AP applications, but not nearly enough for visual as most 2" diagonals are 100+mm of optical path. Only way I see it being used is in straight thru configuration (not sure if anyone would like to use long refractor in straight thru configuration), with suitable 2" extender and 2" eyepiece.

Thank you f, I think I understand what you're saying here , I will pass all you're comments along .

[joe aguiar]

I have skywatcher 80mm pro apo and if it's for planets its hands down that over st 120 which will have ton of bad color.  And cause that u will never get great views from the planets.

St 120 may be better but only if your in dark skies. If its city skys even that size is too small to see dso.

To see dso decent 8 is min and if you want to see all that from city skies dont even consider anything less then 8.

If its 8 inch I would go for an EQ version over dob.

If its 10 or 12 size then dob

[JOC]

On 26/09/2019 at 08:02, northwalesparry said:

visual use , planets ,DSOs and clusters

From what I read on SGL the standard answer to that list is a Skywatcher 200P, most simply Dobsonian mounted, but I think folks also stick them onto those fancy tripod mounts too.  As a satisfied 200P owner I think the rest of SGL is probably correct 😉

[northwalesparry]

Ok how about this then :

Skywatcher 200p

            Or

celestron 8se

both have the same aperture.

 

[vlaiv]

1 hour ago, northwalesparry said:

Ok how about this then :

Skywatcher 200p

            Or

celestron 8se

both have the same aperture.

 

Huh, that one is tricky.

Not sure if you will reach any sort of consensus on that question. Some people prefer newtonian (dob mounted), and others SCT on either alt-az mount or EQ mount.

Each one has pros and cons, and only if you examine pros and cons will you be able to decide which one is better suited for your you (or your friend, whoever is choosing).

In optical terms, well, they are of the same aperture and in principle newtonian will have very slight edge if both are made to same standard of quality (smaller central obstruction, one less optical surface that can introduce aberrations / light scatter, SCT have some spherical aberration when you focus away from perfect focus position - which is possible by focusing mechanism as it moves primary mirror instead of eyepiece).

However, most sample to sample variations in both scopes have larger optical quality difference than those listed above.

I can start list of pros and cons, and others will probably add to each from their own experience.

Newt (dob mounted) pros

- price

- wider field possible

- faster cool down time, less issues with dew (no front corrector plate)

- above mentioned slight optical edge (which might not be there in actual samples, but I list it anyway because I'm in newtonian camp )

Newt cons (again dob):

- harder on eyepieces (but really not that much) as it is F/6 vs F/10 of SCT

- harder to reach planetary magnifications (often barlow is used)

- Bulkier / heavier

- constant nudging may bother some people (although you can get either goto dob, or eq platform for it, or alternatively mount dob on EQ mount or motorized Alt-Az mount. If you put it on EQ mount - Eyepiece will end up in awkward positions most of the time so you will have to rotate the tube)

STC pros (I'll try not to repeat above ones):

- not sure what to put here that has not been mentioned, but probably weight / compactness / portability of OTA (although I did mention that newt con is that it is bulkier/heavier than SCT).

- comfortable eyepiece position (but again same in the dob mounted newtonian, and looking near zenith is easier with dob mounted newt).

SCT cons:

- might suffer from focus shift (since it is focusing with moving mirror, mirror does not stay perfectly parallel the whole time so image can sometimes shift as you focus, particularly when you change focus direction due to slight backlash in focusing mechanism).

Ok, I'm having trouble describing SCT   (I'm really not a fan, and I never used one and I would personally choose dob instead) so someone who has one and likes SCTs should step in to complete lists.

[joe aguiar]

4 hours ago, northwalesparry said:

Ok how about this then :

Skywatcher 200p

            Or

celestron 8se

both have the same aperture.

 

I would do 8se

2nd choice 200 if on eq5 version almost same tube as the dob version but prefer the eq version due to tracking polar aligment, slow motion controls, better ep level then dob and I can add drives later

last would be the 200p on dob version

[northwalesparry]

Thanks for your reply , I think 8se too.

Only because it's more portable and takes up less storage space .😁

Edited October 2, 2019 by northwalesparry