Focal Reducers ?

[Andy ES]

Hi

Im considering purchasing an F10 SCT. I’m wondering if a focal reducer will be required to achieve  a wider field of view for viewing DSOs or can this be achieved by EPs such as the ES 82 degree series?

Im not going to be doing any photography.

Any advice greatly received.

Thanks

[vlaiv]

Ultimately, maximum FOV will be dictated by telescope's illuminated circle.

Focal reducer just "compresses" that circle into smaller space. If, say, illuminated circle of the telescope is 35mm, using x0.7 reducer will give you 24.5mm illuminated circle. If you use eyepiece that has larger field stop than those 24.5mm - you'll get vignetted field:

Important numbers to consider:

1.25" eyepiece has field stop of max ~27mm (max field stop diameter is related to barrel size and is usually 3-4mm less than that. 1.25" = 31.75mm)

2" eyepiece has maximum field stop diameter of ~47mm (again, 50.8mm - few mm).

Visual observation is less sensitive to vignetting than imaging. Humans can tolerate up to 50% vignetting without noticing it too much (this is because we see "logarithmic", or in power law - like magnitudes, so 50% is really not "half as bright" for us).

Depending on the size of SCT - they might not illuminate very large field. For example C8 has smaller illuminated field than 38mm. With x0.7 reducer it is effectively 26.6mm and that just about fits into field stop of 1.25".

This means that you can see equal amount of the sky by using reducer and 1.25" eyepiece of just using 2" eyepiece that has field stop of ~38mm or more - like ES62° 40mm or AERO ED 35mm.

It also means that using eyepiece with maximum field stop diameter (46-47mm) on C8 will be waste as it will vignette quite a bit.

Smaller cats have even smaller illuminated field.

[Andy ES]

Thanks for your time with this detailed explanation 

cheers

 

[Louis D]

To get widest true field of view, you'll either need the reducer/corrector for 1.25" eyepieces or a 2" visual back and 2" diagonal and 2" eyepieces with widest possible true field of view such as a 40mm Pentax XW or 41mm Panoptic.  Either way, you're working against a 2000mm native focal length that only grows longer with 2" accessories because of the additional back focus required.  At best, you're looking at well less than 2 degree of true field of view either way.

If you want wide fields of view, get either a short focal length Newtonian or ED refractor.  Either scope will complement the SCT very well.

[Andy ES]

Thanks I’ve been looking at the Baader 2” click lock system as a possibility with their diagonal.

Thank you

[Don Pensack]

On 09/12/2020 at 02:17, Andy ES said:

Hi

Im considering purchasing an F10 SCT. I’m wondering if a focal reducer will be required to achieve  a wider field of view for viewing DSOs or can this be achieved by EPs such as the ES 82 degree series?

Im not going to be doing any photography.

Any advice greatly received.

Thanks

Assuming it is an 8" SCT, the choice is a 2" diagonal and 2" eyepieces, maximum true field = 1.3° (40-41mm 68° eyepiece)

or a focal reducer and 1.25" eyepieces, maximum field of view 1.21° (32mm Plössl or 24mm 68°)

Vignetting occurs when the field is over about 1°, but it isn't usually noticeable if you keep the combination under 1.3°

Don't even think about a 2" diagonal and focal reducer together to yield even wider fields unless you want to see what vignetting is like.

At any rate, those are large enough true fields of view for nearly everything, so no real reason to go larger.

If you want giant fields of view of 2°- 4°, I suggest picking up a nice 80mm refractor (my favorite instrument for viewing the Pleiades).

I used an 8" SCT for 11 years, and found that 1.2° was about my limit before I started noticing vignetting.

[PXR-5]

I pretty much keep my focal reducer/corrector on my SCT all the time. 

You enjoy a wider field and a bit flatter.

I took it off for Mars, but I am too lazy to take it off for the other planets.

[Louis D]

1 hour ago, PXR-5 said:

I took it off for Mars, but I am too lazy to take it off for the other planets.

I take my CC out of my Dob and my field flattener out of my fracs for planets because each contributes a bit of spherical aberration at high magnifications.  They stay in place the rest of the time.

[Andy ES]

Thanks all.

This will probably be the Celestron CPC925 or 1100.

So if I use a 2” diagonal there is no point using more than a 68 degree EP ?

[Louis D]

2 hours ago, Andy ES said:

So if I use a 2” diagonal there is no point using more than a 68 degree EP ?

For widest field at 40mm, yes.  If you are willing to give up a bit of true field for higher magnification, you can move up to a 30/31mm 82 degree eyepiece or 20/21mm 100 degree eyepiece.  If you want a larger exit pupil for nebula filter observing, you could drop down to a 55/56mm 50 degree Plossl.  You may end up with all of them after a time as you experiment with what works best for your observing style.

[Andy ES]

Thanks so what EPs would be recommend for CPC 925 and 1100

I understand that Baader Morpheus and ES would be suitable? 

[Don Pensack]

1 hour ago, Andy ES said:

Thanks so what EPs would be recommend for CPC 925 and 1100

I understand that Baader Morpheus and ES would be suitable? 

Because the scopes are both f/10:

40-41mm 68°

20mm

13-14mm

10mm

8mm

And if seeing is spectacularly good, perhaps a 6mm.

