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Yep, you guessed it, First Telescope Advice!


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Hello, fellow gazers!

New guy here, asking the age old question for the bazillionth time.
I've done my best researching the topic, reading through previous posts and watching several hours of youtube content, so I have formed something of a baseline understanding of it.
But first, a little background information:
I've always been fascinated with astronomy and all things space, however for some reason that is beyond me, it has never occurred to me that I could buy a scope and take a look for myself.
I was recently introduced to this world by a friend of mine, who out of the blue showed me pictures of Jupiter and Saturn through his scope. I didn't even know he had one.
I have a good understanding of what is to be expected from someone's home telescope, so instead of being underwhelmed, expecting a Cassini photo, I was in awe.
That basically set me on the path of research that inevitably led me to this wonderful forum. Reading through previous topics everyone seems extremely friendly, patient and open to explain in great detail. Several youtubers warned that astronomy enthusiasts love talking about their gear, so it didn't come to as much of a surprise. Before stumbling on this website however I first discovered a local forum from my country (Bulgaria), however that was absolutely not the case. Reading through "beginner telescope" topics there everyone was being an absolute "elitist" with quotes like "if you're not spending at least 800 pounds (adjusting for currency here) you better quit altogether". It was a quite discouraging, however after seeing all that many of you have had to say, I see that it was also not true.

On to the matter at hand.
Researching types of telescopes, sizes, ratios and what not I've managed to narrow it down a bit.
I'm pretty much set on a refractor. Not only because it "looks like a telescope", which it does btw. The collimation, acclimation and general characteristics of reflectors don't appeal to me as much.
As for budget, I believe I could stretch it to 200 pounds, maybe 250 tops.
Target objects - idk really, I haven't tried anything to know what I prefer. I live in the capital so light pollution is definitely a problem when considering DSOs.
Size doesn't pose a problem. I have space to store it and I'm a big enough man to carry a meter long telescope and tripod. I was really surprised that size was such an often mentioned issue in other discussions.
As far as mounts are concerned. I believe I would prefer an AZ mount, but that is flexible. I checked out some tutorials on EQs and they are really not that complicated and have some clear benefits.
Also completely uninterested in photography at this point. I just want to observe.

Here are several telescopes I've been eyeballing and some questions I have about them:
1. Skywatcher Evostar 90/900
2. Skywatcher Evostar 90/660
3. Celestron Astromaster 80
4. Skywatcher 705 and 707 on the lower end of the spectrum, however as most everyone says, if you can get the bigger aperture, get it.

I understand the difference between the 90/900's f/10 and the 90/660's f/7-ish.
I've been reading up on Barlow lenses and Focal Reducers, of which I see the former being much more prevalent.
Correct me if I'm wrong, but a 2x Barlow would effectively make the 90/660 into a 90/1320 of sorts. In that case would the 660 have more versatility with just that extra Barlow?
Also, does the Barlow lens hinder the quality in a way? For example would a 90/500 with a 2x Barlow be inferior to a 90/1000? If so, in what way? Color? Resolution?

I'd love to hear your thoughts on the listed scopes (especially the first 3). Your experience with them, the positives the negatives, what they are good at and what they aren't. I understand that in this price range for each "+" you have to sacrifice something else. I would like to have a better grasp of the compromises that I'm making. Are there any others that you would recommend?

Edited by ashm4n
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Добър ден @ashm4n and welcome to SGL. :hello2:

SGL's sponsor 'FirstLightOptics have produced this... https://www.firstlightoptics.com/beginner-telescopes.html - very informative, if you have not read it. 

Unfortunately no telescope does all. Depending on what you wish to view/observe and your expectations, then may I suggest either:

 

For lunar and planets, then may I suggest this: 

I have a 're-modded' Meade ETX105 [image below] and it is close enough [in specifications] to the one above, that I use for lunar and planet observing.

A5057402-94DE-4E35-A2DE-D8A6BDEFB67B.thumb.jpeg.2165097e2282e5347993d6249a14bd74.jpeg

 

My other two telescopes that I use are a Celestron C6/SCT-xlt [left] and aTeleVue Ranger [right]... post-4682-0-35025000-1347104185_thumb.jpg

During August 1999, I took the TeleVue Ranger to Varna, to view the solar eclipse on 11.08.1999г. A few nights before and after, I was viewing Jupiter and Saturn from my hotel* room balcony,
which was overlooking the Sea Gardens and Black Sea... and a light-polluted sky.

