Why chose an APO for visual?

[Buqibu]

I know apochromatic refractors are considered the best optical telescopes out there, and I get that they are great for astrophotography because of their lighter weight, lack of diffraction spikes, pin point stars etc... But for visual, why buy a 2000 dollar 4 inch apo when you can get a 14 inch dobsonian for the same amount of money? The dob will have better resolution and light gathering capabilites, so why go for the APO? (we are assuming in this scenario that storage is not an issue). Would love to know and hear some opinions, I'm quite new to this. Thanks in advance

Edited September 3, 2021 by Buqibu

[parallaxerr]

Reaching for the 🍿 🤣

[neil phillips]

19 minutes ago, Buqibu said:

I know apochromatic refractors are considered the best optical telescopes out there, and I get that they are great for astrophotography because of their lighter weight, lack of diffraction spikes, pin point stars etc... But for visual, why buy a 2000 dollar 4 inch apo when you can get a 14 inch dobsonian for the same amount of money? The dob will have better resolution and light gathering capabilites, so why go for the APO? (we are assuming in this scenario that storage is not an issue). Would love to know and hear some opinions, I'm quite new to this. Thanks in advance

Simple answer is some people just dont want a big dob. If there was a small scope that could do all a big dob could do. I guess they would be popular. But for various reasons its not everyones preferance. Especially the weight and size. Grab and go appeals for other reasons than just light gathering and resolution power

[Victor Boesen]

I have previously switched from a 10" dobsonian to a 4" APO and I shared some of my thoughts on my choice in this thread:

It might be able to give you some idea as to why someone would prefer one over the other.

[jetstream]

The other night I had to do some Rigel switching around so I put the cooled and collimated 24" Dob in the neighbourhood of Jupiter to line it up. Just so happens the seeing was excellent eventhough I was DSO hunting- go figure.

With a 10mm Delos in the PCII the scopes mag was around 287x...and my jaw dropped to the ground once focused on Jupiter...yeah seriously, aperture opens up the detail a lot.

At the other extreme is the TSA120 which I should have tested that night- this scope punches through the seeing like no get out and will support unlimited mag.

Large aperture is hindered by seeing and possibly optics quality, but has the potential for more detail.

Smaller aperture like APO typically have vg optics and punch through seeing more often than large aperture scopes. Excellent APOs can take seemingly endless mag.

If you want to observe more often get the refractor, espc is seeing is more often avg (or worse) IMHO. Or like me get both a big dob and a refractor.

[Stu]

It’s horses for courses really. Each has their benefits and it totally depends what you prefer and what you want to achieve.

I don’t think you can take storage or convenience out of the equation as these are key elements. Some people (myself included) just love the quality of the views through a top notch 4” apo. No diffraction spikes, beautiful contrast and clarity. You can also get widefield views of large objects which just aren’t possible in a big Dob.

Of course big dobs have their uses too, and can be wonderful if you have access to dark skies and can transport them. For quick sessions though, a quick cooling, manageable refractor is hard to beat.

[jetstream]

1 minute ago, Stu said:

a quick cooling, manageable refractor is hard to beat.

Excellent reply Stu!

My first scope was a 90mm APO that not only gives great lunar/planetary views it has proven itself on countless dark site trips, even showing IC434 before I knew what is was, no filter. This was under super transparent 21.8+ mag skies up north.

[Buqibu]

Thanks for all the replies! So a refractor will always have better contrast even if it is much smaller?

[RobertI]

I am a long time observer but a fairly recent convert to medium sized apo’s. See my recent post detailing my experiences with a new 4” apo over the last year: 

 

[ScouseSpaceCadet]

1 hour ago, Stu said:

It’s horses for courses really. Each has their benefits and it totally depends what you prefer and what you want to achieve.

I don’t think you can take storage or convenience out of the equation as these are key elements. Some people (myself included) just love the quality of the views through a top notch 4” apo. No diffraction spikes, beautiful contrast and clarity. You can also get widefield views of large objects which just aren’t possible in a big Dob.

Of course big dobs have their uses too, and can be wonderful if you have access to dark skies and can transport them. For quick sessions though, a quick cooling, manageable refractor is hard to beat.

Perfect. I've had two 8" dobs and sold them both. My £500 102ED Apo-lite© gives great views of double stars, open clusters, brighter nebula, planets, lunar, solar etc. It's also a reasonable scope to try my hand at astrophotography. Under my Bortle 7 light polluted skies the dob just wasn't worth having. Plus the refractor can be popped out onto an unpowered AZ mount and get going within minutes or for longer sessions, is very comfortable to use mounted on a goto EQ mount with relaxing tracking while the setup time only really taking as much time as the dob cool down. One day I may get myself a decent dob around 12"  and regularly enjoy DSO delights, but I'd have to move house first!

