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Pitch Black Skies

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Posts posted by Pitch Black Skies

    Guiding accuracy

    in Getting Started With Imaging

    Posted

    4 hours ago, ONIKKINEN said:

    Like above, i think less than half of imaging resolution and similar errors in both axis is good guiding. No obvious excursions in declination and a periodic error that is in check is key here. I use the predictive PEC algorithm in PHD2 and it seems to be doing a decent job of keeping my somewhat poorly figured RA worm gear in check.

    But looking at the guide graph can be misleading, especially with a newtonian and you may have good guiding without round stars. That is, i believe, due to mechanical issues with most off the shelf newtonians including: Focuser slop, tube deforming, mirror cell stability issues, secondary holder issues and more. All of these issues will lead to effectively taking your scope out of collimation randomly and during the session. Your guide scope (if using a guide scope) can still report decent guiding because its not aware of the other issues, unless the guide scope attachment method is also sub par (also common with newtonians). Experts here recommend OAGs regularly for newtonians for these reasons, and im beginning to see why since some issues still persist for me even though i dont think they are related to guiding exactly.

    I will probably make the jump from a guide-scope to an OAG some time in the near future because i see "good guiding" but still varying decrees of issues with the actual subs.

    Another really useful reply. I was considering upgrading my 130pds to maybe a 200pds but now realise that the difficulties you mentioned might even be more noticeable at the longer focal length and added weight.

    I have noticed that the RA corrections are much greater than the DEC ones. I might have to adjust aggressiveness. I use an ASIair and not sure if there is PEC training capability with it.

    I might invest in a longer dovetail to reduce flexure down the line. I ensured not to use the finderscope shoe for the guidescope, it didn't seem sturdy enough. Instead I attached a second dovetail on top of the rings and bolts the guidescope to that.

    It makes sense to use the OAG. I don't really understand them, they seem a bit more complicated.

    Guiding accuracy

    in Getting Started With Imaging

    Posted

    5 hours ago, The Lazy Astronomer said:

    Much as I enjoy Rory's videos, I think that was poorly explained. Basically he's saying for widefield imaging, you'd generally be shooting at a lower resolution and mount tracking errors would be less noticeable as each pixel captures a larger portion of the sky. When using longer focal lengths, generally you'd be at a higher resolution and tracking errors become more apparent.

    There's a bit more to it than that, but someone like @vlaiv will explain it much better 😁

    Generally, you'd want your total RMS to be less than or equal to 0.5x your imaging scale (in arc seconds), and ideally, a roughly equal error in RA and DEC - if one is significantly larger than the other, you may see star elongation even if the total RMS is good. 

    Your image scale is ~1.2"/pixel (although it seems the general consensus now is that the sampling rate for colour cameras should be doubled from what the calculation suggests, so your actual scale is ~2.4"/pixel). Honestly, I am unsure whether this means you should be aiming for an RMS of 0.6" or 1.2" (someone more knowledgeable than me will confirm, I'm sure).

    If the latter, that should be relatively easily achievable with your setup; the former will likey be more difficult.

    Thank you, lots of useful info there. You did a great job of simplifying it for me.

    Sometimes my RMS is above 1.2". Does that mean my image is potentially blurring, provided the seeing is below 1.2"?

    F5 Vs F4

    in Getting Started With Imaging

    Posted

    5 minutes ago, iantaylor2uk said:

    I had a 12" Orion Optics (DX300) f/4 scope and personally wouldn't go more than 60 sec exposures with an astro camera. I got some good photos of M51 with just 10 sec exposures, after stacking two or three hundred of them.

    Nice, have you still got the pictures?

    F5 Vs F4

    in Getting Started With Imaging

    Posted

    1 hour ago, Clarkey said:

    The 200 is around 1000mm focal length which is quite a bit above the 130. Also, in terms of pixel scale, above this you would usually be binning data anyway so there is not a huge amount to gain from the longer FL. However, a 200pds or quattro will be pushing the limits of a HEQ5.

    In reality F5 is pretty quick, especially at 650mm FL. F4 would theoretically slightly better but more difficult. There are newtonians produced by Sharpstar that go as fast as F2.8 but they are expensive. They are also not the easiest to work with and would require some modification. I now have an F4 that I will endeavour to work with in the autumn when the nebula's come back out to play. However, with hindsight I should have gone for an F5 or a better scope such as the quattro. But these would have gone an an AZ-EQ6.

