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Parabolic Mirrors

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#1 stanley willheap

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Posted 25 November 2021 - 08:22 AM

Hi there: -)

 

I am looking into getting my first telescope but I am struggling to tell which have parabolic mirrors. Just wondering how vital that is - I understand the different affect each type of mirror has but not sure how significant it is? I have been looking at a few second hand Mak/reflector 127's but so far not found the one. I am also not sure if there is a difference in the performance of mak vs reflector telescopes so any advice on that would also be much appreciated. I am absolutely obsessed and definitely have seen every single Mak 127 currently on the internet: -)

 

Hope everyone is well

 

cheers

 

Stanley

 

 


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#2 rob1986

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Posted 25 November 2021 - 08:26 AM

Hi there: -)

 

I am looking into getting my first telescope but I am struggling to tell which have parabolic mirrors. Just wondering how vital that is - I understand the different affect each type of mirror has but not sure how significant it is? I have been looking at a few second hand Mak/reflector 127's but so far not found the one. I am also not sure if there is a difference in the performance of mak vs reflector telescopes so any advice on that would also be much appreciated. I am absolutely obsessed and definitely have seen every single Mak 127 currently on the internet: -)

 

Hope everyone is well

 

cheers

 

Stanley

not all telescope types use parabolic mirrors. many maks use eliptical mirrors, and scts use spherical mirrors.

 

parabolic mirrors are for newtonian/dobsonians and "classic": cassegrains,


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#3 astrokeith

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Posted 25 November 2021 - 08:27 AM

It can be confusing!

 

Newtonian reflectors, if faster than f10 really should be parabolic, and most will be.

 

Other optical designs, especially Maksutov, are specifically designed not to need parabolic primary mirrors.

 

If its described as a Mak, then you should be confident it will be OK.


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#4 SeattleScott

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Posted 25 November 2021 - 08:36 AM

Unless you buy something super cheap and completely unknown, like say an Abercrombie and Fitch telescope, you can generally be confident these days that it is parabolic, or slow enough not to matter much, for reflectors.

F8 Russian TAL reflectors had spherical mirror and are said to be very good. So you really only need parabolic for the moderately fast F6 and lower models.

Scott
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#5 Bill Jensen

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Posted 25 November 2021 - 08:42 AM

Hi there: -)

 

I am looking into getting my first telescope but I am struggling to tell which have parabolic mirrors. Just wondering how vital that is - I understand the different affect each type of mirror has but not sure how significant it is? I have been looking at a few second hand Mak/reflector 127's but so far not found the one. I am also not sure if there is a difference in the performance of mak vs reflector telescopes so any advice on that would also be much appreciated. I am absolutely obsessed and definitely have seen every single Mak 127 currently on the internet: -)

 

Hope everyone is well

 

cheers

 

Stanley

Stanley, welcome to the fun of getting your first scope, and to CN. 

 

A Mak will normally have a much longer focal length than a standard reflector. As such, you may want to ask yourself what targets you are looking to observe. If you are interested in planets and the moon, a Mak may be a good fit. If you are looking for more general targets including deep sky objects, perhaps a 6-8 inch dob may be a good fit for you, allowing a bit more light and a wider field of view for the deep sky objects. 

 

Perhaps more important is how the scope is mounted. Some of the 5 inch scopes use equatorial mounts that are... not the best. High powers on a shaky mount leads to frustration. 

 

If you have not already done so, you may want to check out your local astro club. They are often very welcoming to new members, provide discounts on the astro magazines, and my local club even has loaner scopes so you can try before you buy. Even if they don't have loaner scopes, you could attend one of their observing sessions, and see what other owners use, and how they work. Hands on is pretty good. 

I also recommend looking at Ed Ting's website, scopereviews.com, as he has videos and great explanations. 

 

Have fun!


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#6 Jon Isaacs

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Posted 25 November 2021 - 08:43 AM

It can be confusing!

 

Newtonian reflectors, if faster than f10 really should be parabolic, and most will be.

 

Other optical designs, especially Maksutov, are specifically designed not to need parabolic primary mirrors.

 

If its described as a Mak, then you should be confident it will be OK.

 

You can calculate the spherical error in a spherical mirror using the equation:

 

Lambda = 22.55 x D (inches) / Fratio3  

 

Diffraction limited is considered to be lambda = 1/4 wave or less.  A 4.5 inch F/8 is 1/5 wave.

 

Generally, one wants a parabolic mirror in a Newtonian. 

 

Figuring out which mirrors are parabolic is not so easy.  Also, some Newtonians are actually Jones-Bird telescopes, these have a fast spherical mirror with a Barlow-like "corrector" in the eyepiece.  These are best avoided.. You can identify a Jones-Bird because it's focal length will be about double the length of the tube.  A common Jones-Bird is a 127mm x 1000mm.

 

Unless you buy something super cheap and completely unknown, like say an Abercrombie and Fitch telescope, you can generally be confident these days that it is parabolic, or slow enough not to matter much, for reflectors.

 

 

Unfortunately, this is not necessarily the case.  For example, the Celestron Astromaster 130 EQ is a 130mm F/5 Newtonian and has a spherical mirror.  There are some 4.5 inch F/4's out there with spherical mirrors.

 

My recommendation to Stanley is that if is looking at a scope, post the question to this forum, to this thread. 

 

Jon


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#7 stanley willheap

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Posted 25 November 2021 - 11:37 AM

Hi everyone

 

Thanks so much for the fast replies. I love the maths in it hehe. In terms of mounts - that was I why I was put off pretty much all the Celestron telescopes. Would you recommend an equatorial mount or AZ? I think GoTo is too expensive for me and also searching the skies sounds like the best bit:) I've mostly been coming across Skywatcher Mak 127's and that's what I am looking for but not really sure what is a good price. Any pointers of what I should expect price wise would be much appreciated. I've been looking on eBay, gumtree, astrobuysell and Facebook marketplace. I'm also not sure whether to try look for kit's or buy everything separately as I'm starting from scratch - I'm a student so definitely on a budget haha.

 

I'm just looking at my local astronomy group now: -) 

 

Thanks again for the warm welcome and all the advice

 

Stanley



#8 TheUser

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Posted 25 November 2021 - 03:42 PM

one think you should know - parabolic mirrors are more difficult to make than spheric. so telescopes with true parabolic mirrors are not cheap. maybe it is better to have good spheric mirror telescope than parabolic of low quality.

 

actually the difference between these two types of Newtonians is not about good or bad, parabolic mirror just allow to get the image with lesser sizes of the tube (i.e. focal length), Newtonians with spheric mirror should have length not less than ~8x of aperture.



#9 SeattleScott

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Posted 25 November 2021 - 04:30 PM

AZ mount generally preferred unless you get an Eq mount with tracking. Then it is a choice of tracking (or potentially GoTo) versus the simplicity of a manual Alt Az. Or just get a GoTo Alt Az.

Scott

#10 sevenofnine

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Posted 25 November 2021 - 04:38 PM

The Orion Apex 127 Mak is listed as "in stock" on the manufacturer's website. During these pandemic times, you just have to get it while you can. There are no guarantees that anything will be back any time soon. I would grab this one now. I have one and it works very well on solar system objects and brighter DSO's. I recommend a small go-to mount for it. Best of luck to you and your choices! waytogo.gif

 

https://www.telescop...cope/p/9825.uts.




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