 

* Hotel Odessos/Хотел Одесос

Edited by Philip R
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My personal choice would be the skywatcher 130p heritage that Philip has linked to.

In my mind refractors are great, but are worse than reflectors at these low price points. Chromatic aberration and star bloat are going to be more noticeable. Also consider that an 80mm refractor might be a lot better than the human eye, but the 130p is a massive step up again from that, and with the right eyepieces can be a deep sky and planetary telescope while also being portable and easy to use!

For visual use I wouldn't stress too much about telescope collimation. The secondary mirror is usually good enough out of the box for visual, and the primary is easy to adjust on skywatcher scopes with a low cost Cheshire eyepiece. Just shine your phone torch down the cheshires mirrored hole and look down to see the cheshires eye shadow and the primary mirror mark, which need to line up. It takes me about a minute or so on my 10" dobsonian although these days I cheat by using a laser!

 

I hope you find the right scope for you, happy hunting : )

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25 minutes ago, Philip R said:

Добър ден @ashm4n and welcome to SGL. :hello2:

SGL's sponsor 'FirstLightOptics have produced this... https://www.firstlightoptics.com/beginner-telescopes.html - very informative, if you have not read it. 

Unfortunately no telescope does all. Depending on what you wish to view/observe and your expectations, then may I suggest either:

 

For lunar and planets, then may I suggest this: 

I have a 're-modded' Meade ETX105 [image below] and it is close enough [in specifications] to the one above, that I use for lunar and planet observing.

A5057402-94DE-4E35-A2DE-D8A6BDEFB67B.thumb.jpeg.2165097e2282e5347993d6249a14bd74.jpeg

 

My other two telescopes that I use are a Celestron C6/SCT-xlt [left] and aTeleVue Ranger [right]... post-4682-0-35025000-1347104185_thumb.jpg

During August 1999, I took the TeleVue Ranger to Varna, to view the solar eclipse on 11.08.1999г. A few nights before and after, I was viewing Jupiter and Saturn from my hotel* room balcony,
which was overlooking the Sea Gardens and Black Sea... and light-polluted.

 

* Hotel Odessos/Хотел Одесос

I think I faintly remember that eclipse, however I was almost 8 years old at the time, but I'm pretty sure the one I'm remembering right now is the one you are talking about :)

Also, yes, I've read the FLO topic. Just looking for some on-hand experience from actual people who have used the scopes.

Edited by ashm4n
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8 minutes ago, pipnina said:

My personal choice would be the skywatcher 130p heritage that Philip has linked to.

In my mind refractors are great, but are worse than reflectors at these low price points. Chromatic aberration and star bloat are going to be more noticeable. Also consider that an 80mm refractor might be a lot better than the human eye, but the 130p is a massive step up again from that, and with the right eyepieces can be a deep sky and planetary telescope while also being portable and easy to use!

For visual use I wouldn't stress too much about telescope collimation. The secondary mirror is usually good enough out of the box for visual, and the primary is easy to adjust on skywatcher scopes with a low cost Cheshire eyepiece. Just shine your phone torch down the cheshires mirrored hole and look down to see the cheshires eye shadow and the primary mirror mark, which need to line up. It takes me about a minute or so on my 10" dobsonian although these days I cheat by using a laser!

 

I hope you find the right scope for you, happy hunting : )

Thank you for your reply.
Do you think CA would be too apparent even in the f/10 scope?

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2 hours ago, ashm4n said:

I understand the difference between the 90/900's f/10 and the 90/660's f/7-ish.

I've been reading up on Barlow lenses and Focal Reducers, of which I see the former being much more prevalent.
Correct me if I'm wrong, but a 2x Barlow would effectively make the 90/660 into a 90/1320 of sorts. In that case would the 660 have more versatility with just that extra Barlow?
Also, does the Barlow lens hinder the quality in a way? For example would a 90/500 with a 2x Barlow be inferior to a 90/1000? If so, in what way? Color? Resolution?

Your calculations are correct on the Barlow use. But (there's always a but!) a 90/900 would have lower chromatic aberration, assuming equal quality. So the Barlow would not correct for that. A good Barlow should be "invisible" in the sense that you shouldn't notice it besides the increased magnification. There's more glass involved, so a loss is inevitable but it's slight.

