Does an inch make a difference?

[Johnboy]

Hi folks,

Up until about August last year I owned 12" Lightbridge which I really enjoyed using, but because of limited viewing time I decided to swap for a scope with Goto. I thought for a while about buying the CPC 800 but was a bit worried that I might regret going from a 12" down to an 8" aperture scope, so I pushed the boat out and went for the CPC 1100 thinking that dropping to 11" would be unoticable. Unfortunately I seem to be finding fainter DSO's to be quite disappointing in the 11". For instance in the Lightbridge I could just make out one of the 'eyes' in m97, but viewing from the same spot, using the same eyepieces and either an 0111 or UHC filter I only get a faintish, undetailed blob with the CPC 1100. I was wondering if a 1" reduction in aperture really makes a difference or is it more to do with the type of scope?

Thanks

John

[Ben Ritchie]

You've dropped from f/5 to f/10 as well as dropping an inch in aperture, so if you're using the same eyepieces you're working at (almost) twice the magnification

[Doc]

The two scopes are completely different, you cannot really compare them at all.

[spaceboy]

I also found the Focal ratio to be the big player in actual image size and the detail your eye can take in. I had side by side an Explorer 200p F/5 and a TAL 200k F/10 and at the same magnifications I personally found the F/5 to be the better scope visually with what I found to be a larger brighter image. The images in the F/10 were far more sharper but smaller and duller. I think this is why you have seen such a dramatic change in what your seeing at the EP and not due to the small drop in aperture.

SPACEBOY

[Johnboy]

Thanks for your replies guys.

The two scopes are completely different, you cannot really compare them at all.

I wonder if I have made a mistake buying this scope. I didn't realize the f ratio made such a big difference. Whoops!

I know there isn't a 'jack of all trades' with telecopes, so as this scope has a long focal ratio I presume it will be better suited to some objects rather than others, if so what?

Cheers!

John

[brantuk]

A focal reducer might make a big difference on the CPC-1100 with dso's. If that doesn't work out then just give the scope to me - I love the CPC's lol

(It should give superb results with planets though without the reducer)

[Johnboy]

A focal reducer might make a big difference on the CPC-1100 with dso's. If that doesn't work out then just give the scope to me - I love the CPC's lol

Hi Kim,

I tried a focal reducer but I couldn't get my scope to focus with it attached. It was fine with the stock diagonal but as soon as I changed that for a Revelation 1.25/2" the problem started.

My wife wouldn't be very happy if I swapped the scope again lol

To be fair I am enjoying the scope but was a bit disappointed with a few of the DSO's. My binoviewer works a treat with the CPC though.

Best wishes,

John

[brantuk]

Glad you like the binoviewer with it - such a joy to see with both eyes and if set up right adds a whole new dimension to observing.

The thing with Schmitts/Maks is that they do have a narrower fov than say a dob. And of course a longer focal length and hence focal ratio. Dso's really benefit from shorter f numbers and wider fields of view.

But it surprises me what you say about the reducer - not tried mine in earnest yet so now I'm worried. Was the problem with "in-focus" or "out-focus"?

[CGolder]

as a owner of a large SCT i feel your pain, they are both a delight and a burden, i have been in constant battle for FOV with mine since owning it. I would never go back to a reflector mind.

My 10" will not focus at 36mm with a reducer so i am restricted to 1.07 degree's

But the scope is still a joy to use and i found the views very similar to a equal size reflector.

I require the c6 for portable and widefield as for some reason the c6 with focus with the reducer and a 36mm eyepiece (go figure)

are you tooking insane perfectionist amounts of time ensuring collimation is perfect?

[Johnboy]

But it surprises me what you say about the reducer - not tried mine in earnest yet so now I'm worried. Was the problem with "in-focus" or "out-focus"?

I'm sorry but I can't remember . I ended up selling it on Astro Buy and Sell so I can't check for you. I was disappointed with it because everybody seemed to think highly of them (saying it was like having two scopes).

[brantuk]

I'll let you know how I get on at Kelling and wether it's like having two scopes, or more like having one scope and a reducer that won't focus lol

[Johnboy]

Hi folks,

I was browsing FLO earlier and spotted this: http://www.firstlightoptics.com/proddetail.php?prod=AE_focal_reducer

I wonder if it's worth giving this a try. A quick search on the web and people seem to think they are good for the price. I think FLO have a returns policy if it didn't work like my last FR.

