Newts are a pain - what alternative refractor for 130PDS

[BrendanC]

Hi all,

I really, really hate collimating my 130PDS. I also don't like the way the focuser tube impinges on the image. And I'd rather not have to use a coma corrector. So, I'm just wondering, what would an equivalent refractor be? There is another thread similar to this on the forum but it sort of diverged from the original post.

By similar, I mean a roughly similar field of view, for a roughly similar amount of money. I do understand that reflectors tend to be faster and have wider fields of view, but what could about £200-£300 buy me (ideally second-hand)?

And to answer the age-old question about what I want to shoot: DSOs eg M42, Horsehead, globular clusters etc.

Thanks, Brendan

Edited November 24, 2020 by BrendanC

[Alien 13]

Price wise you would be looking at a scope around 70mm but it would have a much shorter focal length than you have now, with refractors you will also need a field flattener (£120-200) so there is no escape from the additional optics required for imaging.

Example here and the FF here

Alan 

Edited November 24, 2020 by Alien 13

[vlaiv]

There is almost exact replacement refractor scope and at the price you are willing to pay.

https://www.firstlightoptics.com/startravel/skywatcher-startravel-120t-ota.html

Although this scope has "only" 120mm of aperture, in reality it is very similar to 130PDS in light gathering ability. This is mainly due to the fact that 130PDS is reflector with central obstruction.

We can do simple math to see what is "equivalent clear aperture" of 130PDS. We can take 94% reflectivity per mirror and 30% secondary obstruction.

(65 * 65 - (0.3*65) * (0.3*65) ) * 0.94 * 0.94 = 3397.2211

sqrt( 3397.2211 ) *2 = ~ 116.57mm

So equivalent clear aperture would be 116.57mm

120mm on the other hand has 4 glass/air surfaces, and if multicoated - every one of those will have transmission of about 99.5%, so we have 120 * 0.995 * 0.995 * 0.995 * 0.995 = ~ 117.62 of clear aperture

So they are equivalent in light gathering, and this scope has 600mm of focal length which is very very close to 650mm FL of 130PDS.

Obviously, only problem will be chromatic aberration, so odds are you won't like your images, but this is only solution to sub £300 equation.

Here is very good review of this scope by Ed Ting (btw, he started a YouTube channel, and I think it is certainly worth checking out) - Orion version (same as SkyWatcher):

Btw, there is another similar scope form Bresser that is a bit more expensive (but probably has better focuser):

https://www.firstlightoptics.com/bresser-telescopes/bresser-messier-ar-127s-635-refractor-ota.html

 

[BrendanC]

Wow. You know, @vlaiv, you really do make a great contribution to this forum. And I love the way you take the rational, maths-based approach. I wouldn't call it 'simple' myself! Very impressive though. Thanks.

[SmokeyJoe]

2 hours ago, Alien 13 said:

Price wise you would be looking at a scope around 70mm but it would have a much shorter focal length than you have now, with refractors you will also need a field flattener (£120-200) so there is no escape from the additional optics required for imaging.

Although a field flattened is great for producing 'perfect' images there are quite a few of us that manage to produce decent images without one.  Especially if you are learning. 

[isolli]

On 24/11/2020 at 22:50, SmokeyJoe said:

Although a field flattened is great for producing 'perfect' images there are quite a few of us that manage to produce decent images without one.  Especially if you are learning. 

Wouldn't the same be true of a coma corrector?