2" correct image diagonal recommendation

[Pharian]

Hi all,

Can anyone recommend a 1.25 to 2" correct image diagonal for my C8?

Don't want to have to trade my firstborn, but also can't take the mental gymnastics of working out mirror images of the sky while I am learning my way around up there.

Any advice?

Cheers!

[Pharian]

This should probably be in beginners equipment, can someone pls move it if appropriate? Apologies.

[Camelopardalis]

Chris, with a normal diagonal on a C8, the image is just flipped horizontally, so easier to work with than when it's flipped vertically :thumbsup:

[Pharian]

Awesome thanks.

Any ideas on a full correct image diagonal though? Ideally I'd love to get both, though I guess in a pinch I could find things with a 1.25 correct image diagonal then swap to a 2".

I'm sure that as I get more familiar I'll do better with a standard diagonal but at the moment I'm finding it hard enough trying to work my mount and find anything, trying to do it all backwards is recipe for frustration!

[Merlin66]

A correct image diagonal uses a special prism and as far as I can tell is only available in 1.25" - a 2" prism would be VERY expensive.

[doppler]

This is the sort of diagonal you are looking for. http://www.ozscopes.com.au/skywatcher-45-degree-1-25-erect-image-diagonal.html or a 2" diagonal a bit more pricey http://www.ozscopes.com.au/baader-2-erecting-prism-90-diagonal.html

[Tinderboxsky]

Williams Optics used to produce a 2" model - I have one that I bought about 3 years ago from MyAstroShop. It is quite good. I recall it was about twice the price of the 1.25" version. You might need to try for a second hand version if you you cannot find a new one.
I bought mine for much the same reason in my early star hopping days with paper maps. With practice it did not take long to gain the skills to star hope with a standard diagonal. Now I use Sky Safari on a tablet where the one can invert the image to suit your scope set up. So I don't use it for Astro anymore. I still use it for some terrestrial work.

Steve

[Pharian]

Thanks for the input chaps. Basically my C8 takes 1.25" diagonal. I have a 1.25" correct image diagonal and it's great, I find it much easier to use than the normal uncorrected 1.25" diagonal.

However, I would like to be able to use 2" eyepieces. I think I saw a diagonal that fits a 1.25" focuser/output but then takes 2" eyepieces. At least that's what it looked like to me. However, it was not a correct image diagonal, just a normal diagonal.

So I am wondering if there is a correct image diagonal version of that, one that will fit in my 1.25" C8 and then take 2" eyepieces, while being correct image.

Preferably without having to sell my firstborn to pay for the thing.

I know I am probably grasping at straws here, but if such a thing does exist I would love to nab one.

Tinderboxsky, was that 2" Williams Optics for a 2" focuser output? As in 2" in and 2" out?

Thanks again for your help chaps and chappettes! It is really greatly appreciated.

Chris

[UniPol]

I've had one of these ( http://www.williamoptics.com/diagonalPrism/erectPrism2in90deg_features.php# ) for some years now, simply brilliant. I don't think they are available any longer but worth looking out for a second hand one.

[doppler]

Another possibility? https://www.telescopeadapters.com/hybrid-adapters-extensions/112-hybrid-125-2-eyepiece-adapter.html?gclid=Cj0KEQiAqdLDBRDD-b2sv6-i6MsBEiQAkT3wAiyZtnkllwgNfwRQsqABNN EKl8dELm8QYjN0ubnydhYaApKd8P8HAQ

[Tinderboxsky]

Hi Chris,

Yes, "2 inch in and 2 inch out". It is the one that UniPol has provided a link to.

The 2" nose piece simply screws out, so it would not be too difficult to machine a 1.25" nose piece to screw into the existing thread.

[UniPol]

The beauty of the WO 2" erecting prism is its ability to be attached to 2" SCT or 2" "slip in" visual backs. WO supplied both adaptors with most of their 2" diagonals.

[Pharian]

Is there a way to convert my C8 to 2" from 1.25" with relative ease?

Also, this 1.25 to 2" adaptor, if I was using this wouldn't it reduce the effectiveness of the 2" eyepiece, having it cut down to 1.25"?

https://www.telescopeadapters.com/hybrid-adapters-extensions/112-hybrid-125-2-eyepiece-adapter.html?gclid=Cj0KEQiAqdLDBRDD-b2sv6-i6MsBEiQAkT3wAiyZtnkllwgNfwRQsqABNN EKl8dELm8QYjN0ubnydhYaApKd8P8HAQ

[UniPol]

I thought C8's had a 2" visual back i.e. 2" SCT threaded back or is it an early model with just a 1.25" back? Perhaps you could post a pick of your scope.

[Pharian]

Hi Unipol,

The 1.25" visual back comes off, but I can't tell if the thread on the scope takes a 2" or not? It is certainly bigger than the 1.25" so maybe! That would be grand.

I assume if that is the case I can just buy a 2" visual back from Celestron? When I bought the scope second hand it didn't come with any other visual backs, just the 1.25" and a T Adapter (which I believe is for AP for backfocus, although I am not 100% on that, haven't delved into that area yet).

Here are some images of the scope.

Thanks for your help! It is greatly appreciated.

