King of image quality - A deep dive into the refractor telescope design

[@HowCow7]

Thanks for making this rabbit hole of astronomy easier!!!

[@raypace6981]

Great video Bogdan! I learned something new even though I am an experienced amateur. Thanks for sharing!

[@BogdanDamian]

@raypace6981 Thank you! Glad the video was helpful!

[@Ajajambo]

Great explanation! My 4” achromat is an F6 and with a Baader semi apo filter mounted permanently, my scope has become a keeper. The semi apo virtually eliminate all the CA and give and amazing contrast on planets.

[@frans_d7989]

I have found that at the end of the day “aperture size counts”, only if it’s practical to use. My 100mm refactor just doesn’t compare in light gather and resolution to my 250mm Dobson. Where the refactors come into its own is that it can be used at the spur of the moment, taking only a few minutes get up and go, where the Dobson sit in the garage waiting for a planned activity.

[@edufau815]

I think it is also important to mention two advantages of refractors compared to the rest of the optical designs, aspects that play in favor of their image quality... 1. Their low sensitivity to decollimation, which makes them practically tubes free of maintenance, while in other designs (although not so much in MCTs), to obtain maximum performance, we must regularly worry about the proper alignment of the optics. And 2. Their reduced acclimatization time, which minimizes the effects of the so-called instrumental turbulence and allows observation sessions to begin with good visualization quality shortly after taking them outside.

[@Youtuber-ku4nk]

It depends on what you want from your telescope. Everything is a tradeoff when speeking of optics, which include regular photography. When improving on one thing you loss in other areas. One major disadvantage in refractors is less resolution than newtonian since the newtonians/dobs often have larger aperture. We also get brighter views and more magnification. With my 8" f/5 I can easily fill the view with the moon with very high resolution and free of aberation.

[@edufau815]

@@KZbinr-ku4nk What you say is undisputed, the theoretical resolution capacity depends only on the aperture, and with almost all non-refracting optical systems you can obtain larger apertures for a price lower than or similar to that of the largest commercial refractor telescope (which if I'm not mistaken is 150mm). But we must also keep something in mind, we should not absolutely correlate the theoretical optical resolution with the ability to define details of an optical system. I'm not saying that they don't have anything to do with it, of course they do, but other variables also come into play, such as the ability to transmit contrast (which in the case of refractors is maximum, due to the absence of central obstruction) or the lower sensitivity to not optimal seeing conditions (due to its smaller aperture). What I mean is that, under the same sky, at the same time, and with both tubes having reached their thermal equilibrium, it may happen that if there is not much difference in aperture between the refractor and the other optical (assuming similar manufacturing qualities), the refractor may give better views of the object in the resolving magnification.

[@allancopland1768]

Excellent video Bogdan. I use a 127mm Maksutov and an SvBony 80mm ED scope with the field flattener. They work well for me.

[@mikehicks4411]

Happy Holidays....as always very informative. Looking forward as to what you bring for 2024.

[@danielpetzen]

I really enjoyed these three videos about the three main types of telescopes, thanks! I just wanted to add that Askar is really stirring up the reflector market with their APO series. Even though their 140 APO Triplet isn't anywhere close to the image equality of the Takahashi, it's still about 2,800 USD and a lot minor optical shortcomings can be corrected in post processing nowadays.

[@ronm6585]

Thank you Bogdan. Merry Christmas to you and yours. 👍🏻🎄🎅🏻❄😊

[@jpastroguy]

Outstanding explanation! Thank you & Happy Holidays!

[@alandyer910]

Just a correction - Huygens didn’t invent the achromatic doublet. He’s famous for using very long aerial telescopes because he didn’t have achromatic lenses. The achromat was invented a century later by Chester Moore Hall in the UK.

[@elray4932]

Great objective review!

[@jtepsr]

Absolutely correct, refractors are much more expensive but I think for astrophotographer, they are my favorite. I have owned a Meade lx200 in 10 in and I saw much fainter objects than my televue 101. Sharper images with better contrast comes with the refractor.

[@scottm599]

Thank you, Bogdan!

[@tuunaes]

Job of corrector lens in Maksutov and Schmidt-Cassegrain is correcting spherical aberration of their spherical primary mirror. And refractor's superior performance applies only for that rough aperture size class. Enough bigger aperture negates disadvantage from reflector's obstruction through sheer higher raw performance to resolve smaller details.

[@GabrielB-mr7jp]

Bun canal, 🐰! Mulțumesc.

