4" F/10 achromat with focal reductor

[vlaiv]

I did a little experiment and it seems to have worked, so if anyone wants to do similar, here are details.

I adopted CCD47 which is CCDT67 copy - a telecompressor / reducer originally intended for imaging - but works with F/9 and slower scopes with relatively flat field, to my achromat refractor.

Issue with this and any other reducer is that it requires certain distance from eyepiece to be of prescribed reduction factor. It also moves focus inward - which makes them hard to use for visual.

I decided to try to adopt it to 2" diagonal that I have together with my go to low power eyepiece - ES68 28mm. Adaptation required removing both nose pieces - that of diagonal and that of Eyepiece and also removing 2" receptacle of diagonal.

Diagonal has 2" SCT female thread, while both eyepiece and reducer have male M48 on their side. Since there are no readily available adapters for this combination - I resorted to 3d printing (and yes, even if there were adapters to be purchased - I just love 3d printing ).

Here is the result:

It did not come without some issues - I over tightened one of the adapters and besides some minor tool marks on equipment - there might be issue with diagonal now (It was heated up to certain temperature to soften the plastic to be able to unscrew it - not sure if this messed up with the mirror due to thermal expansion) and also - let's say that Argon purged title on the eyepiece is no longer telling the truth

Trying to unscrew adapter lead to actually unscrewing internals of the eyepiece. I'm sure it's still fine - but I'm also sure that its no longer argon filled (if it was before).

Overall - reduction works. I only managed daytime tests and some lunar testing. Center of the field is sharp and there is minor sharpness fall off towards the edge - maybe outer 15% or so is not as sharp. Craters on the moon are still there and all - it is just that image is a bit softer in that region. Not sure what the stars will be like - hope to test it soon.

In this configuration there is no significant change in focus position as eyepiece is moved forward by about 50 mm - length of 2" receptacle that was removed.

Nice way to get wide(r) field scope from 100/1000 achromat - here is FOV diagram:

Red is ES68 28mm without reducer, yellow is with reducer and green is max FOV for 2" eyepiece in this scope (56mm 52 degree plossl - with assumed 47mm field stop).

[RobertI]

Thanks for that Vlaiv. I have a 4” F10 Tal which I find a very “un-versatile” scope because it will not quite come to focus with 2” diagonals so can only use 1.25” diagonals and eyepieces, limiting the FOV to around 1.6 degrees I think which is not much better than my C8! I’m guessing that there is not enough in focus in the Tal to use your solution?

[vlaiv]

1 minute ago, RobertI said:

Thanks for that Vlaiv. I have a 4” F10 Tal which I find a very “un-versatile” scope because it will not quite come to focus with 2” diagonals so can only use 1.25” diagonals and eyepieces, limiting the FOV to around 1.6 degrees I think which is not much better than my C8! I’m guessing that there is not enough in focus in the Tal to use your solution?

Well, if you can't use 2" diagonal - that is going to be a problem. This solution depends on that and I think that reducer would be vignetted on 1.25" connection.

My scope is SkyWatcher Evostar 102 achromat. I did upgrade focuser on it - I replaced the stock one with M90 TS monorail that I had left over from my RC8 focuser upgrade. This focuser is a bit shorter than stock focuser so I use 35mm 2" extension before diagonal because focuser has only 50mm of travel (stock one has at least double that). This is for 2" diagonal of course. For 1.25" I would surely need longer extension to be able to focus.

Good thing about this adaptation of reducer is that it virtually does not need focus position change on my setup. Reducer requires about 50mm of inward travel when working on prescribed reduction (x0.67) and removal of eyepiece barrel and direct connection to diagonal recovers those ~50mm - so focus pretty much remains in same place (only minor tweak needed).

With Tal - you'll probably need to shorten the tube and I guess you'll be reluctant to do that (I know I was on my scope - it was an option, but I decided against it).

[RobertI]

40 minutes ago, vlaiv said:

Well, if you can't use 2" diagonal - that is going to be a problem. This solution depends on that and I think that reducer would be vignetted on 1.25" connection.

My scope is SkyWatcher Evostar 102 achromat. I did upgrade focuser on it - I replaced the stock one with M90 TS monorail that I had left over from my RC8 focuser upgrade. This focuser is a bit shorter than stock focuser so I use 35mm 2" extension before diagonal because focuser has only 50mm of travel (stock one has at least double that). This is for 2" diagonal of course. For 1.25" I would surely need longer extension to be able to focus.

