DSLR Exposure Length - Tracking, Not Guiding

[Mikey0368]

Assuming my Polar Alignment and 2 Star Alignment are spot on, what length of exposure can I expect to take?

The mount is a SkyWatcher HEQ5 with Rowan Belt upgrade and the camera is a modified Canon 600D. I don't have a guide scope so I'll be relying on tracking, and skies around here are Bortle 6 (I think).

Thanks.

[Alien 13]

It all depends on the focal length of the lens/scope, a 50 mm lens for example could go for hours..

Alan

[Mikey0368]

It's a SkyWatcher Esprit 100ED PRO Triplet, f5.5.

[vlaiv]

Not very long I'm afraid.

Two major things that you will be facing is periodic error and polar alignment error.

Out of the two, I think that periodic error is going to give you more issues than polar alignment error. Let's run some numbers so you can get the idea of what sort of exposures will be attainable.

You are imaging at ~ 1.6"/px, so let's set upper limit to 2px, or about 3" as maximum drift per exposure that will produce acceptable stars (a bit elongated, but still round enough).

With regular polar alignment, you are looking at something order of 5 minutes of arc or PA error. In worst case scenario this translates to drift of about 1.3"/minute.

Stock HEQ5 can have as much as 30-40" P2P periodic error. Since you've done belt mod, this tends to drop quite a bit, but it is still in range of let's say 12-14" P2P. Period of HEQ5 mount is 638s. If we assume perfect sine wave then max RA drift rate due to PE will be 4.13"/minute - in reality PE is never sine wave and you can expect it to have a bit higher drift rate at some point, but let's go with 4"/minute because P2P might be even less like 7-8".

This shows that PE will be limiting factor, rather than PA error (unless you did a very poor job of polar alignment and error is something like 16 minutes of arc - which is quarter of a degree, so yes very large indeed).

With drift rate of about 4"/minute you are looking at about 45s exposures. This is worst estimate, so not all frames will be distorted (it depends where on period of mount you are at the moment, what is the DEC of the target, etc ...).

I think that you can use 1 minute exposures and expect to throw away something like 10% of frames if your target is close to equator. Higher up in DEC you can maybe do 90 seconds and still keep most of your subs.

[Mikey0368]

Fantastic reply, thanks very much that's really helpful.

[Whistlin Bob]

Mikey- I can appreciate that you might want to keep things nice and simple, and guiding is both another level of complexity and cost to your AP. For me, though, it made an enormous difference to my imaging. My kit (130pds f4.5, HEQ5, mod'ed 600d) is pretty close to yours and I was losing around 10-20% of frames (so, roughly in-line with @vlaiv's excellent model) and realistically could never go above 2 minutes. I went for a very basic guiding solution (2nd hand 9x50 guide scope, finder guider adaptor, 2nd hand ASI224 camera which doubles up as an excellent planetary camera, free PHD2 software). You'll need a laptop with you and it generally adds a few minutes to the setup after frame and focusing to put the guide camera on and focus, but it'll move you forwards quite a long way: not only was I no longer losing frames, but the ones I was keeping were better and sharper

Cost wise, it was about £220 all in, although I have some friends who have picked up generic guiding cameras from ebay for around £80 so it can be done more cheaply than I did it.

[Mikey0368]

I'm dragging this up again as I've been trying longer exposures using the Skywatcher HEQ5 with Rowan Belt modification, no guiding. It was really clear and calm a couple of nights ago so I tried 5 minute subs and didn't lose any through star trails or satellites.

The image is M81, 80% of 50 x 300s stacked in DSS with flats, darks and bias and processed in StarTools. The next time we get a night with no wind I'll try for even longer to see how it goes.

[newbie alert]

Guiding will probably be one of the best things you can do.. yes, setting up can be abit of a faff but once you understand it it's pretty simple..

[Mikey0368]

6 weeks later I finally got another night that was good enough for long exposures. This is 3hrs 20mins in total, all exposures were 5 minutes.

[TerryMcK]

Not bad at all. You can see some elongated stars which is understandable unguided. You will see a difference when you get a guide scope and guide cam.

[City9Town0]

As someone who has moved from unguided to guiding an unmodded HEQ5 (with cheap, second-hand guiding kit) I have to say that for me guiding beats unguided out of sight.... The extra setting-up takes ten minutes and with practice I'm now being critical of performance that only a few months ago I couldn't have dreamed of. I've gone from DSLR 60 seconds exposures to 300 seconds, and could easily go longer, with fewer rejects.

[Mikey0368]

Just adding to this again as I'm now getting 10 minute exposures unguided. This is 85% of 16 x 600s stacked in DSS and processed in StarTools.

 

[Marvin Jenkins]

Hang on a minute there Mikey0368. Did you really say ten minute exposures? What happened to the probable limit of touch more than 60 seconds? If the periodic error of the worm drive 638 seconds you should be seeing that effect in your images.

All I can say is you are doing a lot right from what I can see. Well aligned and calibrated equipment can get you results far in access of the norm.
Keep up the good work

Marv

[Mikey0368]

Yep, I'm getting 10 minutes out of it. I have a Polemaster on order so might manage more once it gets here!

 

[vlaiv]

9 minutes ago, Mikey0368 said:

Yep, I'm getting 10 minutes out of it. I have a Polemaster on order so might manage more once it gets here!

Out of interest - could you post a crop on some stars of single sub without scaling it down?

A lot will depend on resolution that you are using to present your image. For example, above image of M81/M82 is presented at about 6.44"/pixel. Any periodic error that is large as 6-7" will cause one pixel elongation - very hard to spot.

 

[Mikey0368]

Here ye go.......

[vlaiv]

4 minutes ago, Mikey0368 said:

Here ye go.......

Yes, excellent - you see, very small periodic error only - just couple of pixels total not more than that. If you look at direction of RA - which is almost parallel to galaxy - you will notice that stars are trailing in that direction.

This also means that your polar alignment is very good and there is almost no drift due to PA. And of course it shows the benefits of doing belt mod - it reduces P2P periodic error quite a bit - I'm guessing not more than 6-7" in this case.

 

[Mikey0368]

It'll be interesting to see how much more I can get when I'm using the Polemaster.

Cheers.

[Mikey0368]

I was experimenting with exposure lengths again last night and this is 900s (15 minutes). I only took 2 lights at that exposure but there's enough here to suggest that I might be able to try it again in a proper imaging session. This is the full image and a cropped version.

 

[Mikey0368]

Even although I've only used 1 light I tried processing the 900s image. I'm not fussed about the image itself, it's the shape of the stars I'm interested in.

So it's 1 light, 1 flat, 1 bias and no dark.