Adventures in imaging using Powermate(s) - is it worth it?

[ram]

TLDR: Do I gain something when using a large sensor and magnifying a small page on a small (say 4" to 2.75") scope using Powermates 4x and 2x? Or is it just the same as blowing up an image? Alternately, should I buy a smaller chip camera with smaller pixels? (See paragraphs at end below for my rough understanding of image resolution both from the scope and from the sensor/photo).

Longer version:

Hi all, since I got back to imaging this month, I've been spending a lot of time on a target I left off, sh2-132, The Lion Nebula and I've collected more than ~60 hours of data so I think it should be good enough. That was all done using my classic technique which I've been doing for two years largely unchanged. Plus I've set up my new Mac so I will redo all the processing since taken fall (upside down after meridian flip) and spring (right side up) appear to be slightly rotated relative to each other even though I was super careful at lining them up - any idea why and will this pose a problem in PixInsight?).

Anyways, I decided to try out something new (this would be my first change since I started imaging with proper CMOS cameras for AP) and since there were several targets, particularly galaxies, that are normally too small for the sensors I use and the scopes I'm now capable of easily moving in and out each day (the C925 doesn't have this problem of size but I can't lift it as easily as I could before and I don't have a permanent set up yet though that's in the vague planning stages---i.e., my wife has agreed to it). So it's mostly the FC100DF and the SV70T.

It turns out that M81 (Bode's galaxy) is really nice and fills up my QHY247C APS-C sensor perfectly with a 4x Powermate. It all looked nice on telescopius but then when I connected it all up I couldn't get any stars to focus. I checked it visually and it looked fine but I centered on Deneb and it was a bit off centre. My guess is the 4x was playing havoc on focussing and I was going too fast so I went to the 2x Powermate. This time I could get focus and I could get the galaxy but my stars weren't round and fuzzy (in fact they were shifting shape all the time - every second it was oval, circle, oblong, etc.). I had this problem due to the distance between sensor and reducer before and I used the same distance which the Powermate should be able to handle I thought but it wasn't to be. Also the fuzziness of the stars bothered me though it could be due to the fact that this FC100DF doesn't have a microfocusser yet.

I went to try it on my FC100DF since I felt I'd have more light to bring in with a 100mm/4" and presumably higher resolution instead of the 70mm/2.75" SV70T but I spent an entire clear night unscrewing and screwing back stuff (which was a PITA and is what led to my cleaning optics post) so I'm going back to imaging narrowbnad with the FC100DF without the reducer which will let me get slightly bigger (plus the mono QHY183M sensor I have is smaller) and putting the Powermate setup on the SV70T and trying that out first.

I'm unsure as to what could be my issues. Is it the Powermate? At 2x Powermate, I'm increasing focal length to 1480mm and 4x it is 2960mm on the FC100DF. I understand Powermate technically isn't a Barlow but for the purposes of making things bigger, I'm assuming it functions the same - am I wrong?

I understand the effective resolution of the image being seen from the scope doesn't change but there are more pixels now covering the important part of the image directly as it appears rather than being blown up. If my normal resolution is say 1 arcsecond/pixel (using CCD calculator) or higher, then using the Powermate 2x should be slightly better than blowing it up 2x (especially if I'm having good seeing)?

Is this is the best way for me to use scopes+sensor combinations of resolution 1"/px (i.e., 3.8uM pixel size and 740mm focal length) or should I get a sensor with smaller pixel sizes?

I went through the diagonal (HighPoint) - could this be causing problems? It looked fine visually through an eyepiece of coure.

It could be my seeing, which was fair to good through the night according to cleardarksky.

Basically I'm wondering what happened to the sharp stars I was getting from my refractor. Some of it could be due to the lack of a microfocusser but even when I thought I had things focussed I still wasn't a lot of love from the setup. I will try again with the SV70T on a night which isn't so close to the new moon and keep these beautiful nights for proper imaging instead of learning new things.

Thanks all!

--Ram

[ram]

There's this PowerMate chart plotting "focus position from the top surface" (is that roughly the distance of the attachment between sensor and powermate)? I was trying to get 55m as usual) and it shows how magnification changes as a result of this distance. So this implies the distance between sensor and top surface doesn't really matter for Powermate 2x and largely not for 4x?

[ram]

Never mind, in case anyone else is interested, I got my questions answered with this (this is what happens when you forget all the useful URLs after having been away all winter). This and other calculators together answered my question (telescope resolution):

https://astronomy.tools/calculators/ccd_suitability

Basically I was oversampling with a 4x on top of 740mm focal length with okay seeing but it has the right resolution. I think things will work better with the 420mm as long as the aperture of the scope can handle it which might be dicey with excellent seeing. So I have to spend time to figure out what to do (change sensors or bin with my FC100DF which seems like it would work).

--Ram

[Graeme1858]

Hello Ram.

I don't use a Powermate. But since trying to use my GPCAM2 290C with my 9.25 SCT I have discovered the concept of over sampling! My 0.5 reducer tends to cause the loss of focus you describe.

This is another useful calculator:

https://www.bintel.com.au/tools/astronomy-calculator/

M81 fills the frame nicely using my Canon 600D which is described in the Bintel calculator as slightly oversampled. I had a go at that one the other night for the June Astrophotography challenge!

Capturing an image with the best fit optically will always be better than magnifying digitally and loosing resolution but a 4 x barlow/powermate will really stretch the limits of the optics.

Perhaps a range of cameras to cover the range of target sizes is the answer but who can afford that!

Glad you got your laptop sorted.

Regards

Graeme

[ram]

Thanks Graeme for the calculator Actually for me it's just about not having a permanent setup and being too weak as I get older to lift up the heavier scopes like the C925 day after day which is what I do. The 4" is the easiest to move in and out with camera attached so all I have to do is mount it, PA and then I'm off. Even the 5" is a bit cumbersome but the 925 is too difficult for me now. Plus the longer FL means more futzing around and less forgiving. So basically even a 2x barlow on the 4" is the same FL as the C925 with the 0.7 reducer and it appears to give a similar FOV. Except the 4" is easier to handle the C925... which is why I asked. But as those calculators show, the resolution of the scope (aperture) and the resolution of the setup and seeing all play off each other into whether it's manageable or not and whether oversampling causes issues.

--Ram

[Graeme1858]

Yes, I know what you mean. My C9.25 sits on a CGX mount which I have to move 5m into position each time I use it. I keep the mount and the tripod together to save doing a PA every time but I would struggle with the counterweights as well! I feel good to go for a few years yet but 50 press ups twice a day was a long time ago, not sure I could do 5 now!

Matching the telescope to the target/additional optics/camera/guide setup seems to be like a lot of things in AP, there's a trade off.

Regards

Graeme