With a small budget, I agree that you'll get more for your money with a dobsonian reflector, as mentioned above. Don't let collimation scare you, it's intimidating when you read all the stuff about it but the reality is that its not scary at all after a couple of times. Anyway, if your scope doesn't get knocked about it'll never be more than a small tweak and often nothing required.

If you're dead set on a refractor, you might find one second-hand?

Oh and welcome to SGL, keep asking the questions.

Edited by wulfrun
typo
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29 minutes ago, ashm4n said:

Thank you for your reply.
Do you think CA would be too apparent even in the f/10 scope?

Chromatic Aberration [CA] is usually seen on 'cheaper' refractor telescopes and/or spotting-scopes and eyepiece. It can be reduced by use of filters, (i.e. a 'contrast' or 'fringe killer') and are available in 1.25" or 2" and are screwed into the eyepiece or star diagonal nose-piece/barrel.

Edited by Philip R
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35 minutes ago, wulfrun said:

Your calculations are correct on the Barlow use. But (there's always a but!) a 90/900 would have lower chromatic aberration, assuming equal quality. So the Barlow would not correct for that. A good Barlow should be "invisible" in the sense that you shouldn't notice it besides the increased magnification. There's more glass involved, so a loss is inevitable but it's slight.

With a small budget, I agree that you'll get more for your money with a dobsonian reflector, as mentioned above. Don't let collimation scare you, it's intimidating when you read all the stuff about it but the reality is that its not scary at all after a couple of times. Anyway, if your scope doesn't get knocked about it'll never be more than a small tweak and often nothing required.

If you're dead set on a refractor, you might find one second-hand?

Oh and welcome to SGL, keep asking the questions.

I've also come to understand that, CA aside, a refractor would produce a sharper and overall more pleasant image than a reflector. And that in more polluted areas the bigger reflector also takes in much of that pollution as well, resulting in a not-that-great (that I believe is the technical term) image.
I've been checking out the second hand market in my country but it's a pretty small one. Not much variety, especially when I filter for my requirements.

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39 minutes ago, Philip R said:

Chromatic Aberration [CA] is usually seen on 'cheaper' refractor telescopes and/or spotting-scopes and eyepiece. It can be reduced by use of filters, (i.e. a 'contrast' or 'fringe killer') and are available in 1.25" or 2" and are screwed into the eyepiece or star diagonal nose-piece/barrel.

In that case I guess CA shouldn't scare me as much. For the beginning it will be fine, and if I get hooked further I could just invest in a good filter.

Sidequestion.
The collapsible Heritage can retract and extend the focal point. However the sides of the extender are empty, so does that mean it has a focal length of 650, but the field that you see is as if the focal length was ~400 (or however long the tube is) since that's the actual depth at which the mirror is located under the entrance of the tube, and the real restriction regarding the maximum angle at which light can enter?

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25 minutes ago, ashm4n said:

Sidequestion.

The collapsible Heritage can retract and extend the focal point. However the sides of the extender are empty, so does that mean it has a focal length of 650, but the field that you see is as if the focal length was ~400 (or however long the tube is) since that's the actual depth at which the mirror is located under the entrance of the tube, and the real restriction regarding the maximum angle at which light can enter?

No, the focal length of the 130 Heritage is indeed 650mm, f/5, it's determined by the mirror curve and nothing else. If you don't fully extend the upper part you would simply push the focal point so far beyond the focuser* you would not get an eyepiece anywhere near in-focus. Likewise the Heritage 150 is a 750mm, f/5.

They are simply collapsible for storage, so they are half tube (roughly) and half open in use. A shroud is easily made and very advisable, to enclose the extending part when in use.

* and simultaneously lose a lot of light since the secondary mirror is now too small for the light-cone

Edited by wulfrun
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Hi and welcome to SGL.

11 minutes ago, ashm4n said:

Sidequestion.
The collapsible Heritage can retract and extend the focal point. However the sides of the extender are empty, so does that mean it has a focal length of 650, but the field that you see is as if the focal length was ~400 (or however long the tube is) since that's the actual depth at which the mirror is located under the entrance of the tube, and the real restriction regarding the maximum angle at which light can enter?

Collapsible heritage can extend and retract secondary mirror cage / assembly but it can't do anything to its focal point / focus plane.

It is feature of primary mirror and is fixed (unless you bend primary out of shape / re-figure it by grinding).

In order to understand maximum angle at which scope can project image - you need to consider several things.

