Jump to content
Sign in to follow this  
wingeing Pom

Is the Hyperstar really a fast optical system or is it hype?

Recommended Posts

You will see people on several different Forums stating quite forthrightly, that the Hyperstar is a gimmick and that all this talk of faster imaging with an f#2 system is just hype advertising. The reason they feel like this (apparently) is because you can have an 11" SCT (say) operating at either f#10 (native) or f#2 (Hyperstar). Now the incorrect line of thought says if you have the same size collecting mirror collecting the photons - then you will get the same photon density (photons per unit area) on your sensor. This is incorrect!! At f#2 your focal length is much smaller than at f#10 and that smaller focal length gives you a much bigger field of view. Since you are now collecting photons from a much bigger region of space, you will get more photons per unit area on your sensor at f#2 than you do at f#10. f#2 is FASTER than f#10.

Below you will see two pictures of the Pelican nebula. One is taken by an f#4.5 Sky90 array and consists of 12 x 10-minute subs, i.e. 2-hours of total integration time. The other Hyperstar 4 image is taken at f#2 and is only 3 x 10-minute subs, i.e. 30-minutes total integration time. What is the difference between the two? Well the Sky90 image is a bit bigger as it has a shorter focal length than the Hyperstar, and the HS & Sky90 sensor sizes are the same. Also the Sky90 image is a bit closer to natural colour - but that's simply because I haven't got the colour processing sorted out properly for the new Hyperstar sensor yet. But the BIG difference between the two images is that the Hyperstar 4 image is a LOT DEEPER than the Sky90 image. How can that be?

Well it all comes down to that difference in speed between an f#2 and an f#4.5 optical system. The f#2 Hyperstar 4 is 5x faster than the f#4.5 Sky90!! That means a 10-minute sub on the Hyperstar requires a 50-minute sub on the f#4.5 Sky90 to achieve the same depth. It doesn't matter how many 10-minute subs you put together on the Sky90, you will only increase the DEPTH by going to longer sub lengths.

If this is true then you end up with a situation that defies common-sense. The 8-foot diameter Hubble Space Telescope mirror is f#24. Are you really telling me that Hubble is 144x SLOWER than the telescope you have in your back garden. Yep - strange but true :)

Greg

Figure 1_Forums.jpg

Figure 2_Forums.jpg

  • Like 1

Share this post


Link to post
Share on other sites
On 10/8/2021 at 2:41 AM, wingeing Pom said:

all this talk of faster imaging with an f#2 system is just hype advertising.

Great write up Professor. However I am surprised that people are thinking along these lines. I had a Takahashi Epsilon which was F3.3, and it was like a Ferrari compared to my F11 C14. In fact for years I have been eyeballing the Tak F2.2 for this reason. That would give me two rigs, an F11 for super close up work and an F2.2 for widefield super fast imaging. 

I went up to Payne's Find over the weekend with some amateur astronomers from Perth Observatory. We had some incredibly dark skies. First time I've been into the outback for a few years. One day  I will have to setup a permanent observatory out there, the dark skies are quite addictive. 

Ray

  • Like 2

Share this post


Link to post
Share on other sites

I think it is "obvious" to those of us who have used fast optics in astronomy and maybe not so much for those that haven't. I do see their problem with huge mirrors being "slow".

Greg

 

  • Like 1

Share this post


Link to post
Share on other sites
On 10/11/2021 at 6:43 AM, Radar said:

Great write up Professor. However I am surprised that people are thinking along these lines. I had a Takahashi Epsilon which was F3.3, and it was like a Ferrari compared to my F11 C14. In fact for years I have been eyeballing the Tak F2.2 for this reason. That would give me two rigs, an F11 for super close up work and an F2.2 for widefield super fast imaging. 

I went up to Payne's Find over the weekend with some amateur astronomers from Perth Observatory. We had some incredibly dark skies. First time I've been into the outback for a few years. One day  I will have to setup a permanent observatory out there, the dark skies are quite addictive. 

Ray

Why did you get rid of the Tak Epsilon? I've always wanted one of those :)

Greg

 

Share this post


Link to post
Share on other sites

I probably should have kept that scope. However I am mesmerized by getting up close and personal with various objects. For example, take a look at this pillar inside the Rosette -

http://thecosmicartgallery.com/TwistingPillarsRosette.php

The next is the Rosette through the Tak.

http://thecosmicartgallery.com/The Rosette Nebula.php

The C14 captures only a tiny section of the Rosette and reveals intricate structures, which for me, I personally find very rewarding (but it is a mission to shoot this close up). However in the near future I will pick up another Tak or fast scope because I do miss capturing some of the widefield objects out there. Lower FL's are more relaxing to shoot with, the tolerances at F11 (4000mm FL) are extremely temperamental to the conditions, but it was a great learning curve. Long focal length AP pushed me to learn a lot more about AP than I would have otherwise experienced. 

Ray

Share this post


Link to post
Share on other sites

Ahh - yes - I get that!! Zooming in is great, and if you want decent resolution of the small galaxies, it's the only way to go of course.

I suppose nowadays instead of the Epsilon it would be one of those RASAs :)

Greg

 

Share this post


Link to post
Share on other sites
On 10/15/2021 at 3:52 PM, wingeing Pom said:

I suppose nowadays instead of the Epsilon it would be one of those RASAs

Actually that is a great idea. The C14 does have Hyperstar. I'll have to take a close look at that option. 

Ray

Share this post


Link to post
Share on other sites

I have never taken a close look at the RASA or how it differs from the Hyperstar. I'll have to take a close look at that option though. You have actually triggered me to start looking at fast optics again.  

Ray

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×