What's the focal point movement from various coma correctors?

[pipnina]

After identifying that my new scope (TS-Photon) which is supposed to have a 25mm 100% illuminated zone and a 38mm 90% zone, actually has (based on putting the properties I've measured into newt-for-the-web) a 0mm 100% zone (diagonal too small) as well as focuser vignetting on account of the camera being too far away (flats show 33% loss of brightness at 31mm diagonal sensor), I have considered how far further out different coma correctors REALLY move the focal point.

The TS-GPU in use on the TS-Photon at present claims only 20mm, but this cannot be true as (based again on newt for the web) I would need to move the camera at least 80mm closer to the secondary mirror in order to achieve the 100% zone claimed by the manufacturer.

I have just been at the dining table with my 130-PDS and my nikon, to do some basic tests on my old setup when pointed at something some considerable distance away (order of 7 miles).

I first affixed my camera to the scope using a blank M42-focuser adapter, so no lenses involved, only mirrors. I then focused on this radio tower about 7 ish miles away, measured (as best as possible given the odd shape of a DSLR) the distance between the focuser knob and the camera lens flange. About 33mm give or take. Then I did the same but with the Baader MPCC MK3 I had left over. This pushed it out to about 42mm, so with my error in measurement that's a 1cm outward movement of the focal plane.

I am yet to test if the effect differs for my f4 newt in the shed, as tonight I am planning to collect some O3 data and don't want to disturb the camera's rotation. But I intend to do the same test on the f4 scope and also include the TS GPU cc that I have and see how they all differ.

 

Does anyone else have any correctors to test? I have heard that Wynne-style correctors produce a VERY well corrected image but push the focal plane a very considerable ways out.

[ONIKKINEN]

The Televue Paracorr type 2 that i just bought apparently moves the focal point outwards by 46mm. I have not tested this though as all of my kit is in various stages of repair/maintenance right now.

[Captain Scarlet]

3 hours ago, ONIKKINEN said:

The Televue Paracorr type 2 that i just bought apparently moves the focal point outwards by 46mm. I have not tested this though as all of my kit is in various stages of repair/maintenance right now.

I have a PC2 and yes that seems about right. I should measure it though I don’t be able to for 6 weeks or so.

[markse68]

I don't think the focal plane being shifted by a CC will affect field illumination unless the CC vignettes the light cone at its lower edge? The distance from the primary to the secondary is important though- is it too close on your Photon? Can you adjust the primary further back down the tube? 35% co should give plenty of 100% field illumination

Mark

[Dan_Paris]

17 hours ago, pipnina said:

The TS-GPU in use on the TS-Photon at present claims only 20mm, but this cannot be true as (based again on newt for the web) I would need to move the camera at least 80mm closer to the secondary mirror in order to achieve the 100% zone claimed by the manufacturer.

 

I measured this carefully some time ago: the GPU corrector moves the focus plane outwards by 8mm.   Note that the housing of the corrector has changed recently (I have the older version) so you might get different results if they tweaked the optical formula between the two versions. 

I use now the Wynne-Riccardi corrector in 2.5" format sold by Teleskop Service (https://www.teleskop-express.de/shop/product_info.php/language/en/info/p10692_TS-Optics-NEWTONIAN-Coma-Corrector-0-97x-Riccardi-Design---M68-Connection.html). It moves the focus plane inwards by 2mm. It gives very good results.

 

Wynne correctors are long but  usually move the focal plane by a small amount (unlike Paracorr-style correctors which have a big outwards focus shift).

 

An semi-accurate calculation of the illumination profile takes into account various parameters:

• size of the secondary mirror
• distance between the secondary mirror and the focus plane
• diameter of the entry lens of the corrector
• distance between the entry lens and the shifted focal plane
• magnification of the corrector
• focus shift by the corrector

In many situations the corrector is the limiting factor, rather than the secondary size.

 

Here's a very useful freeware (in French) that takes into account this data to provide illumination profiles:

http://www.astrosurf.com/durey/vignettage.html

There's a file named correcteur.txt that contains the coma corrector data but lacks some of the recent models. Here's some data that you can add to the file:

 

GPU ,43.4 ,147.0 ,1.0 ,8,0.0
TS 0.73X  reducer-corrector,44.0 ,118 ,0.73, -15.5 ,0
TS Maxfield 0.95X ,44.0 ,125.7 ,0.95 ,0 ,0
Vixen Corrector PH ,50 ,132 ,0.95 ,4.5 ,0
TS Wynne Riccardi 2.5" ,54 ,161 ,0.922 ,-2 ,0
TS Wynne Riccardi 3"  ,64 ,163.5 ,1.0 , 17.5 ,0

 

Dan

 

Edited June 1, 2022 by Dan_Paris