"Universal" 2" reducer-flattener for 105mm f7 generic triplet?

[GTom]

I am looking for recommendations, if any 2" reducers could illuminate an APS-H (KAF-16200, diag: 35mm) FOV with a 105 f/7 APO without significant vignetting? Any flat samples with APS-h or larger cameras are most welcome.

The scope is as far as I remember an early but very decent fluorite triplet in a carbon tube. As I do not know the designer and the seller can't be contacted after 15+years, can't go the "choose the matching reducer" path, need a more universal solution.

 

Just repaired the 2" Crayford focuser, if possible I'd stay with it instead of shoveling out the price of a new 2.5" one including the troublesome removal of the narrow tube adapter.

Edited September 11, 2023 by GTom

[vlaiv]

It can't be done the way you want it to work.

Most FF/FRs are at least x0.8 reduction factor and you want to illuminate 35mm diagonal in 2" focuser.

With x0.8 reduction, you will effectively squeeze 35 / 0.8 = 43.75mm of focal plane onto 35mm sensor. 2" Has at most about 47 - but closer to 46mm of clear aperture. It is 50.6mm tube and you need to put some sort of FF/FR inside that holds lenses in its body. That body will likely by at least 2mm thick - so we have 2mm on each side 50.6 - 4mm =  46.6mm of free aperture at best.

Most FF/FRs work at 55mm away from sensor (or more) and are at least 40-50mm long, so it is safe to assume that there is at least 100mm between FF/FR aperture and focal plane. At F/7 (this is very approximate calculation) - you need 100/7 = ~14mm additional millimeters for beam to narrow down to focal plane which we have calculated to be 43.75mm - so you need at least ~58 of free aperture on flattener entrance to get what you want. Not something you can do with 46mm max.

First order of business, if you want to go that route - would be to replace focuser for 2.5" unit. Then maybe get one of these:

https://www.teleskop-express.de/shop/product_info.php/info/p12210_TS-Optics-REFRACTOR-0-8x-corrector-for-refractors-from-102-mm-aperture---ADJUSTABLE.html

or this one:

https://www.teleskop-express.de/shop/product_info.php/info/p11122_Riccardi-0-75x-APO-Reducer-and-Flattener-with-M63x1-Thread.html

[GTom]

Thank you, just realized myself and checked the "filter sizing tool" here: https://astronomy.tools/calculators/ccd_filter_size

The replacement of the focuser seems to open a huuge can of worms, as it is not a standard M90 but a cemented tube adapter piece narrows down the tube to 70mm (69.8 to be exact) dovetail/grubscrew connection. So far I haven't succeeded removing the tube adapter. Might be willing to cut it off, but that's definitely a longer breadth project.

Instead of a reducer, I'll look for a simple flattener, 1.0x should be easier on vignetting and albeit slowish, f7 is still OK for AP. Following the maths, front element of the flattener should offer 49mm aperture. Still narrow a bit for the extreme corners, but I can live with that.

Found one candidate here, a user with a full frame camera already gave a positive feedback:

 https://www.firstlightoptics.com/stellamira-telescopes/stellamira-2-field-flattener-with-m48-adapter.html

However, no idea what is the difference compared to this product, with an originally higher price tag (now sold @the same price):

 

[vlaiv]

Take a look at this as well:

https://www.firstlightoptics.com/reducersflatteners/ts-2-inch-1x-field-flattener-for-f4-f9-refractors.html

It might be a bit more expensive, but it has been around for quite some time and it has good track record (people even use it for visual because of long working distance)

See this as example of how it performs:

 

[Clarkey]

I use the StellaMira1x flattener on a 115mm f7 scope and it is fine. I would certainly give it a go.

[GTom]

42 minutes ago, Clarkey said:

I use the StellaMira1x flattener on a 115mm f7 scope and it is fine. I would certainly give it a go.

If you have it at hand, can you tell the clear aperture? It is not shown in the specs unfortunately.

Have you tried it with a larger (e.g. FF) sensor?

[Clarkey]

The inside measurement at the thread is 42mm on an M48 connector. The internal lens is actually slightly larger at approximately 44mm. I have not used it on a FF sensor, only APS-C and it does vignette slightly in the corners.

If you remove the M48 adaptor you could screw directly to the flattener and add 2 or 3mm extra backfocus. I'm not sure of the thread size though, possibly M54.

In the comments for the flattener, someone states they are using it with a FF mirrorless camera with a 62ED.

Just for info, here is a single uncalibrated sub (just stretched a bit). Not perfect in the corners, but not too bad. Probably needs an extra mm backfocus.

[GTom]

Many thanks!

Edited September 12, 2023 by GTom

[GTom]

I have to dig deeper (literally) in this: just got a response from FLO, that their stellamira 2" flattener's open aperture should be only 38mm! Body length is 51mm, that would place the "baffle" 106mm from the sensor, far too narrow in my book if correct.

However, the TS has an extremely long backfocus, 111mm recommended. Its aperture is said to be 48mm. Not much better than, ~160mm from the sensor.

 

Will try to confirm the aperture of the stellamira/check other options, as short backfocus+larger aperture is the winning deal here obviously if the quality is all right.

[GTom]

15 hours ago, Clarkey said:

The inside measurement at the thread is 42mm on an M48 connector. The internal lens is actually slightly larger at approximately 44mm. I have not used it on a FF sensor, only APS-C and it does vignette slightly in the corners.

If you remove the M48 adaptor you could screw directly to the flattener and add 2 or 3mm extra backfocus. I'm not sure of the thread size though, possibly M54.

In the comments for the flattener, someone states they are using it with a FF mirrorless camera with a 62ED.

Just for info, here is a single uncalibrated sub (just stretched a bit). Not perfect in the corners, but not too bad. Probably needs an extra mm backfocus.

Forgot to specify, I thought about the telescope-side aperture. Camera-side is indeed great telescope side should be as big as possible. Still debating the Stellalyra and the TS 2": latter goes inside the drawtube, which is an advantage. Double price though.