 

Edited September 3, 2021 by ScouseSpaceCadet

[jetstream]

44 minutes ago, Buqibu said:

So a refractor will always have better contrast even if it is much smaller?

Well....

If the central obstruction of the newt is around 20% the contrast loss is not noticeable (IMHO). Contrast is an interesting subject in itself as is the effect of central obstruction.

Technically an optically perfect refractor will have less "contrast" loss than an optically perfect newt. The problem is there is no such thing as perfect optics and it is possible that an aberrated frac can have more "contrast" loss than a high spec newt, obstruction and all.

Wheres @vlaiv?

[Captain Scarlet]

1 hour ago, Buqibu said:

… So a refractor will always have better contrast even if it is much smaller?

For a given like-for-like aperture, yes, because the central obstruction of a Newtonian reduces contrast, and it’s also probably fair to say that most APOs are better collimated than most newts, and mis-collimation also seriously degrades contrast. But it doesn’t take much of an increase in aperture to overcome that like-for-like contrast difference  

Suiter says that the easiest way to increase contrast, as defined by “MTF”, is to buy a bigger scope!

But contrast is but one of many factors, and likely not the most important one in choosing between scopes.

M

Edited September 3, 2021 by Captain Magenta

[Peter Drew]

Not wishing to be controversial but my visual only experience is different.  I have good quality APO refractors, a 4" fluorite doublet, a 5" Triplet and a 6" ED.  The first two I hardly ever use, the 6" ED gets some use as it's piggybacked on a 16" SCT which is handy for comparisons.  In the main I'm prepared to wait for good enough seeing to exploit the greater resolution of a large telescope, I've been observing for well over 60 years and it takes a good night to make an observation worthwhile for me.  I have some large Dobsonians if I want a DSO view and 8" and 12" reflecting binoscopes for the showcase objects.  I fully agree that the refractors give the crisper images and better star images but for me, a non imager, it's only part of my  enjoyment.  Solar Ha observation, my main interest at present is where a good refractor really shines!    🙂  

[vlaiv]

1 hour ago, Buqibu said:

Thanks for all the replies! So a refractor will always have better contrast even if it is much smaller?

Not quite, there are a lot of variables in play here.

There are also different types of contrast to be discussed. Planetary type contrast when there is enough light and we are talking about small features on planets are one type of contrast. There is other type of contrast - in extended objects.

First one has to do with aperture size, seeing and central obstruction.

DSO contrast depends on other things. Here refractors do have edge - but not all refractors. Achromatic refractors lower contrast because of defocused light. Good apo (visual) will have edge there. Mirrored systems have less light throughput than would be suggested by their F/ratio (central obstruction and mirror reflectivity).

Mirror surface can scatter more light around - that reduces contrast. Newtonians are often poorly baffled scopes. For example it is recommended that tube extends above focuser about x1.5 tube diameter. For 200mm scope this means at least 300mm above focuser position on the tube - and this is just to prevent stray light hitting area behind secondary and causing contrast issues.

Refractors are well baffled by design - they have dew shields that also shield objective lens from stray light and they often have baffles inside OTA to improve contrast.

Back to original topic - could you fit M31 in FOV with large newtonian? Could you take your large newtonian in one hand and mount in other? Could you mount Herchel wedge on large newtonian to do some white light solar observing? Maybe put camera on the other end and take some images with it? Add quark and do Ha solar observing?

For some tasks - large dob is better, but for other tasks - well, small apo is simply more sensible solution.

[jetstream]

2 minutes ago, vlaiv said:

here are also different types of contrast to be discussed. Planetary type contrast when there is enough light and we are talking about small features on planets are one type of contrast. There is other type of contrast - in extended objects.

Aw c'mon, wheres all the charts and graphs!

[Alien 13]

You could argue why choose any scope over the naked eye, there will always be far less total light entering your pupil.. For me I cant abide any magnification over X7 so has to be a refractor.

Alan

[Buqibu]

Thanks for the answers, very helpful🙂

[RobertI]

2 hours ago, Peter Drew said:

Not wishing to be controversial but my visual only experience is different.  I have good quality APO refractors, a 4" fluorite doublet, a 5" Triplet and a 6" ED.  The first two I hardly ever use, the 6" ED gets some use as it's piggybacked on a 16" SCT which is handy for comparisons.  In the main I'm prepared to wait for good enough seeing to exploit the greater resolution of a large telescope, I've been observing for well over 60 years and it takes a good night to make an observation worthwhile for me.  I have some large Dobsonians if I want a DSO view and 8" and 12" reflecting binoscopes for the showcase objects.  I fully agree that the refractors give the crisper images and better star images but for me, a non imager, it's only part of my  enjoyment.  Solar Ha observation, my main interest at present is where a good refractor really shines!    🙂  

16” SCTs with 6” ED finders?  12” mirror bino scopes?  You live in an astronomy wonderland Peter! I really must visit the Astronomy Centre soon.  😊

[Stu]

3 hours ago, ScouseSpaceCadet said:

Apo-lite©

Love it 😀👍

[neil phillips]

3 hours ago, jetstream said:

Well....

