Thinking of a project refractor

[Lady Fraktor]

This morning I was doing some cleaning in the astro shed and I started thinking about the 115 - 150 mm gap I have in my refractors.
Of course when a person starts thinking other ideas arrive!
120 mm is a good size but only 5 mm larger than the 115 mm and a 140 mm is only 10 mm smaller than the 150 mm...
So a 125 - 130 mm would be a nice fit, but should it be longer than f/7 or shorter?
Permanent use with a set of binoviewers would be a interesting idea so removing about 100 mm from the tube would be required.

What do you think? Possible refractors to sacrifice, long/ short focal length?

[JayTee]

I think it should be a focal length that covers any gaps you may have right now. So if you go from say 500mm to 1200mm there's a definite gap there that should be filled.

[Bigzmey]

I had for a while ES 127mm APO triplet F7.5. It was a very nice scope, very capable optically and not too heavy to handle. It required 50mm or 100mm (don't remember exactly) extension tube to reach focus with EP and 2" mirror diagonal, so it might have enough back-focus for BV with 1.25" prism.

[helicon]

Having a midrange fraq might be a good idea. My 152 is sort of portable, my 102mm much better. Not much in between.

[Bigzmey]

Having a midrange fraq might be a good idea. My 152 is sort of portable, my 102mm much better. Not much in between.

120ST F5 (Orion, SW) is a nice form factor for grab and go. Very compact and lightweight with quite noticeable step up in light gathering from 102mm.

[Lady Fraktor]

I think it should be a focal length that covers any gaps you may have right now. So if you go from say 500mm to 1200mm there's a definite gap there that should be filled.

500 to 1200 mm is fairly covered, the gap would be between 1200 - 1575 mm.

[Lady Fraktor]

I had for a while ES 127mm APO triplet F7.5. It was a very nice scope, very capable optically and not too heavy to handle. It required 50mm or 100mm (don't remember exactly) extension tube to reach focus with EP and 2" mirror diagonal, so it might have enough back-focus for BV with 1.25" prism.

A interesting choice but even I would have to question my sanity of taking a saw to the tube of a € 2.400 refractor!

[Lady Fraktor]

Having a midrange fraq might be a good idea. My 152 is sort of portable, my 102mm much better. Not much in between.

70 - 115 mm is covered with the gap between 115 - 150 mm
Portability is great now, before I would have to carry the 150 mm (13kg) up/ down 6 flights of stairs to use it.
Now I just take 3-4 steps out of the shed.

[Bigzmey]

I had for a while ES 127mm APO triplet F7.5. It was a very nice scope, very capable optically and not too heavy to handle. It required 50mm or 100mm (don't remember exactly) extension tube to reach focus with EP and 2" mirror diagonal, so it might have enough back-focus for BV with 1.25" prism.

A interesting choice but even I would have to question my sanity of taking a saw to the tube of a € 2.400 refractor!

You might not to since it comes with shorten tube already.

[Lady Fraktor]

The problem is spending € 2.400 to discover if I had to or not

[Bigzmey]

If you must saw, how about this guy then? I had it briefly. The optics is good. The hardware is your typical Synta, basic but serviceable. Basically, shrank-down version of the 150mm you have.

Omni XLT 120 F8.33
https://www.celestron.com/products/omni ... -telescope

[Bigzmey]

If you must saw, how about this guy then? I had it briefly. The optics is good. The hardware is your typical Synta, basic but serviceable. Basically, shrank-down version of the 150mm you have.

Omni XLT 120 F8.33
https://www.celestron.com/products/omni ... -telescope

Never mind, too close to your 115mm.

