Astrophysics CCD x0.67 reducer spacing

[Brian28]

Hi , would anyone have a spacing diagram for this telecompressor  ..  

Edited February 3, 2020 by Brian28

[vlaiv]

If I remember correctly FL of that FR is 303mm, so you need to place your sensor 101mm away from the lens - which turns out to be 85mm from 2" thread at the back.

What magnification are you aiming for?

equation for reduction factor would be: 1 - distance_from_lens / FL = 1 - ( distance_from_2"_thread + 16mm) / 303

 

[Brian28]

Hi Vlaiv , its going into the imaging train of a RC 250   .

I want to use the 0.67 reduction ..

Edited February 3, 2020 by Brian28

[Fellside]

Brian,

I have used 85mm on a GSO RC10. as Vlaiv says. CCD to the theard on the Camera side of the Reducer. What scope is this to be uesd on?

Graham

[Brian28]

it’s going on Ikharus 10”RC ..which was also based on GS optics 

Edited February 3, 2020 by Brian28

[kirkster501]

You don't need a diagram, the attached tell you all you need to know in terms of getting your spacing right.  It just installs onto the nose piece of the camera.

telecompressor-techdata.pdf

[Davey-T]

I use one on my 10" SCT and spacing doesn't appear to be very critical, not sure exactly what reduction it actually gives me.

Those RCs do seem to be a bit more finicky though 😁

Dave

[Fellside]

59 minutes ago, Brian28 said:

it’s going on Ikharus 10”RC ..which was also based on GS optics 

Graph here gives a good idea of what spacing work. I will be refitting my RC back on the mount soon (Galaxy Season). I think I will deduce the distance and reduce the reduction. I have a feeling that 0.67 may be to much on my setup. I have seen a US RC10 owner who runs at 0.5 or there abouts.

https://cdn.shopify.com/s/files/1/0869/6486/files/ccdt67-27tvph.pdf?438

Long read but has some interesting info:

http://www.davidcortner.com/slowblog/20110629.php

Graham

[vlaiv]

This is simple reducer and it has very mild field flattening effect - so exact distance is not crucial.

You can change distance to change reduction factor as per above formula I gave (except I was wrong FL of this reducer is not 303mm, but rather 305mm - it won't change much in terms of results - but it's better to be precise).

So if one for example wants to have lower reduction factor of x0.7 instead of x0.67 - spacing needs to be decreased to:

0.7 = 1 - (distance - 16) / 305 => 0.7 * 305 = 305 - distance + 16 => 213.5 - 321 = -distance => distance = 107.5mm

0.7 = 1 - (distance + 16) / 305 => 0.7 * 305 = 305 - distance - 16 => 213.5 - 289 = -distance => distance = -75.5mm

We can also calculate needed distance between 2" thread and sensor for other reduction factors following above method.

Now you may ask, why would one want to use this reducer with smaller reduction factor? It is important to remember that this is neither corrector nor field flattener. It does have very light field flattening effect - but it will not correct your field. Therefore all aberrations in outer part of the field will just be "compressed" into smaller area. This is the reason why this reducer is recommended for flat fielded scopes with F/8 and above.

On TS website for example 10" RC is said to be well corrected up to APS-C size chip. This means diagonal of about 28mm.

I see from signature that KAF8300 sensor is likely to be used - it has diagonal of 22.5mm. From that we can calculate what sort of reduction we need to "pack" 28mm diameter field onto 22.5mm diagonal sensor very easily 22.5 / 28 = x0.8.

So using x0.67 with KAF8300 will actually image original field with diameter of 33.6mm - that is almost full frame sensor  - that level of reduction is going to show both curvature and astigmatism (and possibly other aberrations from reducer since this is no longer flat field).

I think it should not be pushed past 0.75 - 0.8 with that sensor on that scope. What distance needs to be for x0.8? We can again calculate 0.8 = 1 - (distance - 16)/305 => distance = 77mm          0.8 = 1 - (distance - 16)/305 => distance = 45mm

Edited February 3, 2020 by vlaiv

[vlaiv]

Made an error in sign in above post - now corrected.

[vlaiv]

There are better options out there to do reduction (and field flattening at the same time) for this scope, namely:

https://www.teleskop-express.de/shop/product_info.php/info/p11145_TS-Optics-RC-0-8x-Corrector-Reducer-for-Ritchey-Chretien---big-sensors---M68-connection.html

that one is x0.8 FF/FR, but I have heard that this one is also very good:

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

(it is cheaper as well and can be used with refractors as FF/FR so it is more versatile)

 

[DaveS]

I have the 0.75x APM-Riccardi for my 130 f/7 'frac, and yes, it is very good.

[theastrodragon]

I'm using 85mm on a GSO 10" RC, and its pretty much spot on at 0.67.

Using a SX-814, chip size 12.5x10mm and the corners are OK. I'm not sure how large a chip you can go to before aberrations become an issue