First is - field of view of scope is usually very small - just few degrees. Fact that you have open tube design is a plus for transportation storage, but minus for viewing. Many people use shroud to shield the scope internals from stray light.

Now onto the field of view.

First thing that determines field of view is focal length. Here is little diagram to help you understand what is going on:

image.png.a11653a6b44bf11789b6ed685bcfb632.png

This is image of reflector system and ray hitting center of the mirror. We can just look at one ray - don't need to look at all rays as they all focus at same point.

With this ray it is easy to see that distance in focal plane from center of the field is function of incoming angle and focal length.

Now, other things come into play. With newtonian, you need to consider size of secondary mirror. If angle is big enough - reflected ray will simply miss secondary and go out at the front of the scope.

Size of secondary also limits the field that can be viewed.

In the end - there is size of eyepiece / focuser tube. Eyepieces have certain size and only those points on focal plane that fit inside eyepiece will be shown. This is called field stop - and when you view thru the eyepiece - it is black edge of the view. That is physical ring in eyepiece that sits in focal plane and blocks any light that does not fall into opening.

This is how field is formed in nutshell.

Size of tube has nothing to do with it.

Back on your question about beginner scope.

If you want to view deep sky objects - which means frequent visits to dark site (you'll need a car and you'll probably need to go at least 50Km away from your city) - then get 130p Heritage.

Here is a good resource for you to check how far away you need to drive:

https://www.lightpollutionmap.info/#zoom=7.14&lat=42.6889&lon=24.4784&layers=B0FFFFFFFTFFFFFFFFFFF

image.png.bfa12032bfc575f5c480c4871e668f80.png

You really need to get to at least green zone to fully enjoy deep sky objects, but darker the better and see if you can get to a higher altitude as well.

If you want telescope that will do it all - then get this one:

https://www.firstlightoptics.com/evostar/sky-watcher-evostar-90-660-az-pronto.html

That scope will give you wide enough field and still be able to show to moon and planets. It is no fuss refractor which you seem to prefer.

Only drawback is chromatic aberration and that az mount. Any mount that you get with a scope with your budget is not going to be very good. Most will have issues with stability, but there are few tricks that you can use to get them to perform better.

In the end, if you plan to do most observing form your town and you want to look at planets and the moon most often - then get this scope:

https://www.firstlightoptics.com/maksutov/sky-watcher-skymax-102s-az-pronto.html

That is excellent little scope for planets and the moon - which does not mean you can't observe DSOs with it (I managed to detect NGC7331 with my Mak102 in dark site).

It is like 4" F/13 refractor with no chromatic aberration.

Lastly, don't be hung up on idea that refractors give sharper image than reflectors. In the budget you have, that is simply not true.

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1 minute ago, wulfrun said:

A shroud is easily made and very advisable, to enclose the extending part when in use.

I know the focal length is measured only when extended. However usually the focal length corresponds with the depth at which the mirror is located in the tube and hence the maximum angle at which light can enter. Therefore bigger focal length results in a narrowed angle and hence smaller FOV. So if we do not cover the extended part, while focal length is 650, the mirror is in the bottom of a smaller 40ish cm tube, so doesn't that result in a larger angle and therefore larger FOV?

I might be wrong, I'm just stating my reasoning for you guys to correct my mistake if I am.

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6 minutes ago, ashm4n said:

I know the focal length is measured only when extended. However usually the focal length corresponds with the depth at which the mirror is located in the tube and hence the maximum angle at which light can enter. Therefore bigger focal length results in a narrowed angle and hence smaller FOV. So if we do not cover the extended part, while focal length is 650, the mirror is in the bottom of a smaller 40ish cm tube, so doesn't that result in a larger angle and therefore larger FOV?

I might be wrong, I'm just stating my reasoning for you guys to correct my mistake if I am.

No, wrong, sorry. Take the mirror out of the tube altogether if you want, the focal length is not changed. Any light reaching it from wider angles will simply not reach the focal point. Indeed, any such light will be only a nuisance.

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27 minutes ago, ashm4n said:

I know the focal length is measured only when extended. However usually the focal length corresponds with the depth at which the mirror is located in the tube and hence the maximum angle at which light can enter. Therefore bigger focal length results in a narrowed angle and hence smaller FOV. So if we do not cover the extended part, while focal length is 650, the mirror is in the bottom of a smaller 40ish cm tube, so doesn't that result in a larger angle and therefore larger FOV?

I might be wrong, I'm just stating my reasoning for you guys to correct my mistake if I am.

