Image correction prism for skywatcher dobsonian

[wok]

I will soon have a SkyWatcher 8'' dobsonian (first telescope). I want to use it mainly for astronomical viewing,  but I do have a nice view of some mountains from my backyard, and up there there are condors (vultures), cougars, and other stuff I'd like to try to see. I understand this scope is not well suited for terrestrial viewing, but if it's possible I'd still like to try it. Since these types of telescopes show an inverted image, I guess I will need an image erecting prism.

I read somewhere (I can't find the link right now) that this prism from siebertoptics (http://www.siebertoptics.com/SiebertOptics-correctionprisms.html) it the only one that would work, because others will go inside the unit and wont give enough room to focus or something like that.

Is this true? Does anyone knows any other sites that sell an erecting prism I could use in my telescope? I'm asking this because, since I will be importing it to my country, I may want to take the opportunity to also buy some filters and other accesories that sieberoptics doesn't seem to have (at least I couldn't find them in their awesome 1990s web design).

[vlaiv]

Don't know much about it but here are two things that might help you out:

1. If you worry about focus - get low profile 2" to 1.25" reducer, something like this:

https://www.teleskop-express.de/shop/product_info.php/info/p281_TS-Optics-adapter-from-2--to-1-25----2--filterthread---compression-ring.html

SW dob has one, but that one is tall (about 40-50mm) and if you want to use 2" eyepieces you need 50mm extension for 2" (also comes with scope). So there is about 40-50mm of travel that you can save if you don't use any of supplied ones and just use low profile reducer.

2. Do your research about what type of prism is needed. Refracting telescopes (that usually use prisms / mirrors) when used with diagonal invert image left/right, while reflectors invert image up/down (or actually rotate it 180 degrees). So I'm not sure that standard erecting prism will work correctly with newtonian on dob mount.

Also you might not need diagonal at all if you are willing to do some gymnastic - Provided that you look at far away mountains and telescope is pointing low - you can approach it from the other side (than for regular astronomical viewing) and try to look thru eyepiece with your head upside down - it is not comfortable but it should work.

I'm not even sure, but even regular diagonal mirror might be of some use, depending how it is placed/oriented. It also swaps one direction - when normally used it is up/down.

I have same dob, so I can do a simple test when the weather clears (there is a lot of rain at the moment so I don't feel like taking the scope out) and let you know if regular diagonal mirror works and at what orientation.

[Peter Drew]

Having made several correct image reflecting binoscopes I can say that a correct image diagonal will do what you wish. It needs to be fitted with the eyepiece facing the primary mirror end. The main problem you are going to come up against is having enough inward focus travel, raising the primary mirror and/or a very low profile focuser will help. Another problem is if you use too low a magnification terrestrially the shadow of the secondary mirror will show. You will be amazed at the quality of view. 

[vlaiv]

56 minutes ago, Peter Drew said:

Having made several correct image reflecting binoscopes I can say that a correct image diagonal will do what you wish. It needs to be fitted with the eyepiece facing the primary mirror end. The main problem you are going to come up against is having enough inward focus travel, raising the primary mirror and/or a very low profile focuser will help. Another problem is if you use too low a magnification terrestrially the shadow of the secondary mirror will show. You will be amazed at the quality of view. 

So, erecting prism will work if eyepiece end is turned toward main mirror? That is a bit awkward viewing position - reminds me of aiming a tank gun

It would not work for me - I use left eye to observe - so my head would be pressed against OTA . Also it's probably better to put dob mount on a table or something for easier access in this case.

Would regular diagonal mirror/prism work? I'm thinking in following configuration - instead of eyepiece pointing towards main mirror one should point it up - 90 degrees from erecting prism position. This would also give a bit more comfortable position for terrestrial viewing - seated but looking down from above the telescope (ok for Dob as it would be relatively close to ground). But I suspect that in this case left/right would be inverted (might not be issue for wildlife)?

[Louis D]

The Zuka spotting scope had that arrangement with the eyepiece looking in the same direction as the primary mirror.

[Peter Drew]

@Vlaiv, you can rotate the whole OTA to use the other eye, it still gives the same erect orientation.    @Louis D, the manufacturer of the Zerochromat also made and marketed a similar reflecting spotting scope.  

[Stub Mandrel]

3 hours ago, vlaiv said:

So, erecting prism will work if eyepiece end is turned toward main mirror? That is a bit awkward viewing position - reminds me of aiming a tank gun

 

I have an erecting prism diagonal that came with my mak, it only has a 45 degree angle and would be comfortable, unlike a standard 90-degree prism.

It's one of these:

https://www.harrisontelescopes.co.uk/acatalog/celestron-45-degree-erect-image-diagonal-1.25.html?gclid=CjwKCAiA24PVBRBvEiwAyBxf-d8rgz40i_h5mka52bum7-JpYzDIbgpyKhG9Rq1PHKwEJ5W8JCwvfRoCaUAQAvD_BwE

[Peter Drew]

Yes, these would work, not on a binoscope though.  

[lenscap]

Celestron supply a 20mm Erecting Eyepiece to allow terrestrial viewing with their newtonian scopes. The amici prism set is inside the barrel of the eyepiece, so there is no problem with back focus and no need for a diagonal. You can Barlow it if you want higher magnification. I have used one in my 200mm Newt so it would work in your Dob. The EP is about £20  second hand in the UK so it may be the cheapest solution if you can get one. https://www.rothervalleyoptics.co.uk/second-hand-celestron-20mm-erecting-eyepiece-125.html

[cletrac1922]

You should just be able to use your spotting scope, provided have got a right angle spotting scope

Just move to dob on its base to adjust viewing

Spotting scopes are direct view, not inverted