    Nice, I was looking at the 8" Quattro too but probably too much to ask for the HEQ5. Knew I should've got the EQ6R 😂

    F5 Vs F4

    in Getting Started With Imaging

    Posted

    1 hour ago, Nik271 said:

    Sorry! my statement  was not very precise, I meant to achieve the same SNR with the new F4 scope you need  64% of total integration time of the old F5 scope. So 100 hours  total at F5 equals 64 hours at F4. This new camera looks very sensitive and with extremely low read noise so the length of the individual frames should not matter too much.

    With the F4 scope you will be saving 36% of the  total time compared to  the130PDS, to me this does not seem worth the extra hassle.

    I never had an F4 so not sure how much hassle it would be, but I'll take your word for it. I can only imagine what something like f2.8 is like then.

    F5 Vs F4

    in Getting Started With Imaging

    Posted · Edited by Pitch Black Skies

    2 hours ago, Nik271 said:

    To answer your question, the difference of the speed of F4 vs F5 scope (using the same camera) is the square of their F-ratios. So the F4 scope will be 16/25= 64% faster:

    all other things being equal you will need only 64% of the total integration time with the F4 scope. If you use a dedicated astro camera it's slightly better to keep the same length of individual frames, but decrease their number. If you use DSLR which usually generate thermal noise in long exposures it might be better to decrease the length of the frames instead (only applies if you use frames of length several minutes).

    Most people find the difference in speed between F4 and F5 is not worth the extra need for manufacturing precision, collimation and corrections of aberrations the F4 mirror will require.

     

    Cool, thanks Nik.

    I'm using a 533MC with Aplanatic CC.

    64% faster, so does that mean less than half the total integration time?

    Edit - 25/16 = 1.56 times faster. Is that correct?

    So a 300s sub with F5 would be 200s with F4.

     

    Do you recommend sticking with the F5 then?

     

    F5 Vs F4

    in Getting Started With Imaging

    Posted · Edited by Pitch Black Skies

    On 30/03/2022 at 19:23, Clarkey said:

    Personally I would stick with your 130PDS. I have the TS F4 and it took quite a bit of additional expense to get it to give decent images. Firstly, the Baader MPCC I had for my F5 scope was not up to the task of giving good stars to the edge of the sensor. I ending up paying for a Skywatcher F4 aplanatic CC. In addition, the focuser was not really up to the job so I replaced it with a Steeltrack. There were also a number of other changes required to make it an effective imaging scope.

    Yes it is quite fast, but for the amount of work needed to get a 'good' scope I would look at something different. The F4 also requires very careful collimation - MUCH tighter than the F5. The other problem I had was awful internal reflections. Although not directly linked to the scope - it was the filters - to use the mono camera would require me to buy a whole new set of filters. I will try it with a DSLR as the reflections may not be an issue.

    I don't know what mount you have, but you could look at a 150 or 200mm F5.

    I bought a StellaMira F6 90mm ED triplet instead..... but I am nearly £1000 worse off!

    Okay so that's yourself and Alacant warning against it. Will probably give it a miss so. There's a true saying, you get what you pay for.

    Maybe a Quattro 8s? I have the HEQ5, not sure if it could handle it?

    I have the Aplanatic CC. The 150 and 200 will only bring me a bit closer to the target though won't they. What I'm really after is shorter imaging times.

    F5 Vs F4

    in Getting Started With Imaging

    Posted

    I'm considering upgrading my F5 130PDS to a 6" TS F4.

    I know F4 will reduce the exposure time, but by how much? How is it calculated?

    Also, does it reduce my total integration time too, or do I still need the same integration time but now I have more subs to stack so better SNR?

     

    Guiding accuracy

    in Getting Started With Imaging

    Posted · Edited by Pitch Black Skies

    Hi guys,

    What RMS should I be hoping to achieve and what is unacceptable?

    How are my guiding requirements calculated?

    I'm probably not explaining this right.

    I am referring to part 3:15-3:50 in this video.

    I don't understand how that's calculated.

    Any help is greatly appreciated.

    I use 533MC, 130PDS & HEQ5.

     

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