[JayTee]

This one appears to fit right in the aperture range (probably not the FL range) of which you speak. But I think it may be too expensive by the time it arrives to you.

https://www.telescope.com/Orion-EON-130 ... 132454.uts

[Lady Fraktor]

This one appears to fit right in the aperture range (probably not the FL range) of which you speak. But I think it may be too expensive by the time it arrives to you.

https://www.telescope.com/Orion-EON-130 ... 132454.uts

I imagine by the time shipping, import and DPH are added the cost would likely be minimum 30-35% more

[Lady Fraktor]

a few that I have found so far, pros and cons for all.
https://www.teleskop-express.de/shop/pr ... embly.html
https://www.teleskop-express.de/shop/pr ... --OTA.html
https://www.teleskop-express.de/shop/pr ... ctive.html

The search continues!

19/04/22 Edit:
I emailed TS about my idea and #2 is out of consideration, I had forgotten the telescope has a sub-aperture corrector directly in front of the drawtube.

[Lady Fraktor]

If you must saw, how about this guy then? I had it briefly. The optics is good. The hardware is your typical Synta, basic but serviceable. Basically, shrank-down version of the 150mm you have.

Omni XLT 120 F8.33
https://www.celestron.com/products/omni ... -telescope

Never mind, too close to your 115mm.

It was actually the first consideration, if I did not have the 115 mm that would like be the one.

[SkyHiker]

Instead of sawing off a piece of the tube, can't you just use these binoviewers that claim to not affect the optical path and keep the focuser distance the same? It's only for 1.25" but at least you can keep your frac resellable if what they claim is true.

TS also has a nice collection of astro binos if you want to avoid using binoviewers. Should be F/4 or so with 10 mm eyepieces.

[Lady Fraktor]

A interesting BV though looking at the specifications I see 7 elements and a clear aperture of only 17.4 mm with a eyepiece field stop size of 22 mm.
The BVs that I use have been re-worked so are a bit better than that. 3 elements left side 26 mm CA, right side 28 mm, and 28 mm CA at the nosepiece.

The idea is that by modifying the telescope tube you would not need to use any OCA (optical corrector assembly) to reach focus, everything stays at its native f/l. This way there is no having to match OCA, long/ short diagonals and being stuck using limited range of eyepieces.

My Stellarvue 80 mm is setup for this but requires removal of the focuser then a 102 mm section of the tube then reattach the focuser. Not something you want to be doing in the field.

Thank you for the ideas though

[SkyHiker]

The idea is that by modifying the telescope tube you would not need to use any OCA (optical corrector assembly) to reach focus, everything stays at its native f/l. This way there is no having to match OCA, long/ short diagonals and being stuck using limited range of eyepieces.

My Stellarvue 80 mm is setup for this but requires removal of the focuser then a 102 mm section of the tube then reattach the focuser. Not something you want to be doing in the field.

I had to look up OCA and found this very helpful binoviewer link from my favorite astro shop, Agena Astro. It sounds like you removed the OCA from your binoviewer and are making an adapter for your focuser that lets you take out its housing. Keep us posted on your success!

For those who have an astrograph, I presume there would be zero problems attaching binoviewers since an astrograph pushes the focal plane out far enough? And for any truss Dobsonians that have a truss hole for AP it would be OK too? That is my situation, I have been considering getting a BV.

[Lady Fraktor]

Nothing that complex, the OCA is screwed onto the nosepiece of the BV. Many people use a 2x barlow for their BVs to work but there is a difference between how a barlow and OCA work. Because I have different focal length refractors I have 4 different OCA: 1.3x,1.4x,1.6x,1.8x to get closer to native f/l.

By removing a section of telescope tube I will be removing the lightpath length of the BVs, and no need to modify a diagonal.
This way a regular diagonal can be used and no OCA is required. BVs typically add about 100+ mm to the focal length so the OCA is required to reach focus.

Many people use BVs with newtonians but I am not sure what OCA would be needed or if modification to the mirror position is required,
a lot of BVs come with a 1.6x

They are relaxing to use since you are using both eyes. When the planets are out I can sit and view for hours without fatigue.
I am really not sure how they would work with a astrograph though.