Lens and Teleconverter Versus Small Scope Generic equipment discussions · bluespeck · ... · 13 · 379 · 0

bluespeck 1.51
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Hello.  I am relatively new to astrophotography and would welcome some equipment advice.I have been experimenting and getting some decent beginners results with an unmodified Nikon D750 and a Sigma 70-200mm f2.8 lens on a Sky-Watcher Star Adventurer tracker.  I am having a lot of fun and learning more all the time.I would like to be able to stretch my reach a little further and am contemplating whether I’d be better off getting a 2x teleconverter for the Sigma lens or small telescope.I am far from an expert and would welcome some opinions and suggestions.  Here is what I am thinking:
  • The lens is the Sigma AF 70-200mm f2.8 DG OS HSM (Sport).  It is a fast lens with great optics.  If I add the Sigma TC-2001 2x Teleconverter, it will stretch my reach to 400mm.  I know that I will loose two full stops, but the resulting f5.6 actually seems slightly better than many small telescopes.  On the surface this seems like a good idea because it is good quality, affordable and expands the use of equipment I already own.[/*]
  • As an alternative, I am also thinking about getting a small telescope to use instead of the lens/teleconverter combo.  Aside from budget, I have no preferences and the only real limitation is weight.  My Star Adventurer tracker has a payload capacity of 5 kg (11 lbs). [/*]
  • I recently watched a Youtube video reviewing the William Optics Zenithstar 61II APO. It seem to be highly recommended and with an aperture of f5.9 and a focal length of 360mm, it should allow me to reach a little deeper into the night sky than my current set up.  This scope is well within the payload capacity of my tracker and is within budget.[/*]
  • William Optics also has the Zenithstar 73 APO which while a little more expensive has as 430mm focal length and still should be within the payload capacity of my mount. I am not sure if the added length might be an issue for my star tracker and might have to do a little more research here.  If it is compatible, the longer reach might be good.[/*]
  • Last point.  My Sigma 70-200mm has an 82mm diameter.  The Zenithstar 61II is 61mm and the Zenithstar 73 is 73mm.  Does this impact benefits of one over the other?[/*]
A big thank in advance for any guidance and assistance.

   is
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Intheswamp 0.00
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Giving your message a bump here to see if someone might respond with some thoughts.  I'm curious to what folks might recommend as I'm in a budget Star Adventurer situation myself.
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JO_FR_94 6.49
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Hi,
Apart from mount considerations and weight, in terms of pure optics, here is what I can say :

- lenses usually can’t be used wide open, due to too much aberrations (coma, chrimatic etc...), you have to stop them down, usually at least to f/4 to start getting nice stars, while a small refractor that has the same diameter is made to be used « wide open » (no diaphragm). I am of course taking about flat field corrected refractors made for astrophotography.

- if you start to increase the diameter, you can get much better resolution, due to diffraction phenomenon : punctual light sources at infinity are not points on your sensor; the image is an interference figure called Airy figure (concentric circles fading away). The higher the diameter, the best your scope will be able to distinguish two stars very close together (otherwise, their Airy figures will overlap too much to really see contrast between them). That’s also why it is preferable to get a bigger refractor (higher diameter) when you want higher focal length, than just using Barlow lens / 2x or 4x teleconverters. It is not just for FD ratio considerations.

- Teleconverters and Barlow lenses are used mainly for bright objects (planetary observation for example) when you already have a long focal length. You have FD ratios that are « ugly » for a classic photographer but OK to image ver small and bright objects. For faint deep sky objects well, most uses fast FD devices (except again for small bright deep sky object like planetary nebulas, using techniques of short exposures to limit guiding problems and seeing limitations).

That’s the general optical considerations. Now if you seek portable setups, many lenses are used for deep sky photography with GREAT success. But if you start seeking higher focal length with portable setup, well, I would personally avoir teleconverter or Barlow for deep sky images : I was a bit on your case with a Skyguider Pro and I chose a camera with a small sensor (ASI183MM Pro), because it is 2,7x smaller than a full frame sensor and thus the « equivalent focal length » I get is 2,7x higher. On top of that I invested in the lightest 70mm diameter reafractor I could get (TS Optics Imaging Star 71/347). It is a flat field corrected refractor of 347mm of focal length, so FD ratio of 4,9. With my small sensor it would be equivalent to a 936mm focal length lens on a full frame sensor (in terms of FOV).

But at this focal, with a small sensor, you need guiding, particularly on a small mount.

Last advantage of the refractor over lens : I find it easier (especially at this equivalent focal length) to focus, with or without electronic focuser. I also use some of my lenses (Mitakon Speedmaster 85mm, Sigma 105 EX Macro, Samyang 8mm...), and each time, focusing is a pain in... you get it :-)

Hope this feedback will help you a bit.
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leviathan 4.72
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Get yourself a small scope like Redcat or EDPH II 61mm. It will provide better flexibility later on, better focuser, color and field correction, etc.
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andreatax 7.22
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If the lens is well-corrected at full aperture then going for the lens would be the easy choice. Nothing beats aperture, if optical correction is (about) the same. 200mm is a good focal lens to work with too considering that you have a D750. Adding a teleconverter could be a good choice if said teleconverter is a good match optically, which there are few around. Stick with Nikon as this is best option in my experience. If you can test them and return of not happy that is the way to go. I suspect that going at longer focal length would be too much a strain for you current mount so you might need considering getting a proper mount of you set up for a small APO instead (not my choice, to be clear).
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bluespeck 1.51
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Thanks everyone.  I really appreciate your thoughts and comments.  

