Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Most Versatile FOCAL LENGTH? (Max number of targets)

  • Please log in to reply
4 replies to this topic

#1 ant-man

ant-man

    Lift Off

  • -----
  • topic starter
  • Posts: 14
  • Joined: 11 Aug 2020

Posted 20 September 2020 - 09:34 PM

Hello! I am planning on my first APO refractor purchase, looking to image deep sky (more or less wide field--that's what you get with refractors anyway) with my mirrorless (Sony FF stock, APS-C modded). I have a Star Adventurer currently, but can upgrade to something like Celestron Advanced VX or the like. 

 

Here's my question: 

 

In your experience, what focal length would you consider the most versatile, that is, will hit the most number of good quality targets if I were to only have ONE refractor, at least for the time being? In other words, the object, be it nebula or galaxy, would fit nicely in the frame and get some nice detail inside. Obviously, having the FF and the APS-C will give that extra reach, so its a benefit for me. 

 

I think (Rokinon) 135mm and (Red Cat) 250mm might be too wide. Will focal length ~360mm be zoomed in enough to get the most targets? Or should I aim for around ~480mm? Not sure if going much deeper than that might be too expensive or difficult for a newbie to work with. 

 

I just heard about the Red Cat 71... that might be something to wait for. 

 

Anyway, thank you for the input in advance! 



#2 alan.dang

alan.dang

    Mariner 2

  • *****
  • Posts: 260
  • Joined: 15 Dec 2011

Posted 20 September 2020 - 09:40 PM

The hard part is just sticking to one. The challenge is always going to be cost and size. The larger the scope, the harder it will be to maintain tight tracking.

With your mount being the limiting factor, I would look for a cheap used 400/5.6 Canon or a used Pentax 400/4.

400 is on the wider side but a more appropriate focal length for your mount.

#3 xonefs

xonefs

    Vostok 1

  • -----
  • Posts: 139
  • Joined: 23 Aug 2020

Posted 20 September 2020 - 11:07 PM

Maybe something around 700 that you can throw a reducer/flattener on to go down to ~500 and switch between the two. Would need a bigger mount though. Redcat is definitely too wide for a lot of things even with APS-C. 

 

This is a single full frame shot of how small M42 is on a redcat. On a 24mp APS-C sensor it is fine way cropped in, but it's a bigger target. The redcat is sharp enough to get a lot of detail on a small pixel sensor which is good but you will probably want more reach and usually still have a lot of extra space. You will almost never take advantage of full frame on it.

 

5Lt3f18.jpg


Edited by xonefs, 20 September 2020 - 11:16 PM.

  • drd715 likes this

#4 HydrogenAlpha

HydrogenAlpha

    Viking 1

  • -----
  • Posts: 509
  • Joined: 02 Oct 2014
  • Loc: Singapore

Posted 20 September 2020 - 11:32 PM

I personally like my refractor at 780mm FL. It goes down to 585mm with a reducer. 

 

Other factors also matter. If your camera has a large sensor and small pixels, a mid-range focal length can open doors to objects on both sides. 

 

Also, creating mosaics is an option if you want to shoot larger objects. You'd get both a good resolution and FOV that way, at the cost of being more time consuming and complex to handle. But it is still a worthwhile endeavour to keep in mind as you progress. 


  • drd715 likes this

#5 alan.dang

alan.dang

    Mariner 2

  • *****
  • Posts: 260
  • Joined: 15 Dec 2011

Posted 21 September 2020 - 12:27 AM

The biggest problem is going to be your mount. That puts a limit on aperture/focal length which is going to drive how much nebulosity you can get.

400-800 is a sweet spot, and that is where you have the FSQ-85/106/130, TEC110, VSD100, AP130 EDF/GT/GTX, NP101is.

The problem is weight and precision of the AVX. You will need to auto guide beyond 400mm and you may still need to guide at 400.

Dollar for dollar, it’s hard to beat a used Canon 400/5.6 paired with a EF to FE adapter. The 380/3.8 of the VSD100 is $6000, so the 400/4 Pentax is a way to get something similar but faster, allowing for shorter subs.

A Sony 400/2.8 might be interesting :)


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics