Add tele-converter to 200mm lens or buy small scope?

[bluespeck]

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:

1. 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.
2. 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). 
3. 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.
4. 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 nice.
5. 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 thanks in advance for any guidance and assistance.  

   is
 

Edited August 26, 2020 by bluespeck

[RT65CB-SWL]

Hi @bluespeck and welcome to SGL. 

Personally, I think you are better off purchasing a 'small' 'scope. Reason being, you are adding more glass in the light path which you do not want, which in turn, is slowing the exposure time.

I am not an astro-imager, but first, I would recommend you purchase a copy of 'Making Every Photon Count'.

Other astro-imaging SGL'ers will be along to assist you making the correct/perfect choice.

[Alien 13]

I use my f/4 Canon 70-200mm with a X2 teleconvertor a lot for normal daytime shots and Lunar when I want a quick grab and go but ultimately for DSO I would opt for a small scope if using the SW adventurer something like the 61mm WO or the Red Cat..

Alan

[SiriusB]

I Dunno about Astrophotography & Dunno if technologie's moved on~ but back in the days of 35mm film, a zoom lens AND a teleconvertor was a big no no  in terms of picture quality.

Ok ,if it was a case of taking a  shot vs no shot, like if you was a photo journalist running 1600asa or something, but not for quality....& that's what you astroimagers crave isn't it?

Of course, i'm still stuck back in 1980's 😉 or was it 1970's ?😁😁

[bluespeck]

On 26/08/2020 at 14:17, Philip R said:

Hi @bluespeck and welcome to SGL. 

Personally, I think you are better off purchasing a 'small' 'scope. Reason being, you are adding more glass in the light path which you do not want, which in turn, is slowing the exposure time.

I am not an astro-imager, but first, I would recommend you purchase a copy of 'Making Every Photon Count'.

Other astro-imaging SGL'ers will be along to assist you making the correct/perfect choice.

Thanks for the advice Philip.  What you say makes sense.  I think I'll do a little more research before I decide either way. 

[bluespeck]

On 26/08/2020 at 15:36, Alien 13 said:

I use my f/4 Canon 70-200mm with a X2 teleconvertor a lot for normal daytime shots and Lunar when I want a quick grab and go but ultimately for DSO I would opt for a small scope if using the SW adventurer something like the 61mm WO or the Red Cat..

Alan

Thanks Alan.  Good advice.  I am leaning towards the WO Zenithstar 61 as it is well with my trackers payload capacity and has a little larger reach than the Red Cat (which I hear is terrific).

[bluespeck]

23 hours ago, SiriusB said:

I Dunno about Astrophotography & Dunno if technologie's moved on~ but back in the days of 35mm film, a zoom lens AND a teleconvertor was a big no no  in terms of picture quality.

Ok ,if it was a case of taking a  shot vs no shot, like if you was a photo journalist running 1600asa or something, but not for quality....& that's what you astroimagers crave isn't it?

Of course, i'm still stuck back in 1980's 😉 or was it 1970's ?😁😁

Thanks SirisuB.  I agree... back in the film days teleconverters were considered somewhat of a no no.  I think technology has improved since and it might be less taboo today.  That said, I am starting to thing that going with a small telescope might be a better idea.