focal reducers

[rowan46]

My little 5inch sct isn't up to the job of getting many nebulae, especially from my not especially dark site. Consequently I have decided to give up on them and concentrate on clusters and doubles. My f10 and a barlow is fine for doubles but I definately need a wider field for clusters. So which is the best focal reducer? Celestron, Meade or Antares. Antares are significantly cheaper but, are they significantly worse?

[Brinders]

I use a Hirsch f6.3 focal reducer/corrector and see no problem with it, plus on Ebay it's a lot cheaper than any of the others:-

Telescope, Focal reducer f/6.3 fits Meade Celestron SCT on eBay (end time 11-Apr-11 15:31:57 BST)

See also my image of M42 here:-

http://stargazerslounge.com/imaging-widefield-special-events-comets/137474-orion-nebula-m42.html

Brinders

[rowan46]

thanks for that, does it perform as well as the others?

[E621Keith]

Sorry it this for photography or visual?

If it is for visual, wouldn't getting a longer eyepiece be a better option?

I haven't used the Antares, but read it has a similar performance to the Meade and Celestron units. You should note C5 and C6 have 1.25" back with an even smaller opening, so using a reducer may cause vignetting.

[rowan46]

Sorry it this for photography or visual?

If it is for visual, wouldn't getting a longer eyepiece be a better option?

I haven't used the Antares, but read it has a similar performance to the Meade and Celestron units. You should note C5 and C6 have 1.25" back with an even smaller opening, so using a reducer may cause vignetting.

thankyou for that, it is for visual and wasn't aware of the vignetting issue, so which eyepiece is going to give me a wider view than a 32m plossl on a 6.3 reducer?

[E621Keith]

I don't really understand exactly how the eyepiece field stop limits the FOV, but I believe a 32mm plossl with 50 deg AFOV is at the limit of what 1.25" barrelled eyepiece can produce. This is the reason why a 40mm plossl has the same real FOV as a 32mm plossl.

A 32mm plossl on a C5 yield a real FOV of 1.3 deg. Assuming the eyepiece is not affected by the baffle issue, a 0.63x reducer will bring the scope focal length down to 788mm, and gives a real FOV of 2.1 deg.

Again, assuming the C5 does not have the baffle issue, a 36mm Hyperion Aspheric will produce the same 2.1 deg FOV on a 1250mm C5. The Hyperion will yield a 72 deg apparent field of view instead of the 50 deg on the plossl.

However, when you consider the C5's limited baffle tube diameter (<1"), the max real FOV of the C5 will most likely be limited to 1.3 deg, as seen through a 32mm plossl (limit of 1.25"eyepiece). The rest of the field would be cut off by the baffle tube. Neither adding a reducer or using a 36mm Hyperion will increase real FOV, as the extra field would be cut off by the baffle tube and result in a reduced AFOV.

On the other hand, if you are looking for greater AFOV, a 24mm Hyperion (AFOV 65deg) will show the same real FOV as a 32 mm plossl.

If you want really wide FOV, consider getting a ST-80. It will have some aberration issues, but for wild field low power application, CA shouldn't be a problem. A 32mm plossl will show a 4.2 deg field. (note, I didn't like my 32mm plossl on my 80 f6 apo, I suspect it may be worse on a f5 ST80.)

[rowan46]

thanyou for a useful reply and taking the time to explain fully. I am much obliged.

[E621Keith]

Sorry, I think I may be mistaken. I just tested my C6 with a focal reducer and a 32mm plossl, and it does produce a wider real FOV. I don't have any 2" eyepiece to test with, but I think a Hyperion Aspheric and SW Aero ED should also producer wider FOV than 32mm plossl without reducer.

I am not sure if those 2" eyepieces will reach their advertised AFOV on the restricted baffle tube of a C5 or C6 though.

The field stop diameter on the 32mm plossl appears smaller than the opening on my C6, so it may explain why the real FOV can be increased by using a reducer.

If you don't have 2" accessories at the moment, then the focal reducer may be a cheaper option. If you do go for 2" eyepieces, you'll need to budget for 2" diagonals and 2" visual back as well as the eyepiece itself.

[rowan46]

Keith you are a star I was fairly sure that 2" wasn't the way to go because of the baffle. I was hoping that someone would say I've tried a focal reducer and it works or vice versa. Thankyou for taking the trouble to test it for me. Thats what I love about this site so many people do go the extra mile to help others.