209142

209143

209144

[Pharian]

https://www.bintel.com.au/product/bintel-2-inch-mirror-star-diagonal-sct/

I just screw one of these bad boys on where the visual back was don't I. Yep, feeling a wee bit foolish right about now.

Cheers!

[UniPol]

Looks like you are on the right track Christopher. Keep an eye out for the WO erecting prism, it would work very well on your scope.

[PeterAnderson]

Okay, the price is likely to be high, and check out the 'clear aperture' of the light passing through the unit before you jump. (I am unsure of the characteristics of the diagonal.)

This left to right (mirror reversal), is a real pain in the aXXX for someone who grew up with Newtonians. Upside down, I can handle, but mirror reversed totally loses it! (Leonardo Da Vinci wrote mirror reversed so his noted were coded. That give you an idea how hard it is to relate stuff when mirror reversed.) Okay I could take out the diagonal, look straight through, and crick my neck!

Now there is a cheap, but inferior alternative that does not involve sacrificing first- borns. This is the 45 degree correct image versions. I did an assessment some time ago on my unit:
Celestron 45-Degree Erect Diagonal (1.25") Review
By PeterA VERIFIED BUYER Would be great for terrestrial viewing and the low price attracted me, but...
The main item of note is that the undersized prism has a clear aperture 18mm in diameter compared with 28mm for the standard 1.25" 90 degree model supplied with Celestron SCT's. Also,contrary to the illustration it has only one screw to affix the eyepiece and no grubb screw in the side to adjust the prism. The point of focus is quite different than when using the standard 90 degree prism.

This 18mm diameter prism means that when using low magnification eyepieces you suffer vignetting around the edges. For example the field lens of the 40mm Plossl that came with my C11 uses the full inside diameter of the barrel of 28.5mm and vignetting is bad, and the edge of the field is not visible. A standard 25mm plossl has a 23mm wide lens and vignetting is very apparent. It is not until you get to the 8-24mm Celestron zoom with a small field lens of 16mm (narrow low power field) or higher powers like a 12.5mm plossl (11mm wide lens) that vignetting is not apparent.

So this effectively rules out low power wider angle views. Examining the image critically I formed the view that the vignetting encroached a tad more at the lower edge of the field. The images also appear to vary very slightly in focus and quality over the field, but not on one axis as you might expect if it was an alignment issue.

I felt that the image was not quite as bright nor quite as sharp as with the (standard) 90 degree model, but it was a very close call. I was trying to be as critical as I could, switching from one diagonal to the other using the same eyepieces. I believe, that the differences were subtle, but apparent. I used Jupiter and the galilean satellites on my C11 for my comparison and they are always a good test.

[Wavytone]

Christopher from your photos, no that scope does not have a 2" back. It is the usual 8" SCT Celestron back unchanged for the past 60 years.
If you are really determined to use a roof-prism diagonal to give a correct upright image I would suggest you don't, and instead learn to get used to seeing a reversed image.
While you can put bits and pieces on that will take 2" eyepieces, the real limitation is the inside diameter of the hole on the back of the OTA and you can't change it.

PS this is from someone with 50 years experience.

[Merlin66]

The Celestron seems to have the standard SCT 2" thread.
You can get, as mentioned,2" diagonals to directly fit this thread.

[astro744]

The amount of sky you will see through the eyepiece at low power can be represented by the width of a fingernail at arms length. You do not need to do any mental gymnastics as any star patterns you see with the unaided eye will not be recognisable at the eyepiece as the field of view is so small.



I agree. Get a quality 1.25" mirror diagonal and if you want lowest possible power then add an f6.3 focal reducer or get a 2" visual back which is not very expensive and often found used (one for sale now) and add a 2" mirror diagonal and use either 1.25" (with 2"-1.25" adaptor usually supplied with diagonal) or 2" eyepieces for lowest possible power. Note using both 2" low power eyepiece and f6.3 focal reducer does not always attain focus and I recommend against the f6.3 reducer for visual if going the 2" path.

Note the Celestron 93653-A visual back has a single screw lock with no compression clamp (unless they've added a compression clamp very recently). Without the compression clamp you are relying on that single screw to lock the star diagonal. The 93653-A has a lock ring that screws onto the thread at the rear of the telescope and the idea is to unlock this when rotating the diagonal and eyepiece so you don't have to loosen the screw locking the diagonal but in actual use the small lock screw can become loose.

You can get a 2" SCT diagonal which also has the lock ring or a SCT-2" visual back that then takes a standard 2" refractor diagonal. Whichever way you go you want the visual back length to be long enough so that if you place a large body eyepiece into the diagonal you can still rotate the diagonal and eyepiece without it fouling the focus knob. The SCT-2" visual back is likely to produce a longer path than the dedicated 2" SCT diagonal and you will find most observers use the 2" visual back.

Another option is replace the focuser with an SCT Crayford style but this does not work if the SCT is fork mounted due to clearance issues.

Whatever you choose, enjoy!

[Fox]

Very interesting thread cos I have been thinking the same thing for my refractors, ie. can I get a 2 inch erecting prism to suit my 27 mm Panoptic. The OZ scope 2 inch is 10x the price of the 1.25 inch!!! It may be easier for me just to good 1.25 inch 24 mm SWA eyepiece instead... Then go for the 1.25inch prism route, Fox