Good thing about this adaptation of reducer is that it virtually does not need focus position change on my setup. Reducer requires about 50mm of inward travel when working on prescribed reduction (x0.67) and removal of eyepiece barrel and direct connection to diagonal recovers those ~50mm - so focus pretty much remains in same place (only minor tweak needed).

With Tal - you'll probably need to shorten the tube and I guess you'll be reluctant to do that (I know I was on my scope - it was an option, but I decided against it).

Thanks Vlaiv. I have often thought about shortening the tube, I would be prepared to do it as the scope isn’t worth much, but probably will never find the time! Could always try your solution for straight through viewing! 🙂

[vlaiv]

1 minute ago, RobertI said:

Thanks Vlaiv. I have often thought about shortening the tube, I would be prepared to do it as the scope isn’t worth much, but probably will never find the time! Could always try your solution for straight through viewing! 🙂

I think that simplest way to get max field on that scope (even if it means straight thru viewing) is to get 56mm plossl EP.

Exit pupil won't be much bigger than using 32mm plossl on F/6 scope - and that, depending on how dark are your skies, is quite acceptable (to me at least).

FLO has that EP in Astro Essential variant.

I just used what I had on hand, but it looks like that reducer is no longer affordable - it costs about x3 as much as mentioned EP.

https://www.firstlightoptics.com/reducersflatteners/ts-optics-ccd-telecompressor-for-ritchey-chrtien.html

and you still need to pair it with suitable eyepiece - one with ~30mm of field stop (mine has 30.8mm and this reducer fully illuminates ~29mm).

[RobertI]

Thanks Vlaiv. 

[GazOC]

Thanks for the info, Vlad. Very interesting 

I've got an ED100 that I cut down to use as a PST mod. I've been wondering for years whether it would be usable with a focal reducer as an f6 ish scope for visual use.

Unfortunately that model reducer linked above is a little rich for my blood for what would be "fun" project.

 

 

[vlaiv]

22 minutes ago, GazOC said:

Unfortunately that model reducer linked above is a little rich for my blood for what would be "fun" project.

Yes it is. It did not used to be, but with recent price rise it is now quite expensive.

When I purchased it, it was about £150 or so.

[woldsman]

Interesting thread. Tal f10 100RS scopes can use 2” diagonals (mine came with one). 

[Louis D]

Face it.  Experimenting and tinkering with various astro bits and pieces can be a lot of fun.

[vlaiv]

I've since managed one observing session with this combination - and it works great.

Ergonomics is not that great, especially if using diagonal and eyepiece in regular way and then trying to screw in all adapters in the dark, but once properly setup - gives great reduced images. Very nice stars to the edge and can fit whole M31 in the FOV.

[Louis D]

51 minutes ago, vlaiv said:

I've since managed one observing session with this combination - and it works great.

Ergonomics is not that great, especially if using diagonal and eyepiece in regular way and then trying to screw in all adapters in the dark, but once properly setup - gives great reduced images. Very nice stars to the edge and can fit whole M31 in the FOV.

Just buy another diagonal for use with all of your other eyepieces and leave that modified diagonal dedicated to widest field viewing.  Simply swap diagonals as a unit to move upward in power from the widest field unit.

[vlaiv]

1 hour ago, Louis D said:

Just buy another diagonal for use with all of your other eyepieces and leave that modified diagonal dedicated to widest field viewing.  Simply swap diagonals as a unit to move upward in power from the widest field unit.

I'll probably just swap out my F/10 achromat for this:

https://www.firstlightoptics.com/stellamira-telescopes/stellamira-110mm-ed-f6-refractor-telescope.html

For some inexplicable reason, I'm sort of drawn to that scope . It's not color free, but it has something in it ... and also, it seems to be able to deliver very good views with that combination of glasses according to this:

https://www.telescope-optics.net/commercial_telescopes.htm#error

There will be some residual color, but at that level (somewhat more than 4" F/15) - it will be less than 4" F/10 that I already have and I'm not particularly bothered by CA in that scope.

A bit more aperture, less focal length to be able to show wider field views and a bit less color, with potential for very sharp views (e-line Strehl design limit of 0.997 - which is potential to be almost perfect in that line) - what is not to like?

[Louis D]

Sounds like a plan.  Keep us informed if you do actually pull the trigger on such a scope.