If Morpheus, then 6.5mm, 9mm, 14mm

If APM XWA, then 7mm (coming soon), 9mm, 13mm, 20mm

If Pentax XW, then 7mm, 10mm, 14mm, 20mm, 40mm

If TeleVue Panoptics, then 19mm, 41mm

If TeleVue Delos, then 8mm, 10mm, 14mm, 

If TeleVue Nagler, then 7mm, 9mm, 13mm, 22mm

If Explore Scientific 82°, then 8.8mm, 11mm, 14mm, 

If Explore Scientific 68°, then 20mm, 40mm

If Explore Scientific 100°, then 9mm, 13mm, 20mm

If Stellarvue Optimus, then 9mm, 13.5mm, 20mm

It would be quite OK to mix and match.

The goal is a 4mm, 2mm, 1.3-1.4mm, 1mm, 0.8mm exit pupil

 

If you forego the 40-41mm for a 30-3mm, then your choices at that range are:

APM 30mm UFF

Pentax XW 30mm

TeleVue 31mm Nagler

Explore Scientific 30mm 82°

 

Now, I was being judicious in choices.  If you prefer to have more eyepieces and have them closer together, it could be reasonable to have a set of up to 8 or so

but I think it isn't really necessary for you to have more than 2 eyepieces under about 110x magnification (maybe 50-60x and 100-110x), nor is it necessary for you to have more than 1

above 300x (maybe 400x?)  At least 95% of your observing will be in the 100-300x magnification range.  There are very few targets that look better at 50-60x than they do at 100-110x.

 

[Starwatcher2001]

I went for a C925 as a good all-rounder. Knowing the inherent FOV limitations I still did some experimenting to see what the widest FOV I could actually achieve. I am purely visual. Cutting to the chase, I agree with the much more experienced people above.

I used a two inch visual back and diagonal, a f/6.3 reducer and various eyepieces, and compiled a list of stars and angular distances (measuring with SkySafari), to see what would fit into the same FOV.

 

The two charts below show the results and are stuck on my eyepiece box so I can tell what each piece gives me under the stars.

 

Without the F/R

- The 2" Pentax xw-40 gives the widest FOV at 1.1 degrees, and gives a lovely field. This is my goto for wide clusters.

- Due to differing AFOVs the 1.25" 40mm and 32mm eps give exactly the same FOV as each other. I prefer view from the 32mm.

With the F/R

- The 2" Pentax xw-40 gives around 2 degrees. I can see both Sulafat and Sheliak (gamma and Beta Lyra) and the ring nebular between them in the field (1*58'33") ... But it vignettes terribly and looking at anything remotely bright like the moon or planets and the central obstruction becomes very visible. As much as I wish it were otherwise, this combination is not usable for me.

- Looking at 1.25" eps: During daylight with anything in the range 25-40mm, the central obstruction is visible to some degree. At night on bright objects like the moon and planets, likewise. With the 1.25" 32mm the double-cluster fits in the FOV with space around them. It's a nice view but the C/O is still slightly discernable. The "coathanger" asterism fits in the field nicely.

Your mileage may vary as they say, but my favourite combinations are: 2" Pentax sw-40 without f/r (1.1 degrees), and 32mm Bresser with f/r (1.59 degrees).

 

Edited December 12, 2020 by Starwatcher2001
fixed e/p number

[Mr Spock]

I used to have a C925. My widest view was with the 2" 42mm LVW which gave 1.3°. They aren't available any more but I'd expect a 41mm Panoptic to be similar.

Not only does a focal reducer have a smaller field stop, it also introduces a lot of extra elements which will reduce light transmission and contrast. Not what you want if you are trying to tease out faint objects or low contrast objects.

[Don Pensack]

You're looking at it wrong.  An SCT is not a wide field instrument.  It's designed to illuminate a little over a 1° field.

In practice, vignetting doesn't raise its ugly head until you exceed about 1.2°.  But trying for more is just not a practical pursuit.

So whatever combination of eyepieces, with or without reducer, yields a 1.2° field or less will be just fine.

With the 9.25", any 2" eyepiece with a 46-46.5mm full field fieldstop diameter will work fine.

The f/6.3 reducer used with a 2" star diagonal yields an actual reduction to f/5.5, not f/6.3, hence a true focal length around 1300mm.

That means an eyepiece with up to a 27.2mm field stop, and that can be accomplished with 1.25" eyepieces.

Hence, if using a 2" diagonal, you should think of the scope as using either 2" eyepieces or 1.25" eyepieces with the reducer, but not 2" eyepieces with the reducer.

There is serious vignetting when you exceed 1.3°--look at a blank daytime sky and you'll easily see it.

 

A logical approach to getting wider fields is to add an 80mm refractor to your scope mix, where you can get fully illuminated fields as wide as 5-1/2° !

You can see the entire Veil Nebula in one field, or a fantastic view of M31's full extent, or the Pleiades, Kemble's Cascade, et.al.

 

[Andy ES]

Great, thanks again all for a  very thorough explanation and advice.

really appreciate it.

 

[Paz]

I have a focal reducer that just happened to come with my C8 when I bought it second hand.

I tried it a couple of times but since then never use it. I agree with the idea of (if possible) getting a wide field scope for wide field (it need not be an expensive one), and just use the SCT at f10 like nature intended.