It’s best to forget that the top part of the tube is open. The light enters as parallel rays through the top ring which holds the secondary. It heads down the tube and hits the mirror which reflects it and turns it into a set of converging rays which bounce off the secondary  and then meet at the focus point somewhere outside the focuser. That focus point is defined by the curvature of the mirror and is independent of the tube length.

Many people fit a shroud around the open part of the tube which has no effect on the focus point, but does cut down on glare and also tube currents caused by your breath.

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1 hour ago, vlaiv said:

Hi and welcome to SGL.

Collapsible heritage can extend and retract secondary mirror cage / assembly but it can't do anything to its focal point / focus plane.

It is feature of primary mirror and is fixed (unless you bend primary out of shape / re-figure it by grinding).

In order to understand maximum angle at which scope can project image - you need to consider several things.

First is - field of view of scope is usually very small - just few degrees. Fact that you have open tube design is a plus for transportation storage, but minus for viewing. Many people use shroud to shield the scope internals from stray light.

Now onto the field of view.

First thing that determines field of view is focal length. Here is little diagram to help you understand what is going on:

image.png.a11653a6b44bf11789b6ed685bcfb632.png

This is image of reflector system and ray hitting center of the mirror. We can just look at one ray - don't need to look at all rays as they all focus at same point.

With this ray it is easy to see that distance in focal plane from center of the field is function of incoming angle and focal length.

Now, other things come into play. With newtonian, you need to consider size of secondary mirror. If angle is big enough - reflected ray will simply miss secondary and go out at the front of the scope.

Size of secondary also limits the field that can be viewed.

In the end - there is size of eyepiece / focuser tube. Eyepieces have certain size and only those points on focal plane that fit inside eyepiece will be shown. This is called field stop - and when you view thru the eyepiece - it is black edge of the view. That is physical ring in eyepiece that sits in focal plane and blocks any light that does not fall into opening.

This is how field is formed in nutshell.

Size of tube has nothing to do with it.

Back on your question about beginner scope.

If you want to view deep sky objects - which means frequent visits to dark site (you'll need a car and you'll probably need to go at least 50Km away from your city) - then get 130p Heritage.

Here is a good resource for you to check how far away you need to drive:

https://www.lightpollutionmap.info/#zoom=7.14&lat=42.6889&lon=24.4784&layers=B0FFFFFFFTFFFFFFFFFFF

image.png.bfa12032bfc575f5c480c4871e668f80.png

You really need to get to at least green zone to fully enjoy deep sky objects, but darker the better and see if you can get to a higher altitude as well.

If you want telescope that will do it all - then get this one:

https://www.firstlightoptics.com/evostar/sky-watcher-evostar-90-660-az-pronto.html

That scope will give you wide enough field and still be able to show to moon and planets. It is no fuss refractor which you seem to prefer.

Only drawback is chromatic aberration and that az mount. Any mount that you get with a scope with your budget is not going to be very good. Most will have issues with stability, but there are few tricks that you can use to get them to perform better.

In the end, if you plan to do most observing form your town and you want to look at planets and the moon most often - then get this scope:

https://www.firstlightoptics.com/maksutov/sky-watcher-skymax-102s-az-pronto.html

That is excellent little scope for planets and the moon - which does not mean you can't observe DSOs with it (I managed to detect NGC7331 with my Mak102 in dark site).

It is like 4" F/13 refractor with no chromatic aberration.

Lastly, don't be hung up on idea that refractors give sharper image than reflectors. In the budget you have, that is simply not true.

Gotcha on the heritage topic.

What makes you pick the 90/660 over the 90/900 skywatcher? I know the f/10 would be better as far as chromatic aberation goes, so does the larger FOV of the 660 make up for that and then some?

Also the Celestron is nearly 100 pounds cheaper? Would you say it's also 100 pounds worse :D?

Edited by ashm4n
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13 minutes ago, ashm4n said:

What makes you pick the 90/660 over the 90/900 skywatcher? I know the f/10 would be better as far as chromatic aberation goes, so does the larger FOV of the 660 make up for that and then some?

Couple of things.

First, that scope was in "do it all" category, which means, it needs to do wider fields of view as well as higher power.

With 32mm Plossl - it will actually provide you with very decent wide field view, here is comparison on some of the targets to 90/900:

image.png.e0351876434a39340114e116defaa9d8.png

image.png.2fc1226d47d7d6f0c67e2ee49c61ecbe.png

90/660 is red circle - so it will show you more context in wide field of view.

