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So what am I missing, between large aperture reflector versus smaller refractor?


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My telescope happens to be an astro imaging Newtonian at 254 mm f/ 4, this was originally chosen for dual use as in some imaging and some visual. So apart from the weight reduction, what are the pros and cons of a smaller refractor versus a larger reflector?

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59 minutes ago, Cumbrianwolf said:

My telescope happens to be an astro imaging Newtonian at 254 mm f/ 4, this was originally chosen for dual use as in some imaging and some visual. So apart from the weight reduction, what are the pros and cons of a smaller refractor versus a larger reflector?

The main pro of a refractor is no collimation needed. The main con off a refractor is larger apetures are very expensive, also very heavy.

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Just now, AstronomyUkraine said:

The main pro of a refractor is no collimation needed. The main con off a refractor is larger apetures are very expensive, also very heavy.

I am also seeing the FOV winning over on the refractor compared to a reflector. No happy medium, must buy one of each. 😉 if only.

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3 minutes ago, Cumbrianwolf said:

I am also seeing the FOV winning over on the refractor compared to a reflector. No happy medium, must buy one of each. 😉 if only.

Reflectors are great for galaxies, while refractors are probably better for wide angle nebula shots. Then you have the debate about diffraction spikes against no diffraction spikes. Some people love them, others hate them. Some imagers will add diffraction spikes to an image taken with a refractor.

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Having used a big newt for several years and only recently obtaining a short focal length refractor these are my reflections 😄:

Short focal Refractor (70mm fl 336mm)

  1.  It is very rare to reject a sub-frame due to mount tracking and guiding issues if properly setup (HEQ5). So assuming good weather I get 2hrs of subs out of a 2hr session.
  2. The aperture 70mm resolving power is lower (1.5 arc seconds), on the plus side this means that poor seeing is less likely to ruin sub-frames. This means I can image on more nights. (The downside is stars are bigger and finer detail can be missed compared to 250mm.)
  3. The field is much wider of course - great for M31 or M45 or most of the bright bigger nebula.
  4. When you consider the 2.2 arc second per pixel of my camera due to the scope's short focal length the result is a very robust imaging system that is going to give good results under most UK conditions.
  5. Overall less frustrating than a big newt and if I had started with this instrument I probably would have maintained interest more consistently over the years.

 

Long focal length and wide aperture newt (250mm fl1200)

  1. The aperture gives potentially higher resolving power (0.45 arc seconds) which is great for detail on small targets like galaxies.
  2. The F4.8 vs F6/F8 of comparable refractors means significantly shorter integration time.
  3. The focal length is necessary for most galaxies and planetary nebula if you want the target to be a good part of the frame.
  4. Needs an EQ6 or better mount to get satisfactory results (I used an HEQ5 with it for years and I got some good results but it was tricky).
  5. It is very rare for me to get many perfect sub-frames in a row with nice round stars. Seeing is almost always poor where I live, wind blows the scope and I'm still ironing out my mounts performance. I don't want to use an OAG if I can avoid it but that is an expense I may have to try.
  6. Although the scope is quite fast, I still need to do long sessions to build up good sets of sub-frames.
  7. The weather conditions in the UK are almost always limiting resolving to much less than the potential resolving power but even so my stars are obviously smaller and detail is finer than you can get with a shorter refractor.
  8. I aim for 1x binning to get the highest resolution but if you are happy to bin the data 2x2 (to match conditions) then you can get results that are a resolution and fov similar to 125mm F9.6 scope but light gathering the same as 125mm F4.8. (This is very good argument for me to use my 150mm F5 more often - which I can use on my HEQ5 with very good results).
  9. A big newt is very expensive when you factor in the mount you need to take a good long exposure image. If you just want a good visual or planetary imaging experience then a 250mm+HEQ5 is a great combination.

I am emotionally attached to my big newt so I will stick with it to get those higher res images. I appreciate the question👍

 

Edited by paul
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1 hour ago, paul said:

Having used a big newt for several years and only recently obtaining a short focal length refractor these are my reflections 😄:

Short focal Refractor (70mm fl 336mm)

  1.  It is very rare to reject a sub-frame due to mount tracking and guiding issues if properly setup (HEQ5). So assuming good weather I get 2hrs of subs out of a 2hr session.
  2. The aperture 70mm resolving power is lower (1.5 arc seconds), on the plus side this means that poor seeing is less likely to ruin sub-frames. This means I can image on more nights. (The downside is stars are bigger and finer detail can be missed compared to 250mm.)
  3. The field is much wider of course - great for M31 or M45 or most of the bright bigger nebula.
  4. When you consider the 2.2 arc second per pixel of my camera due to the scope's short focal length the result is a very robust imaging system that is going to give good results under most UK conditions.
  5. Overall less frustrating than a big newt and if I had started with this instrument I probably would have maintained interest more consistently over the years.

