Star-field motion in captured light frames - is this supposed to happen?

[GuySt]

Still learning the basics of astrophotography with a GEM mount. What I have:

HEQ5, PoleMaster, ZWO guide scope (<-- NOT in use as yet, next night out for this part of the gear), Williams Optics RedCat 51, Nikon D7500.

Monday night was all about setting up the mount, leveling and balancing it, and polar aligning it with the PoleMaster. I think all of that was correctly done. So, though I'd not planned on taking any images, decided to since the camera was on the mount. I did NOT do the steps to get PHD2 set up. That's for next time. 

But, I do have some a question about the light frames nonetheless. I used APT to run a lights and darks set and Sequator to align/stack them.

It's clear when scrolling the light frames that the star field is moving. The stars stay sharp and do not trail (30s exposures at 800 ISO) but the star field is shifting, left to right.

Q1: Is that supposed to happen?

I thought the point of an EQ mount is that it moves with the earth's rotation so, well, things don't move in the scope's FOV.

Q2: Will using the ZWO guide scope prevent such star-field motion?

Also, the 'black' background is VERY noisy. Horribly so. This images is processed in GIMP/G'MIC to reduce that noise and scaled down 50% (to make it much less that 115Mb in size for posting). But the Sequator output TIFF image is noissy.

Q3: Is this 'normal' for astrophotographs? Recall it's made with only lights and darks (I'd not planned to take images so I didn't have the t-shirt for Flats/Dark-Flats and I forgot about biases entirely).

Q4: Will these additional frame types reduce this noise?

Q5: Am I foolish to think that a non-noisy background obtainable with the Nikon D7500?

Regards.

[vlaiv]

8 minutes ago, GuySt said:

Q1: Is that supposed to happen?

It is not supposed to happen but it can happen and often it does happen.

This usually has two causes:

- polar alignment is not spot on so there is mismatch in rotation of the earth and rotation of the mount which produces slight drift over time - this drift is in DEC direction

- Mount is not perfect mechanical device and it can track slower or faster than the Earth rotates (few seconds over the course of a day or similar). This again produces slight drift - this time in RA direction.

Your drift can be combination of the two, and you can check what is dominant component by looking in which direction is majority of the drift.

12 minutes ago, GuySt said:

Q2: Will using the ZWO guide scope prevent such star-field motion?

That is the purpose of the guide scope. In 99% of cases it corrects for above said drifts, however, be careful in 1% of the cases (numbers 99% and 1% are arbitrary - emphasis was on "rarely") there is something called differential flex. Telescopes sometimes move because they are not 100% tightly secured on the mount. You can't feel this motion by hand and sometimes it is even not related to scope itself - it could be mirror inside telescope that is moving for example.

This motion is due to Gravity - depends where scope is pointing and where center of mass is.

Two telescopes can move at different rate because of their size and way they are attached (one scope could be fine while other has some drift). If guide scope moves differently than main scope - you will again have this sort of drift - in this case this is due to differential flexure.

17 minutes ago, GuySt said:

Q3: Is this 'normal' for astrophotographs? Recall it's made with only lights and darks (I'd not planned to take images so I didn't have the t-shirt for Flats/Dark-Flats and I forgot about biases entirely).

That is normal - each frame contains some signal and some noise, and noise comes from different sources - read noise, thermal noise, light pollution noise and even target noise (shot noise).

18 minutes ago, GuySt said:

Q4: Will these additional frame types reduce this noise?

That is the name of the game - improve SNR or signal to noise ratio. Stacking multiple images improves this ratio. Best to think of it as signal being the same and noise reducing. SNR improves as square root of number of stacked frames.

19 minutes ago, GuySt said:

Q5: Am I foolish to think that a non-noisy background obtainable with the Nikon D7500?

Yes and no.

There will always be some noise in the image it can't be completely eliminated and there is no such thing as non-noisy background.

That was yes.

Any image can be made to look noise free. That was no, and I'll expand. It is about signal to noise ratio. There is some signal in the image and there is some noise in the image. If you make signal stronger than noise - that is good and that is what we are after - higher signal to noise ratio. This enables you to stretch the data in the image in such way as to see the signal while not showing the noise.

Most problems in processing comes from the fact that one "knows" there is signal in the image - after all you imaged M31 - it is there, you can see it, and one tries to bring out that signal. If noise is not low enough compared to that signal - in bringing out the signal you will bring out the noise as well.

It is great skill to stretch the data only to the limit it will let you - show the signal while not bringing out the noise.

[GuySt]

vlaiv - I cannot thank you enough for such clear and direct answers to my questions. I may, perhaps, might, be understanding all this. I started down this path after taking some images with that Nikon D7500 and a regular camera tripod of comet Neowise. I was very pleased that I have images I took, but less pleased because (as a photographer of many years, albeit amateur) I knew the images could be better. And as a new retiree, I needed a hobby.

Looking for how to take better images lead to a Polarie sky tracker, then I dove off the cliff toward full-scale AP with the NEQ5, Williams Optics RedCat 51, etc. (the ZWO guidescope and QHY PoleMaster are the latest two additions). I'm still learning ... but it may be I'm learning the right things. I'm a bit hampered because I live in a condominium and there is nowhere close to me I can set up the mount etc. I cannot just take things to a backyard. So I have to load it all in my truck and drive somewhere. But, that's the life I have and I'll make do.

Again, MUCH THANKS. Just what I needed.

Edited October 1, 2020 by GuySt

[GuySt]

P.S. Here's one of those Comet NeoWise images.