[@johnbarry5036]

hi, can I ask, what kind of alt-az mount is that? Looks maybe like a Vixen Mobile Porta? If so, are you happy with that mount? Thanks!

[@BogdanDamian]

@johnbarry5036 Hi! It's the AZ Pronto mount from Skywatcher. I'm very happy with it as it is lightweight and compact and still can handle a 5kg load.

[@3dfxvoodoocards6]

Excellent video, like!

[@artyombeilis9075]

It is important to note that for that for achromat it isn't only important focal ratio but aperture as well. There is so colled CA queivalent that is defined as focal ratio divided by aperture in inches. So 90/900 achromat that is F/10 would have exactly the same CA level as 60mm/400mm achromat - the smaller they are the faster they can be. So 90/900 and 60/400 would have CA around 2.8 - very little if you get close to 3 it becomes very good visually. For 102/660 is about the same as ST80 80/400 - with coefficient around 1.6 - it has some visible CA - but not as good. And so it goes.

[@BogdanDamian]

@artyombeilis9075 Thanks for your input! The chromatic ratio of a refractor is definitely something to take into consideration as well.

[@joseborges8479]

Very informative.

[@BogdanDamian]

@joseborges8479 Glad it was helpful!

[@hanamichizakuragi5021]

Hey mr bogan damian - nice to see you again my friend -

[@angeltensey]

As i understand, ED and APO designs compared to achromatic for same aperture offer better color and sharpness performance, but same light gathering capability. So, if i want quality improvement, i switch from achromatic to ED or APO, but if i want to see fainter objects, i still better go for aperture. Right?

[@BogdanDamian]

@angeltensey Yes, that is correct.

[@angeltensey]

@@BogdanDamian also, are there any filters that can reduce chromatic halo?

[@BogdanDamian]

@angeltensey Unfortunately there isn't a filter that can really help with CA. There are some products, that advertise a reduction in CA, but in my opinion they aren't worth the trouble.

[@angeltensey]

@@BogdanDamian and what if i just stupidly filter out most prominent colors?

[@BogdanDamian]

@angeltensey Then you would be removing all color information including color fringing.

[@cherielektra]

hi. I see you use a SvBOny. How do you feel about the 6.3 focal reducer of that brand versus the celestron? It's much cheaper. However, I am using an 8se. Thank you!

[@BogdanDamian]

@cherielektra Hi, I don't have the reducer, so unfortunately I can't comment on it's quality.

[@elray4932]

Thanks!

[@Astronurd]

I have a 6" triplet refractor and a 12" dobsonian with a premium mirror. My dobsonian blows the refractor clear out of the water. Aperture is indeed king.

[@chrisg9602]

What's a good refactor to just for viewing

[@BogdanDamian]

@chrisg9602 Depending on your Budget the Skywatcher Evostar 90/910, Svbony SV503 102ED or the Skywatcher Evostar 100/900 ED

[@shakabenji]

First ...clear skies ;-)

[@quitolemutt1062]

NE IN 20 males have colour deficits largely in the red/green so true colour images are moot. For those of us with that genetic abnormality does that mean we can choose to spend $ on other aspects of image quality and sharpness being primary. If, like me , you are also cursed with moderate astigmatism you have difficulty getting a round planet large star to resolve doublets. For the cost of a reasonable 80mm refractor I can buy Televue corrective oculars for each eye but there is no such colour repair. Can you suggest the best scope for the 5% so afflicted?

[@Sw87sw87]

Have you been practicing more astrophotography lately?

[@BogdanDamian]

@koalafishmutantbird4682 Yes, I had a couple of nights outside in December. I'm trying right now to get some decent pictures of DSOs as well. It's a bit more challenging than planetary imaging, but I'm getting there 🙂

[@Sw87sw87]

@@BogdanDamian Nice. I recently bought an HEQ5 and ASIair. I’ve only been able to use it a few times due to weather but I have taken some basic beginner DSO pictures. Just learning and improving as I go.

[@BogdanDamian]

@koalafishmutantbird4682 That is great! Keep at it and you will get better and better. As soon as the weather improves, I'll be getting outside as well. Clear skies!

[@zbnmth]

not every reflector has a central obstruction. In fact, there is a whole world of unobstructed designs (and builds!) out there. Schiefspiegler, Herschellian, Pfund even...

[@orinrichards2465]

Thank You for an excellent presentation. I have learnt a lot. They dont teach this stuff at university. You have to learn it on your own.