If the central obstruction of the newt is around 20% the contrast loss is not noticeable (IMHO). Contrast is an interesting subject in itself as is the effect of central obstruction.

Technically an optically perfect refractor will have less "contrast" loss than an optically perfect newt. The problem is there is no such thing as perfect optics and it is possible that an aberrated frac can have more "contrast" loss than a high spec newt, obstruction and all.

Wheres @vlaiv?

Was going to say something along these lines. But you did it for me. A lot is made of contrast yet DP uses a C14 with huge central obstruction i know contrast can be tweaked post processing and is a different question perhaps. To the one about visual. But hes images speak for themselves. Achro refractors and wavelength blurring especially on short F ratio. can have a huge impact on contrast. I personally couldnt live with a optically perfect 4" refractor. over my 245mm Newtonian. I know for a fact no 4" refractor even the best. could outperform that newtonian on the planets imaging. The camera never lies as they say. Visual is subjective at best. I know this wont be popular because i am splitting hairs. But i believe what i say. Thats not to say if someone said fancy a Tak cheap i would say no lol. 

[jetstream]

13 minutes ago, neil phillips said:

Was going to say something along these lines. But you did it for me

I know you have a lot of knowledge on this subject Neil and I totally agree with you. I think that with visual a persons eyes and also seeing come into play in a big way- not much talk about ones eyes in all these types of discussions.

Your images speak for themselves Neil- keep up the good work!

[neil phillips]

20 minutes ago, jetstream said:

I know you have a lot of knowledge on this subject Neil and I totally agree with you. I think that with visual a persons eyes and also seeing come into play in a big way- not much talk about ones eyes in all these types of discussions.

Your images speak for themselves Neil- keep up the good work!

I get why someone would want a nice 4 apo frac. Hell i am scraping the barrel but i have a 70 mm achro F13. 120mm Achro F8.3. 102mm Achro F11. 127mm Achro F9.5 None as nice as a apo, of course. I have these scopes for different reasons. And may slim down at some point. But i think the point i am making is a lot is made of contrast. But it will not increase the performance to the point where my newtonian is defunct. or inferior. Different yes inferior no.  a pristine optically perfect 4" apo, is likely very nice indeed. But heres the kicker its limited no matter how much contrast it has. Thanks for the nice words btw

Edited September 3, 2021 by neil phillips

[neil phillips]

We had a similar discussion before. I think the conclusion is. its great to have different scopes. No need to make another scope inferior if its use is for something it excels at like double stars for example. But No one scope has it all. If it did it would be very popular

[Andrew_B]

4 hours ago, neil phillips said:

Was going to say something along these lines. But you did it for me. A lot is made of contrast yet DP uses a C14 with huge central obstruction i know contrast can be tweaked post processing and is a different question perhaps. To the one about visual. But hes images speak for themselves. Achro refractors and wavelength blurring especially on short F ratio. can have a huge impact on contrast. I personally couldnt live with a optically perfect 4" refractor. over my 245mm Newtonian. I know for a fact no 4" refractor even the best. could outperform that newtonian on the planets imaging. The camera never lies as they say. Visual is subjective at best. I know this wont be popular because i am splitting hairs. But i believe what i say. Thats not to say if someone said fancy a Tak cheap i would say no lol. 

Imaging is very different from visual, especially when it comes to making use of the theoretical capabilities of a scope.

A C14 under UK skies will likely never get close to being the visual instrument it would be for someone living on a mountain in Arizona. Lucky imaging and other techniques are a game changer but they can give a misleading impression of the relative performance of different scopes for the visual observer.

[neil phillips]

2 hours ago, Andrew_B said:

Imaging is very different from visual, especially when it comes to making use of the theoretical capabilities of a scope.

A C14 under UK skies will likely never get close to being the visual instrument it would be for someone living on a mountain in Arizona. Lucky imaging and other techniques are a game changer but they can give a misleading impression of the relative performance of different scopes for the visual observer.

As can peoples perception with there eyes. Which is even more subjective. Anyway i was only really pointing out the contrast question, often isnt the deal breaker a lot make it out to be from my experience. What also is not misleading. is seeing a image live on screen of lunar and planetary. Often i have seen more detail live on screen than through a scope. Though i agree thats not going to be everyones cup of tea. But you would be surprised what different scopes can show live on a screen. No lucky imaging to mislead in that situation. Often the bigger scope with a bigger central obstruction will show more. Though not always of course depending on many factors like collimation and cooldown seeing conditions ect. There is a lot to the question of contrast. And a lot that is also very misleading. 

Edited September 4, 2021 by neil phillips