Anybody had any experience with these?

Cheers

John

[brantuk]

Are you at Kelling at all this year John? You'd be welcome to try mine out (it's the Celestron one that the retailer guaranteed would work well)

[pbyrne]

I did something similar, I dropped from a 12" dob to a 10" SCT, but I was aware that going from f/4.9 to f/10 was going to make a difference. Okay, open clusters, objects I love, have not got the same impact, they are still impressive, but when you turn the SCT on globulars, then you will see a great difference. On the moon the scope is wonderful and the planets are a joy to behold.

Give the scope some time and you will come to love it.

[Johnboy]

Are you at Kelling at all this year John? You'd be welcome to try mine out (it's the Celestron one that the retailer guaranteed would work well)

That's a very kind offer Kim, thank you. Unfortunately I wont be at Kelling so I wont be able to take you up on it. Maybe next time!

I did something similar, I dropped from a 12" dob to a 10" SCT, but I was aware that going from f/4.9 to f/10 was going to make a difference. Okay, open clusters, objects I love, have not got the same impact, they are still impressive, but when you turn the SCT on globulars, then you will see a great difference. On the moon the scope is wonderful and the planets are a joy to behold.

Give the scope some time and you will come to love it.

Thanks for the advice pbyrne, I am enjoying the scope but it does seem like a whole new learning curve . SGL is great place to help with that learning . Thanks again.

Regards

John

[Mr Spock]

I went from a 250mm f4.8 to a 235mm f10. I can't see any difference in DSOs. With the 250mm I was using a 22mm + 13mm eyepieces (x54, x92), and with the 235mm 42mm + 22mm eyepieces (x56, x107). Magnification isn't quite the same, but near enough to not make any difference. Widest true field is less at 1.28° instead of 1.5° though due to the 22mm being a Nagler. I wouldn't go below x54 on the 250mm, even though it was possible, due to exit pupil size and the secondary becoming visible.

Focal reducers are a waste of time. They add more elements in the light path and restrict the field size. They also have a focal length of about 110mm which makes it unlikely a 2" diagonal and eyepiece will come to focus.

So having to use 1¼" eyepieces instead of 2" restricts your eyepiece choice. I'm using a 42mm 72° eyepiece which gives x56 and a true field of 1.28°. In 1¼" I'd have to use something like a Hyperion 24mm to get maximum field size - it would give x62 and a true field of 1.1°

[Rui]

It's not only 1 inch less and a bigger focal length.

Going from a simple reflector to a SCT also means more glass and more absortion and reflections.

Apart from the EP's that both need, in the reflector you have just 2 reflection surfaces. In the SCT you have those 2 plus one front glass plate and one diagonal in the way. Not mentioning a larger central obstruction...

No matter how good the transmission and coatings are, there is always some light loss. Always.

(needless to say... I'm a dob lover

[opticalpath]

<<I tried a focal reducer but I couldn't get my scope to focus with it attached >>

Which focal reducer did you try? The problem with some short focal length reducers that give a big reduction (like Meade 0.33x or some other 0.5x) is that they can't be spaced too far from the eyepiece or camera ...... otherwise you cannot 'rack in' enough to reach focus.

The celestron or Meade 0.63x reducer has a longer focal length and I have found it not nearly so critical - it will work at quite a wide range of eyepiece-to-reducer spacings and still reach focus. I used mine with a spacing of 160mm on my Meade SCT (giving me about f/5) and could still reach focus. I don't know if the Celestron has less focusing range though.

If you can borrow a Meade or Celestron 0.63x reducer, I would try it.

Adrian

[Rob]

Been here and got the T-shirt. I've had 2 C8's and loved them, but always got frustrated with the narrow view. I did eventually grab a 6.3 Celestron reducer that certainly made the difference.. mind you I would always suggest the Celestron offering.. others have their own short comings.

[Johnboy]

Which focal reducer did you try?

Adrian

Hi Adrian,

It was this one: http://www.opticstar.com/Run/Astronomy/Astro-Telescopes-Celestron.asp?p=0_10_5_0_3_6

It is unbranded and came as a free gift with the scope. Strangely, although none of my eyepieces would focus with the FR in place, my binoviewer (without the FR) is fine. If it was the added length of the FR that was causing the problem then wouldn't the binoviewer have the same problem?

Thanks everyone for your help.

Regards,

John