Imaging/visual coma corrector options for 2 inch focusers - TSGPU/Paracorr/alternatives

[ONIKKINEN]

I have a VX8 F4.5 newtonian and up until this point i have been imaging with a TS maxfield 0.95x coma corrector which initially i liked, but using it more and improving my capturing in other parts the coma corrector comes out as a weak link. I dont think it performs all that well up to my APS-C sensor size and really its not that sharp even in the middle so its time to look for an upgrade. The corrector will also be used for visual.

For now there is no straight up budget limit and i dont really need to order one right now, but im weighing the different options here so that i know what i am saving money for. The GPU i could buy now, but some of the more expensive ones would have to wait.

My Rising Cam IMX571 OSC has 3.76 micron pixels so this is something i have to keep in mind with image scale. Most correctors will need binning, or some other method to half the resolution (like superpixel debayer, splitting etc, when i say bin i mean one of these methods).

 

Top contenders i am thinking of:

TSGPU 1.0x, giving 1.72''/p BIN2

TeleVue Paracorr 1.15x, giving 1.5''/p BIN2

 

Some other correctors that come to mind:

One of the 0.73x correctors or the new starizona 0.75x giving 1.18''/p unbinned

APM 1.5x corrector/barlow giving 1.15''/p BIN2

 

Looking at the best subs i have taken so far cropped to half the frame to lessen the effect to FWHM from the poorly performing maxfield 0.95 i have gotten down to 2.5'' FWHM stars, but most of them are closer to 3''. There is still a chance that my mount muddied the shots further since the EQM35 for me has a seismograph like movement in RA so its possible it was not the best possible frame for my conditions. Now with the AZEQ6 (i dont have imaging data from this yet) this problem should be gone.

This leads me to think that the 0.73x correctors unbinned would probably be a bit optimistic, as would be the APM 1.5x. Also havent really heard much from the APM 1.5x and the spot diagrams are maybe not as good as i would like so probably wont go this way. The real competition here is between the TSGPU which i have not heard really any negatives off and the Paracorr, which i really haven't heard negatives or positives from since it seems less common for imaging. The Paracorr is also twice the price, but includes a handy top for visual use whereas the TSGPU will require adapters (which i have so no cost).

If anyone has first hand experience using the GPU and especially the Paracorr for imaging i would love to hear them, negatives and positives. Also please do point out if my hopes for the resolution i will be getting are optimistic or pessimistic, they are just guesses.

[Clarkey]

I have used the SW Aplanatic which is the same as the TSGPU I believe. I tried the Baader MPCC with my F4 6" Newtonian and it  did not perform as well as I had hoped. The Aplanatic however is spot on but it is sensitive to sensor distance. I had to shim it out a fraction of a mm. I use it on my 1600 pro which is marginally smaller than APS-C and it gives pinpoint stars to the corners. I did consider the paracorr and others but I was put off by the price. The other option is the ES HR, but I don't know much about it.

Here are a couple of images using the kit described. These equate to about 1.3"/px at a FL of 600mm.

[ONIKKINEN]

14 hours ago, Clarkey said:

I have used the SW Aplanatic which is the same as the TSGPU I believe. I tried the Baader MPCC with my F4 6" Newtonian and it  did not perform as well as I had hoped. The Aplanatic however is spot on but it is sensitive to sensor distance. I had to shim it out a fraction of a mm. I use it on my 1600 pro which is marginally smaller than APS-C and it gives pinpoint stars to the corners. I did consider the paracorr and others but I was put off by the price. The other option is the ES HR, but I don't know much about it.

Here are a couple of images using the kit described. These equate to about 1.3"/px at a FL of 600mm.

Looks great!

I cant really see any coma in the corners, which is what i expected from the GPU.

On the topic of the Explore scientific corrector i thought i would have a look at some spot diagrams, below is what i found. These are difficult to compare since some of them do not state what scope they were tested with but its better than nothing.

Explore scientific HR:

TS GPU:

Televue Paracorr:

This is the only graph i could find for the paracorr for some reason.

 

And the TS maxfield 0.95 that i am replacing:

 

Out of these the maxfield 0.95 is obviously the worst, no contest. Between the GPU and explore scientific its more even, but looking at these i would favour the GPU more. The paracorr looks like it would be the best out of these, but the graph is very different looking so not sure how to interpret it. The RMS radius curve would point to significantly lower values compared to the others. Also not really sure what the difference between the RMS radius and GEO radius measurements are. They are quite different between all of these too so not sure what to make of it.

[Clarkey]

The scales on the GPU and the maxfield spot diagrams so it is difficult to compare precisely, so technically they probably not that far apart. Have you adjusted the backfocus to see if you can get an improvement?

I just go by what I see as at the end of the day that's what counts. 

[ONIKKINEN]

1 minute ago, Clarkey said:

The scales on the GPU and the maxfield spot diagrams so it is difficult to compare precisely, so technically they probably not that far apart. Have you adjusted the backfocus to see if you can get an improvement?

I just go by what I see as at the end of the day that's what counts. 

The Maxfield is not that picky about the backfocus it seems. I accidentally forgot to put a 0.5mm shim in that i thought i needed, but imaged several sessions without and noticed no difference afterwards 😅.

Problem with the maxfield is easy to see in the spot diagrams (looking at the RMS radius numbers, i ignore the visuals in the graphs as the scale is arbitrary), the size is very different in the center compared to the edges, which means if one focuses on a star in the center one gets soft stars in the edges but decent ones in the center. If one focuses with image statistics from the entire shot, like NINAs HFR calculations you tend to get a generally soft focus as the ideal HFR value is probably somewhere halfway from the center to the edges. I focus with NINAs HFR values, i find it gives the best result out of the tools i have at hand (no autofocuser) and still this extra coma issue is present.

With the other correctors this issue is much smaller. The GPU stays close to 2 micron RMS spot size so i assume that means the image would be flat and focusing on any point is valid. With the ES HR i see that the spot size doubles compared to the center in the edges, but the size of the spot itself is still very small. Paracorr claims to be between 1 and 1.5 micron RMS on an APS-C sized field so on paper its the best. The maxfield is the clear outlier from these and the spot size quadruples between the center and the edge of the field. If the GEO measurement is used its even worse, but i dont know what that means anyway so maybe not.

[Clarkey]

I used the NINA AF routine with the GPU for the results above so I think your assessment is right.

FWIW I would use the f4 scope more but it gives such awful halos I bought a 90mm refractor instead. Shame because it is fast and gives pretty good results. When I can afford the Chroma’s........