Second thing - it is shorter tube. Shorter tube is easier to mount and will produce less vibrations. I think (but can't be sure) that AZ-Pronto that is bundled with 90/660 is better / more stable mount than Eq2 that is usually bundled with 90/900.

Add to that shorter scope - you get overall better performance as far as mount and vibrations go.

Third thing is related to chromatic aberration. Sure, 90/900 will have less of it, but it will still be there. I have 4" F/10 achromat and sure enough - there is chromatic aberration present when one pushes above certain magnification.

I also had 4" F/5 very short achromat at some point - and yes, it showed much more chromatic aberration, and it was useless on planets - unless you did a few tricks to it. Otherwise, it was very fine scope for what it was intended - lower power views.

You can do some trick to lessen impact of chromatic aberration.

For example - you can create aperture mask for your scope. If you create 70mm aperture mask for 90/660 - 70/660 will probably have less color than 90/900. There is something called color index - that will tell you how much chromatic aberration you can expect from your scope (although it is not 100% set in stone as CA will depend on glass type used, type of achromat and how well lens is figured) and you can compare two scopes for chromatic aberration using that.

It is calculate by dividing F/ratio of the telescope by aperture size in inches.

Use following graph as guideline of performance of achromatic scopes:

post-2597-14073834301145.jpg

On that chart 90/900 is F/10 scope so it gets color index of 2.82 - it still has quite a bit of CA.

70/660 will be F/9.42 scope, and aperture is 2.75. If we divide those two we get CA index of 3.42 so it will perform better.

Only drawback of smaller aperture is a bit less resolution / max power, but it is sometimes worth the trade as CA also lowers contrast / resolution and reduces what you can see.

In any case - you can make different aperture mask - 80mm perhaps, or tune it to your liking.

Same holds true for 90/900 - you can reduce CA by using aperture mask on it (it will even be more effective), but my point is - CA can be controlled in several ways (using filters, aperture mask and so on), so it should not be determining factor at your budget level.

If you want - you can certainly get color free views at 90mm of aperture by using something like 90 or 100 mm maksutov, but you sacrifice field of view more.

34 minutes ago, ashm4n said:

Also the Celestron is nearly 100 pounds cheaper? Would you say it's also 100 pounds worse :D?

I was not aware that Celestron has 90/660 version?

Did you mean 90/900 is 100 pounds cheaper, let me check that.

I've found 90/1000 by Celestron at TS for 300 euro (without tax and shipping), while 90/900 by Skywatcher is 256 euro (on EQ2 mount).

These are all very basic scopes and I might sound a bit like people on your local forum, but do look up price of a decent 2" focuser - you will find that most are as expensive as these basic scopes.

This simply means that you can't expect much in terms of quality of mechanics. Optics won't let you down for the most part - they will show you plenty, but mounts won't be as stable as they can be, focusers won't be as smooth as they can be, eyepieces and diagonal won't be as good as they can be.

For example - Celestron that I mentioned above - has rubber/plastic type of dew shield. Some of these scopes have plastic focusers. This is all done to save on price and allow for decent optics.

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I started with the 90/900 on an eq3-2, then got the Heritage 130 and lived with both at the same time. If space and transportation (even down 1 flight of stairs) comes into the equation, get the Heritage. If not, the refractor is a great shout. It's zero maintenance and ready to go at a moments notice. It will give a great image on planets as it has decent colour correction, much better than a barlowed 900/500. The 90/900 showed me a lot. However at times it felt a little under mounted on the EQ3-2. I wouldn't want to put it on a pronto. An AZ4 would work.

FWIW, i acquired an ed80 and gave the 90 away. I still have Heritage as it's just useful to have around and disappears when you don't need it.

Good luck with your choice.

Edited by lvan
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1 hour ago, vlaiv said:

Couple of things.

First, that scope was in "do it all" category, which means, it needs to do wider fields of view as well as higher power.

With 32mm Plossl - it will actually provide you with very decent wide field view, here is comparison on some of the targets to 90/900:

image.png.e0351876434a39340114e116defaa9d8.png

image.png.2fc1226d47d7d6f0c67e2ee49c61ecbe.png

90/660 is red circle - so it will show you more context in wide field of view.