I ended up going with the WO Zenithstar 61 along with a L-Pro filter.

Unfortunately since the telescope arrived back in October we have only had two or three nights of clear sky up this way, so I haven't really had many opportunities to test things out. In fact between weather and other commitments, I've only been out with the new set up twice, and I am really itching to get out there and spend more time with it under some clear skies!

That said, in the limited testing I've done so far, there were a couple of things that I noticed right off the bat.  Firstly, the scope offers a huge focusing advantage versus the lens. Huge! Also there does seem to be a noticeable improvement in colour and field flatness. I suspect that I will find more advantages once I get to fully use the scope, but even if nothing else, what I've discovered so far make me very glad that I went this route.

The only negative so far is that along with my Nikon D750 they are perhaps pushing the weight limitations of my Star Adventurer tracker. I have MacGyvered an extension bar for the counter-weight shaft and it seems to do the trick for now, but I suspect that adding a guide scope in the future might push the balance over the top.

With this in mind I am hoping to upgrade my mount/tracker in the new year. I have some back issues (physical limitations) and don't think that I could handle any of the big heavy mounts.  I've started to do a little research and am thinking about the Sky-Watcher HEQ5.  It isn't too heavy and seems like it might be suitable.  Any additional comments about the mount (or other recommendations) would be appreciated.

Btw... My only complete session with the new Z61 so far produced what I think is a decent first attempt at IC1805 (Heart Nebula). I am very much a novice and would welcome any feedback and suggestions.  The linked image is an 85 min exposure captured under Bortle 5 skies using the Z61, L-Pro, D750, and Star Adventurer combo.  https://cdn.astrobin.com/images/73667/2020/3be6f0dd-4d39-4016-a3d3-b062e782e06b.jpg

Thanks again
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tim@the-hutchison-family.net 12.30
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Your might want to take a look at the Rainbow Astro RST-135 mount. Very small and light, no counter weights, but very capable.

http://www.rainbowastro.com/rst-135/
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dkamen 6.89
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With a telescope you must take into account the flattener (another 10cm added length and maybe 200g weight).

I have the ZS73II and can guarantee it is too much for the star adventurer.

Also, above 250mm (okay maybe 350 with a full frame) and with slow optics targets are difficult to acquire without goto.  For me at least.

Cheers,
Dimitris
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andreatax 7.22
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Tim Hutchison:
Your might want to take a look at the Rainbow Astro RST-135 mount. Very small and light, no counter weights, but very capable.

http://www.rainbowastro.com/rst-135/


Nearly 4 grands for THAT!? C'mon, the guy is just starting this is well over the top!

If anything get a traditional small mount, a CEM-28 maybe.
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tim@the-hutchison-family.net 12.30
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andrea tasselli:
Nearly 4 grands for THAT!? C'mon, the guy is just starting this is well over the top!

If anything get a traditional small mount, a CEM-28 maybe.


How about a modicum of decency, perhaps?  If you disagree, fine, but there is no call for that.  I happen to own one and I think it is a very nice piece of equipment.  Further, the original commenter made a mention of physical challenges in moving something large.  I didn't put a gun to his head and say YOU MUST BUY THIS!  I made a suggestion and said he might want to check it out.  No reason for you make such an aggressive comment!
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Intheswamp 0.00
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I have to say I had the same reaction as andrea tasselli had in regards to the recommendation of the very expensive tracker.  I certainly don't think there was anything nearing indecency in tasselli's response, it seemed like a natural repsonse.  The OP was asking opinions on using a zoom camera lens and extender along with a few other sub-$1000 options and got a recommendation to spend $3900 on a tracker.  
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tim@the-hutchison-family.net 12.30
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Any additional comments about the mount (or other recommendations) would be appreciated


I was replying to this request... But that's fine. I'll gladly remove myself from this conversation.  Good luck bluespeck.
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Intheswamp 0.00
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Tim Hutchison:
Any additional comments about the mount (or other recommendations) would be appreciated


I was replying to this request... But that's fine. I'll gladly remove myself from this conversation.  Good luck bluespeck.

Fair enough, but  a leap from a $350 tracker to a $3900 tracker is enough to raise one's eyebrows...no need to leave the conversation because someone voices their opinion.
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bluespeck 1.51
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Hi All ,

I really appreciate everyone’s feedback and opinions.  I am sorry that this conversation has stirred some tension, it was not my intention. Please let us a just take a deep breath and regroup.

I agree that the $3,900 option is well out of my current price bracket and possibly even above my current technical needs, but it was nevertheless a suggestion worth my investigation if not now, at least at some point down the road.  I had never heard of the RST-135 and am at least now familiar with it as a possible future upgrade. Thank you Tim for letting me know about it.  I am very grateful that you took my physical limitations into consideration when responding to my original post.  Bulky or heavy equipment is definitely an issue for me.

I also want to thank Andrea and Intheswamp for recognizing and taking into consideration that as a beginner, there might be other options more economically suited to my current and mid-term needs.  We all know that astronomy and astrophotography are equipment heavy pastimes.  Regardless of skill level, there are alway going to be a wide range of options and we could all spend a small fortune and still not have everything that we want or need. Moving up from my existing Sky-Watcher Star Adventurer to the CEM-28 might be a good suggestions for now and I will definitely look into it.  Thank you for the recommendation.

Speaking of the CEM-28... does anyone have any comments about it versus the Sky-Watcher HEQ5?  I recently watched a video about the HEQ5 by AstroBackyard and it seems to be a decent mount as well. (https://youtu.be/jVjp2Krc4c0)

Thanks to all and keep looking up
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