 

Long focal length and wide aperture newt (250mm fl1200)

  1. The aperture gives potentially higher resolving power (0.45 arc seconds) which is great for detail on small targets like galaxies.
  2. The F4.8 vs F6/F8 of comparable refractors means significantly shorter integration time.
  3. The focal length is necessary for most galaxies and planetary nebula if you want the target to be a good part of the frame.
  4. Needs an EQ6 or better mount to get satisfactory results (I used an HEQ5 with it for years and I got some good results but it was tricky).
  5. It is very rare for me to get many perfect sub-frames in a row with nice round stars. Seeing is almost always poor where I live, wind blows the scope and I'm still ironing out my mounts performance. I don't want to use an OAG if I can avoid it but that is an expense I may have to try.
  6. Although the scope is quite fast, I still need to do long sessions to build up good sets of sub-frames.
  7. The weather conditions in the UK are almost always limiting resolving to much less than the potential resolving power but even so my stars are obviously smaller and detail is finer than you can get with a shorter refractor.
  8. I aim for 1x binning to get the highest resolution but if you are happy to bin the data 2x2 (to match conditions) then you can get results that are a resolution and fov similar to 125mm F9.6 scope but light gathering the same as 125mm F4.8. (This is very good argument for me to use my 150mm F5 more often - which I can use on my HEQ5 with very good results).
  9. A big newt is very expensive when you factor in the mount you need to take a good long exposure image. If you just want a good visual or planetary imaging experience then a 250mm+HEQ5 is a great combination.

I am emotionally attached to my big newt so I will stick with it to get those higher res images. I appreciate the question👍

 

I appreciate your response and that was very detailed and surmised the points I needed to here. I already have the imaging Newtonian and the EQ6 mount. Which I have just posted my first time image with moonlit sky on this forum. The reason for the question is that the scope itself is so darn heavy that made me consider the smaller refractor, but I can now see that I need both. 😉 

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2 minutes ago, paul said:

Yes you seem to have guiding sorted 🙂

 

If it wasn't for my RORO  observatory tent I would probably not be using the big newt now. 

image.png.ae9261a2a2b06d1e8f07c0a627ed5caa.png

 

What a good idea on the tent, a little more comfortable than just standing around. The auto-guding seemed to be working well, but since the subs are quite short it is hard to say at this point, the above was only done in 15 second bursts as any more than that, and it would overexpose, no doubt, the moon did not help matters. Is that mount an EQ8, as it looks substantial?

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The mount is a CEM70G. I had been struggling for years first with an EQ5 then an HEQ5Pro to do astro-imaging with my Newt. I decided to invest in a mount that could handle it and any other scope I could conceive of owning in the future. That was a good plan as I added a bracing system (and weight) to the OTA to remove flexure.

 

One consideration for me was portability, the CEM70 head is lighter than the EQ6. But its worth noting that the Tri-pier is heavier than the EQ6 tripod. So in the end the weight combined works out the same 😄.

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2 minutes ago, paul said:

The mount is a CEM70G. I had been struggling for years first with an EQ5 then an HEQ5Pro to do astro-imaging with my Newt. I decided to invest in a mount that could handle it and any other scope I could conceive of owning in the future. That was a good plan as I added a bracing system (and weight) to the OTA to remove flexure.

 

One consideration for me was portability, the CEM70 head is lighter than the EQ6. But its worth noting that the Tri-pier is heavier than the EQ6 tripod. So in the end the weight combined works out the same 😄.

That is a serious bit of kit but as you said a wise long-term choice, these larger mounts are heavy no matter what we chose. I assume it will almost track itself without the need for guiding, I do like the outputs and power ports as that is a very convenient option.

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5 minutes ago, Cumbrianwolf said:

 I assume it will almost track itself without the need for guiding, 

I wish 🙂. From what I have read guiding is on par with the EQ6. Its capacity is greater and its that margin I wanted particularly. I chose the pimped up version as I like the convenience of it's GPS, the USB3 hub and the iPolar. The Tri-pier is fantastic too.

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Just now, paul said:

I wish 🙂. From what I have read guiding is on par with the EQ6. Its capacity is greater and its that margin I wanted particularly. I chose the pimped up version as I like the convenience of it's GPS, the USB3 hub and the iPolar. The Tri-pier is fantastic too.

Cannot blame you for such a choice, I am almost envious, OK I am envious lol. GPS you say, now why oh why did I sell my GPS mouse for this mount, I do not know!

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