Second thing - it is shorter tube. Shorter tube is easier to mount and will produce less vibrations. I think (but can't be sure) that AZ-Pronto that is bundled with 90/660 is better / more stable mount than Eq2 that is usually bundled with 90/900.

Add to that shorter scope - you get overall better performance as far as mount and vibrations go.

Third thing is related to chromatic aberration. Sure, 90/900 will have less of it, but it will still be there. I have 4" F/10 achromat and sure enough - there is chromatic aberration present when one pushes above certain magnification.

I also had 4" F/5 very short achromat at some point - and yes, it showed much more chromatic aberration, and it was useless on planets - unless you did a few tricks to it. Otherwise, it was very fine scope for what it was intended - lower power views.

You can do some trick to lessen impact of chromatic aberration.

For example - you can create aperture mask for your scope. If you create 70mm aperture mask for 90/660 - 70/660 will probably have less color than 90/900. There is something called color index - that will tell you how much chromatic aberration you can expect from your scope (although it is not 100% set in stone as CA will depend on glass type used, type of achromat and how well lens is figured) and you can compare two scopes for chromatic aberration using that.

It is calculate by dividing F/ratio of the telescope by aperture size in inches.

Use following graph as guideline of performance of achromatic scopes:

post-2597-14073834301145.jpg

On that chart 90/900 is F/10 scope so it gets color index of 2.82 - it still has quite a bit of CA.

70/660 will be F/9.42 scope, and aperture is 2.75. If we divide those two we get CA index of 3.42 so it will perform better.

Only drawback of smaller aperture is a bit less resolution / max power, but it is sometimes worth the trade as CA also lowers contrast / resolution and reduces what you can see.

In any case - you can make different aperture mask - 80mm perhaps, or tune it to your liking.

Same holds true for 90/900 - you can reduce CA by using aperture mask on it (it will even be more effective), but my point is - CA can be controlled in several ways (using filters, aperture mask and so on), so it should not be determining factor at your budget level.

If you want - you can certainly get color free views at 90mm of aperture by using something like 90 or 100 mm maksutov, but you sacrifice field of view more.

I was not aware that Celestron has 90/660 version?

Did you mean 90/900 is 100 pounds cheaper, let me check that.

I've found 90/1000 by Celestron at TS for 300 euro (without tax and shipping), while 90/900 by Skywatcher is 256 euro (on EQ2 mount).

These are all very basic scopes and I might sound a bit like people on your local forum, but do look up price of a decent 2" focuser - you will find that most are as expensive as these basic scopes.

This simply means that you can't expect much in terms of quality of mechanics. Optics won't let you down for the most part - they will show you plenty, but mounts won't be as stable as they can be, focusers won't be as smooth as they can be, eyepieces and diagonal won't be as good as they can be.

For example - Celestron that I mentioned above - has rubber/plastic type of dew shield. Some of these scopes have plastic focusers. This is all done to save on price and allow for decent optics.

Thank you for the extremely informative reply. That CA table is a great guide. I was led (by myself) to believe that the 90/900 is a super long and therefore very free of CA scope. However I see now that at that aperture you gotta be over f/12 13 or more if CA reduction is your main goal. And yeah I guess you are right about the price point there are no APOs :D.

One thing about the Celestron. It's not 90/900. It's this one

https://www.firstlightoptics.com/celestron-astromaster-series/celestron-astromaster-80eq-md-refractor-telescope-with-motor-drive-smartphone-adapter.html

was just wondering how you think it compares to the skywatchers.

 

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6 minutes ago, ashm4n said:

was just wondering how you think it compares to the skywatchers.

Optics is likely to be the same (and probably from the same factory), but where things get different is accessories.

With this scope you get erect image diagonal - which means your scope will be easily used for day time observation (like spotter scope) - but diagonal itself is still 90 degrees.

45 degrees erect image diagonal is much more useful for spotter role.

Problem with such diagonals is that they are low quality. Both optically and mechanically.

Regular mirror diagonal had about 30mm mirror (clear aperture of 1.25" is about 28mm), but these erect image diagonal actually use roof prism and usually have only about 24mm or even less of clear aperture. Roof prism will give you vertical spike on bright stars in night time observation and in general image is probably going to be less sharp than with regular mirror diagonal.

Next thing that is different is telescope tube itself - you get that plastic dew shield (I really don't like that).

Mount is EQ1 class mount (at least I think), so not very good.

