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Northernlight

Torn between 6" Apo & 10" Newt

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Hi all,

I have spent the last 6 month trying to decide what my next imaging scope will be. I've went back and forth between 2 scopes from TS in Germany.  One is their 150mm F8 Triplet Apo and the other is their 10" F4 ONTC Newt. So these are the things i have considered so far

  • So the cost between them is very small by the time i include all the collimaiton tools required for the newt (Approx £2900 vs £3000)    - Draw
  • Using an existing reducer on the Frac, it makes the Image scale & Fov near identical (960mm vs 1000mm)  - Draw
  • The Newt it larger aperture (150mm vs 254mm) and much Faster (F4 vs F6.4 with reducer)  - Win Newt
  • I'm scared / worried about colimating the F4  Vs plug and play on the Frac  -   Win Frac
  • Weight - Frac 12.8kg Vs Newt 12kg  - Close enough  - Draw
  • Balance  - Win for Frac
  • Length - Frac (1120mm)  Vs Newt (1000mm)  - Slight win for Newt
  • Image Scale will be near identical @ around 1.1px - Draw
  • My CCD is a QSI 683 and seems a good match for the focal length on both scopes - Draw

So on the main points, it's pretty much a Draw between them

Now given that i'm looking to use this scope to image Smaller DSO's, galaxies, Comets etc, i have read a few different statements over the last 6 months which may or may not be correct in regards to Imaging:-

  • Aperture is less important when imaging  (usually they say aperture rules, but not sure if that holds true all the time)
  • If you have average sky conditions you will get more usable subs from a Frac
  • You need excellent seeing conditions to take advantage of the extra resolution of the larger aperture

Now given that my biggest fear is colimation, i'm just wondering how much i'm would really loose in resolution on the 6" Apo vs the 10" newt given my average seeing conditions. Is the pain of colimation worth the extra resolution and quicker sub lengths

 

Cheers,

Rich.

 

 

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I have owned two very similar scopes 

Fast Newt - when it was good it was very very good but when it was bad it was horrid.

Quality Frac - BORING - no twiddling - nothing to adjust - no wasted nights trying to collimate at -5 degrees - no hindsight issues 'should I have got the carbon fibre model' etc it just gets on with it night after night after night.

I like tinkering but prefer the plug and play of the frac.

 

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I have a 6" f5 newt plus a 72ED f5.5 and an ED80, both FPL53. 

If collimation is your main concern, forget it! With the right collimator (a few tens €) it doesn't take more than 30" to have the primary perfectly collimated. Once aligned properly the first time, the secondary just stays put. 

This being said, I have an ambivalent position towards diffraction spikes. I kinda like them, but when you remove the camera it might be hard to get it exactly in the same position, then the spikes don't match between different nights. 

And, each time i move the camera I have to take flats again, which isn't the case for the fracs. 

Resolution wise, my f5 newt is just great, and the refractors can't compete, but you're looking for a 6"APO, not much up to that Level.. And stars are normally tighter in an APO than in a newt. 

7 hours ago, Northernlight said:

Aperture is less important when imaging  (usually they say aperture rules, but not sure if that holds true all the time)

Aperture rules for high resolution, so on planets it might be. But for DSOs it's the f/ that matters, yet the newt is still faster. 

You mention a reducer (reducer/corrector I guess, as  the triplet won't have a flat field) but not a coma corrector, absolutely a must at f4. While such a fast newton could be hard to fine tune if anything is less than perfect, finding the perfect backfocus to flatten the 72ED (at f4.4, so quite extreme) took its time too, not plug and play at all. 

I know I did not give you a strong push towards either side, but this is also the reason why i have both. 

From urban sky (Bortle 9) the newt with filter gives better images, but the scope i use most often is the small 72ED, @1.4 kg it's hard to beat as a portable setup.

Fabio

P. S. Any chance to try them at a Star Party, before parting  (pun intended 😄) with such an amount of money? 

Edited by FaDG
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I prefer a frac over a Newtonian but the difference between f4 and f8 is quite substantial. I think I'd be leaning towards the newt but I'm probably the only one. 

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A 150mm triplet APO for £3000 ....

I would not hesitate

Edited by dweller25

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You could go halfway with a Tak epsilon 180, bigger that the frac yet much faster than the newt :D

Alan

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Alien13, you did see the bit about being scared about colimation didn't  you lol.

Olly did you take those images at native F7 ?

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If I had the money I'd always go with the frac reduced. As well as the superb image quality, they are just easier. It's hard enough dodging clouds without chasing the knife edge collimation of a really fast scope. I actually prefer f/5 Newts, and if I was on a tight budget would always opt for one. 

With your budget, frac frac frac :) 

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21 hours ago, FaDG said:

This being said, I have an ambivalent position towards diffraction spikes. I kinda like them, but when you remove the camera it might be hard to get it exactly in the same position, then the spikes don't match between different nights. 

Actually this is not true. If the OTA stays in the same position in its rings, the spikes will remain in the same position.

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Yes, unless you extract the comacorr and camera from the drawtube, i.e. for collimating. 

If then you reintroduce it at a different angle than before (even small) , the spider will be rotated wrt. camera reference, and so will be the spikes. 

 

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1 minute ago, Northernlight said:

Surely star alignment & stacking procedures in programs like pixinsight take care of that ??

If the spikes are aligned differently there is no way they can be overlapped. 

Alignment and Stacking will register the two images for sure, and the stars will be coincident, but the crosses won't match with one another, so you'll have 8 spikes Instead of 4.

Actually there is a way to solve this, which is by plate solving and turning the camera until exactly at the same angle than before, but i never went through this process.

Unless moving the newt by car to a dark site (which I won't do anyway, as I use the 72ED for that) , I found that installing on the mount and moving to storage doesn't harm collimation, so I need to redo it only seldom. 

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In pixinsight, I thought it has a routine to register the images to each other and rotates the images to be exactly the same orientation before stacking, by analyzing the stars in the image 

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15 minutes ago, Northernlight said:

In pixinsight, I thought it has a routine to register the images to each other and rotates the images to be exactly the same orientation before stacking, by analyzing the stars in the image 

Indeed. The image will be rotated to the reference and the spikes with it too. Just don't rotate the OTA in the rings.

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38 minutes ago, alexbb said:

Actually this is not true. If the OTA stays in the same position in its rings, the spikes will remain in the same position.

You're right @alexbb and @Northernlight, if the OTA is not rotated the reference is kept as the image is rotated back in the calibration process.

Apparently at least once the tube rings were not tight enough and the scope rotated a bit while mounting/dismounting without me noticing it. 

Sorry for the mistake.

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4 hours ago, dweller25 said:

A 150mm triplet APO for £3000 ....

I would not hesitate

As long as it arrives from Germany in good collimation, yes. Collimating a doublet refractor is one thing, a triplet, something else altogether !

At least with a newt the collimation is straightforward and very much intended to be done by the owner.

 

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That's a fair shout john, but I suppose I could always send the frac back if it's colimation is misaligned.

I'm sure colimation on a well made newt is nothing to worry about and more about being inexperienced with the process.

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12 hours ago, Northernlight said:

Alien13, you did see the bit about being scared about colimation didn't  you lol.

Olly did you take those images at native F7 ?

Slightly slower since the TEC flattener takes the FL from 980 to about 1015mm. 

Olly

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I would go for the refractor but don't be worried by F4 collimation with a Hotech SCA Laser it is very easy. My Dob is F 4.3 which is near and that is only ever 2-3 minutes work.

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I think I'd go with the 6" frac as well, and I say that as someone with a 12" OCD, er sorry ODK incoming. OCD is what you need to have to collimate an ODK .

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I am fortunate to have a couple of nice Fracs and a couple of nice Reflectors, personally I love the diffraction spikes, I'm yearning for a fast Newt Astrograph to grab faster images, I want one around 1000-1200mm which would be perfect to fit into my future plans, Peter at 656Imaging has offered to build me a custom Newt, which I will most probably take him up on in the future, as I know it will just blow my mind.

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