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

Hyperstar VS F6.3 focal reducer question

  • Please log in to reply
5 replies to this topic

#1 Daniele

Daniele

    Vostok 1

  • -----
  • topic starter
  • Posts: 168
  • Joined: 14 May 2020
  • Loc: Tuscany, Italy.

Posted 17 May 2021 - 11:20 AM

Hello guys,

So far I shot deep sky objects with hyperstar with satisfaction, even with smaller galaxies despite the short focal length. This thanks to my small sensor (Atik 420).

My question is: given the same signal, in term of resolution (the quality of small details) for example for M51, is there any difference between Hyperstar and the F6.3 focal reducer? With the F6.3 reducer the FOV will be much smaller and it will take more time to get the same signal, but I’ll get a bigger galaxy. But in this case the galaxy will be just bigger or even with more details compared to the one with Hyperstar?

Thanks!

#2 barbarosa

barbarosa

    Gemini

  • *****
  • Posts: 3,073
  • Joined: 11 Apr 2010
  • Loc: 139 miles W of the Awahnee Hotel

Posted 17 May 2021 - 12:09 PM

I think it comes down to the number of pixels in the illuminated  image area, that is the greater the number of pixels in the image area greater the possible resolution. 

 

Using an ASI224 and a RASA 8 we get as stand ins for your gear.

 

RASA 8

 

Focal Ratio: 1.97 Resolution: 1.93"x1.93" per pixel  Field of View: 0.7° x 0.52° 

C-8 w/.63x reducer

 

Focal Ratio: 6.31 Resolution: 0.6"x0.6" per pixel Field of View: 0.22° x 0.16° 

 

The answer seems to be more detail is possible with the .63x setup. 


Edited by barbarosa, 17 May 2021 - 12:16 PM.


#3 bobzeq25

bobzeq25

    ISS

  • *****
  • Posts: 25,004
  • Joined: 27 Oct 2014

Posted 17 May 2021 - 12:14 PM

Complicated.  One needs to start with image scale, the number of arc sec covered by a pixel.

 

I'll assume your Hyperstar focal length is 500mm.  Close enough.  Your image scale is 4.4 divided by (500/200).  1.8.  OK for wide field, suboptimal for small targets.

 

Now, F6.3.  Focal length about 1500mm.  Image scale about 0.6.

 

Looks like more resolution.  But, seeing comes into play, and tracking.  So you won't get all the theoretical resolution.  You will get a lot of it.

 

I find I get significant improvement, going from image scale 2 to 1, little additional gain below 1.  My skies have chronic bad seeing, though.

 

The difference in total imaging time is substantial.  You'll need about 8 times the total imaging time, going from F2 to to F6.3, to get the same signal to noise ratio.

 

All in all I think it's worth doing.  You'll like the change on small targets, but will need a lot of total imaging time compared to Hyperstar.  That's the main reason I got a C8 RASA, to lower total imaging time.


Edited by bobzeq25, 17 May 2021 - 12:15 PM.

  • mashirts likes this

#4 Daniele

Daniele

    Vostok 1

  • -----
  • topic starter
  • Posts: 168
  • Joined: 14 May 2020
  • Loc: Tuscany, Italy.

Posted 17 May 2021 - 12:30 PM

I think it comes down to the number of pixels in the illuminated image area, that is the greater the number of pixels in the image area greater the possible resolution.

Using an ASI224 and a RASA 8 we get as stand ins for your gear.

RASA 8


Focal Ratio: 1.97 Resolution: 1.93"x1.93" per pixel Field of View: 0.7° x 0.52°

C-8 w/.63x reducer


Focal Ratio: 6.31 Resolution: 0.6"x0.6" per pixel Field of View: 0.22° x 0.16°

The answer seems to be more detail is possible with the .63x setup.
Thank you both guys :)

Given the same sensor (Atik 420) what about hyperstar compared to RASA 8 for galaxies?

On my C9.25 with hyperstar I have 525 mm

Edited by Daniele, 17 May 2021 - 12:30 PM.


#5 bobzeq25

bobzeq25

    ISS

  • *****
  • Posts: 25,004
  • Joined: 27 Oct 2014

Posted 17 May 2021 - 01:10 PM


Given the same sensor (Atik 420) what about hyperstar compared to RASA 8 for galaxies?

 

Not enough difference to go out and buy one instead of using the Hyperstar you already have.  If someone else is reading this, RASA is somewhat better than Hyperstar for wide field, Hyperstar is more versatile because you can take it off.
 


  • barbarosa likes this

#6 gcardona

gcardona

    Ranger 4

  • -----
  • Posts: 398
  • Joined: 24 Nov 2009

Posted 17 May 2021 - 01:19 PM

The 6.3 focal reducer will give you more resolution, regardless of image scale. The Hyperstar system is not diffraction limited, and the spot size is about 2.5 (v3, v4 about 2) times the size of the diffraction limited airy disk. So, if you are interested in minute details of galaxies it would be best to use the scope at its native resolution or with the focal reducer.


  • mashirts likes this


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