Full Frame Camera corrected field circle size?

[ceefna]

Hi I have an ED127 fcd100 scope with dedicated 3" 0.7x reducer/flattener and am having issues when using my asi6200 full frame camera. The stars are horrible in the corners but is this to be expected with this setup? Looking at my images they sudgest I am too close to the reducer at 55mm? I've made a 1 and 2mm spacers but even at 58mm there is little to no improvement. Is this just the size of the corrected circle or do you think its worth increasing the back focus further?

If anyone has this setup working please let me know

Many thanks

Richard

american_frame6_Ha_frame6_-10C_600sec_1x1_2021-09-29_232547.fit

[ceefna]

just to add I have the same curve without the reducer fitted.

[wimvb]

A full format camera has a sensor which is 36x24 mm, with a diagonal of about 44 mm. Only a very limited number of scopes/correctors can handle such a large field.

Edited October 2, 2021 by wimvb

[gorann]

You may also try reducing the distance if it is possible. I notised that the shape of the corner stars does not allways tell what direction to go with the distance.

[ceefna]

Thanks for the replies, I was going off this image that made me think the camera was too close? The flattener does'nt seem to flatten the image at all compared to without it.

[wimvb]

3 hours ago, ceefna said:

Thanks for the replies, I was going off this image that made me think the camera was too close? The flattener does'nt seem to flatten the image at all compared to without it.

I wonder if that diagram is for fully corrected stars. The stars in the corners of your image show coma, a sign that they are not completely corrected.

[symmetal]

I think you have a combination of tilt, spacing and coma. The centre stars are significantly out of focus compared to the edges indicating a spacing error, along with the corner stars pointing towards the centre. The best shape/focus stars are at top centre. The corner star shapes are showing a lot of coma which is the main cause of their bad shape.

With my 6200 I couldn't get good edge stars until I made the tilt test jig which worked wonders. With my FLT98 I now have good star shapes overall apart from the far right edge which shows some coma. Depending on how the camera is fitted to the filter wheel it may be impractical to use the tilt adjustment on the camera so a tilt adjuster before the filter wheel may be better and you can test the whole image train behind the flattener on the jig then as my post shows on the above thread. I spent a long time beforehand trying to adjust tilt looking at stars but with little success. 5 mins on the jig fixed it. Setting the FF distance to the recommended figure plus about 1.5mm for the glass in the way was all that was needed for correct spacing adjustment. I would have thought your scope design/price would not show as much coma as it does. With the tilt and spacing correct the coma should hopefully be at a minimum though it may have to sent off for an alignment check if you're still unhappy with it.

Looking at the CCDI plots it's implying the tilt isn't bad and it's mainly a spacing issue. CCDI seems to ignore coma and reports on the star main FWHM figure. The pink areas are bad with black and dark blue the best stars, although the FWHM figures are high even for the 'best' stars. For some reason it doesn't give a Max FWHM on the 3D plot (lower image).

Alan

 

Edited October 2, 2021 by symmetal

[ceefna]

Hi Alan thanks for your detailed reply. I have been trying to sort this before I head off to Kelling Heath next week for the star camp. Its quite difficult to gauge how far off the spacing is so last night I added another 3.8mm and it seems to have improved the curvature in the right direction but still not enough. The back focus from the reducer is now at 61.8mm???? I have no idea why this should be so far from 55mm? I plate solved the new image on astrometry.net and its now reporting the correct fov 3.09x2.06 deg and 1.16 arcsec per pixle I am having to put the hex focuser back on as I've run out of inward focus on the steeltrack. I will address the tilt when or if I can get the spacing correct, I have thought about putting the asi1600 back on as that worked nicely at 55mm.

So do I just keep adding spacers? Is there any formula to working this out or is it trial and error? I need to make new m54 extensions but how big that is the problem

american_frame7_Ha_frame7_-10C_600sec_1x1_2021-10-03_011431.fit

[gorann]

On 02/10/2021 at 12:44, wimvb said:

A full format camera has a sensor which is 36x24 mm, with a diagonal of about 44 mm. Only a very limited number of scopes/correctors can handle such a large field.

I have to agree with Wim, I think you are asking too much of the scope. My Esprit 150 for example can just about illuminate my ASI6200 using the dedicated 1x flattener, but with the dedicated 0.77x reducer/flattener it can only be used up to 34 mm image circle, https://www.teleskop-express.de/shop/product_info.php/info/p12689_Skywatcher-0-77x-Focal-Reducer-and-Flattener-for-Esprit-150-ED.html. I have hard to believe that the ES127 have a larger corrected field than the Esprits. In the end you may have to crop and settle for something like APS-C format. I seem to remember that you can tell the ASI6200 to run in APS-C format (so smaller files to handle).

[alacant]

On 02/10/2021 at 12:34, ceefna said:

ED127 fcd100 scope with dedicated 3" 0.7x reducer/

Hi. You need to begin with an extra 10mm of spacing to give around 65mm, shoulder of the adapter to sensor. That works for aps-c but we've no experience of full frame with the big ES. Maybe a starting point though? IIRC, Bresser told us 'anywhere between 65 and 70'.

HTH

Edited October 3, 2021 by alacant

[symmetal]

Your latest image Richard is a lot better as far as spacing goes. The centre is much more in focus. Here's the CCDI results. 

I didn't notice on your first post that you were at the recommended 55mm for the flattener. I assume you're using autofocus. If so it was offsetting the focus to try and get an overall 'best' focus and was including stars at the frame edge which are outside the corrected field. Increasing the FF spacing has seemingly made the autofocuser concentrate on the more central stars to your advantage. The improved range in FWHM figures reflect this. If your autofocus routine has an option to use the more central stars and ignore the edges then setting it at 55mm spacing may work OK. If not, increasing the spacing by another couple of mm may give a more optimum central area out to around APSC size.

As others more familiar with this scope have said, the 0.7x reducer/flattener is only intended for APSC size sensors at most. If you crop your latest image to APSC size it looks fairly good. As mentioned, a flattener with less reduction will likely give better corner results than a 0.7x one.

The bottom corners of the image are showing less coma than the top corners, indicating some tilt is present so gettting that right may improve things overall but as it stands you're stuck with coma in the corners.

Alan