Details like mount and included accessories can make difference in price - just check out this scope:

https://www.firstlightoptics.com/scopetech-telescopes/starbase-80-refractor-and-mount-package.html

Very similar in specs to scopes that you were looking - 80mm F/10 achromat refractor - but it is x2-3 more expensive.

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18 minutes ago, vlaiv said:

Optics is likely to be the same (and probably from the same factory), but where things get different is accessories.

With this scope you get erect image diagonal - which means your scope will be easily used for day time observation (like spotter scope) - but diagonal itself is still 90 degrees.

45 degrees erect image diagonal is much more useful for spotter role.

Problem with such diagonals is that they are low quality. Both optically and mechanically.

Regular mirror diagonal had about 30mm mirror (clear aperture of 1.25" is about 28mm), but these erect image diagonal actually use roof prism and usually have only about 24mm or even less of clear aperture. Roof prism will give you vertical spike on bright stars in night time observation and in general image is probably going to be less sharp than with regular mirror diagonal.

Next thing that is different is telescope tube itself - you get that plastic dew shield (I really don't like that).

Mount is EQ1 class mount (at least I think), so not very good.

Details like mount and included accessories can make difference in price - just check out this scope:

https://www.firstlightoptics.com/scopetech-telescopes/starbase-80-refractor-and-mount-package.html

Very similar in specs to scopes that you were looking - 80mm F/10 achromat refractor - but it is x2-3 more expensive.

I understand. Coincidentally while googling I stumbled across thread of yours from 2 years ago discussing the 90/660 around the time of it's release. It has further swayed me in it's direction.

Everywhere I look it's paid with the az pronto. How good/bad of a mount is it?

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3 minutes ago, ashm4n said:

How good/bad of a mount is it?

Honestly, I have no idea.

I used to have AZ3 mount, and it was not very good.

It had limited range of altitude motion (almost no chance to observe near zenith) and slow motion controls worked in very small range (needed to readjust them after a while). I no longer have it. Otherwise it was not very unstable mount.

This AZ pronto seems to be similar class of the mount. Maybe capable of a bit less weight but slow motions are now fixed and work thru the whole range.

I think that it will still have issues near zenith with long tubes as there is a chance of scope hitting mount legs unless you use some sort of extension tube (there are extension tubes to be purchased separately).

Take a look at this review:

http://www.waloszek.de/astro_sw_az_pronto_e.php

for more detail on the mount in real use.

 

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Hi.

I don't know if you know him already but Danislav Kodzhabashev is involved with the local group in and around Sofia and probably know everyone in a 100klm radius that is into astronomy, I am some way from Sofia near Sevlievo and only rarely go to Sofia these days. I could check with him and message you his phone number if it was OK with him  if you want to meet like minded people.

Alan

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Just to add my few words of advice. Don't get hung up on the chromatic aberration in achromatic scopes.  Many people were perfectly happy with their long focal ratio achromatic refractors before the ED and APO scopes were introduced.  Also note that visible chromatic aberration does not = Ruins Your Evening.  Even with the f5 achromats, the chromatic aberration, while definitely present, is more obvious on some targets (e.g. the edge of the Moon) than on others (e.g galaxies and nebulae)

I have a classic brass 70mm refractor with a very long focal ratio which shows no chromatic aberration at all.

Beware of budget telescopes that are 'too cheap'.  At one point I bought a supermarket refractor which, while in most respects excellent value, had an objective lens which (as I eventually realised) was not good enough. I might have tried sourcing an upgrade, but there was no way of getting the objective lens off without using a saw. 

At one point I had both a narrow-field (f12.5) and a wide-field telescope  in the 4" & 5" aperture range. I used the narrow-field instrument far more, and made little use of the 102mm f5 achromat till I discovered I could image with it.

The Startravel 102mm f5 achromat appears to be a short version of Sky-watcher's Evostar, and for the price is a well-made instrument with the focuser, dew-shield, tube etc all made of metal.  It makes a useful budget imaging scope but is not much good for viewing fine detail on planets etc.

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@ashm4n... please forgive me, but I am not sure if you are Bulgarian, but during my visits to Varna/Варна between 1999 & 2003, there used to a magazine called Телескор [Telescope]. I used to buy a copy every time I saw it on sale at the newspaper kiosks, (I am not sure If the magazine is still available/in print). Though Bulgarian is not my first language, I can translate some of it.

@alan potts... thank you for replying. I hope you did not mind me including you in this topic.

Wishing you both, clear skies. :icon_salut:

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