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Hyperstar numbers


Demonperformer
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I have, for some time, been considering getting a hyperstar for my 6SE, and think I might be getting myself confused over some of the numbers, so thought I would check my thinking with you hyperstar &/or imaging experts.

OK, I understand that this converts my f/10 scope into an f/2 scope (in round figures), which means that (ignoring the difference in the fov) I get the same result from a 1 second sub that I currently get from a 25 second sub [(10/2)^2].

My next thought was that, as my mount can track well enough for a 25 second sub, I can still take 25 second subs, giving me the equivalent light grasp of a 625 second sub at f/10. BUT, will the light pollution that currently infests my subs be equivalent to what I am currently getting on my f/10 25 second sub, or 25xworse [actually, long before it got 25x worse, I would have nothing on the image except LP, but you get the idea]? Well, having batted that question around for a couple of days, I have come to the conclusion that I have no idea:icon_scratch:.

That is therefore my first question.

But my thoughts have not stopped there, although I strongly suspect that reality is now about to become an invisible dot on the far horizon.

My scope can track well enough to take a 25 second sub at f/10 [1500mm f/l]. Now it seems logical that the amount any error shows up on the final result is proportional to the focal length of the scope [for a given receptor] and so, if I am now imaging at f/2 [300mm] then I should be able to track for 5x longer and get no worse 'tracking' errors on the finished shot than I currently get. So I would now be looking at 125 second subs, giving me the light grasp of a 3125 second sub on my f/10 setup. Seems to make sense to me if I were using an EQ mount. However, as my mount is the alt-az that came with the scope, I wonder if the main problem would be field rotation rather than proper 'tracking' issues?

Again, having looked at this issue from as many angles as I can think of, my final decision is 'maybe':confused:, so that now becomes my second question.

In reality, I will probably end up purchasing a hyperstar whatever the answers to these questions are, but it would be good to go into it with realistic expectations rather than what I suspect are currently vastly inflated expectations.

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Your figures seem correct to me!

Light pollution will be the same as you are gathering the same amount of light in a shorter amount of time.

Field rotation will be your biggest problem but if you stick to the short subs you should be OK. Might it not be better to get the hyperstar but get an F6.3 reducer and an EQmount?

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What camera would you be using DP?. I'm neither a Hyperstar owner or imaging expert but from what i've seen in some illustrations, some cameras look a bit obstructive on a 6". The 6SE should work well with a F3.3 reducer (if the fit is the same as my 8SE) provided that the camera is a small chip model and not a DSLR.

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Caveat: All the below is assuming you're using the same camera for f/10 and f/2. If you get a new camera which has a pixel size better matched to the shorter focal length, it will cancel a lot of things out.

OK, I understand that this converts my f/10 scope into an f/2 scope (in round figures), which means that (ignoring the difference in the fov) I get the same result from a 1 second sub that I currently get from a 25 second sub [(10/2)^2].

Yes and no. Each pixel now covers 25x larger area on the sky, so for sky background, and extended objects (like nebula and galaxies), then yes. For stars, no - because they're always the same size and telescope aperture is still the same (in detail, depends on your pixel sampling).

My next thought was that, as my mount can track well enough for a 25 second sub, I can still take 25 second subs, giving me the equivalent light grasp of a 625 second sub at f/10. BUT, will the light pollution that currently infests my subs be equivalent to what I am currently getting on my f/10 25 second sub, or 25xworse [actually, long before it got 25x worse, I would have nothing on the image except LP, but you get the idea]?

Well, the sky background per pixel will be 25x higher, but so will the object flux. In this case the S:N is going to be 5x higher with the Hyperstar than f/10. The main advantage is that you'll overcome the read-noise 25x quicker, so can get away with much shorter exposures than at f/10.

Again, that doesn't really apply to stars, so you'll be *less* sensitive for stars probably (because you have the same star flux vs more sky background, but again that depends on details of pixel sampling).

However, as my mount is the alt-az that came with the scope, I wonder if the main problem would be field rotation rather than proper 'tracking' issues?

Bit counterintuitive here, but the effect of field rotation only depends on the number of pixels on the camera -- not the size of those pixels on the sky. So the field rotation limit will be the same at f/10 and f/2.

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Thank you all for your replies.

The camera is an H9, which I have just purchased. This is one of the cameras recommended for the C6 hyperstar.

@TD: I get what you say about using a different camera, but the comparison of what I would get from the H9 with/without hyperstar is really helpful. If I read you correctly, the limitations I have on exposure length with the H9, due to LP and tracking/field rotation, will be the same with or without hyperstar.

It is interesting that Blinky & Peter have both suggested a focal reducer as an alternative. Curiously I actually have a 3.3 reducer that I bought second hand ages ago as a "let's see what I can do with this" purchase. Was totally unimpressed (visually) and consigned it to my 'probably never use this again' box. Completely forgot about it until I read your replies:o. Now I'm thinking this is at least a good 'next step', once I get used to the new camera, before I fork out on a new piece of kit that requires me to dismantle bits of my telescope to use. (The terrors that thought produces are a story for another time:eek:)

Thanks.

Edited by Demonperformer
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If I read you correctly, the limitations I have on exposure length with the H9, due to LP and tracking/field rotation, will be the same with or without hyperstar.

Yep, I think so.

If your limit is that you saturate your camera with light pollution, you'll just reach that limit 25x quicker. That's a good thing though, because it effectively makes your nights 25x longer once you start stacking lots of images together :)

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Yep, I think so.

If your limit is that you saturate your camera with light pollution, you'll just reach that limit 25x quicker. That's a good thing though, because it effectively makes your nights 25x longer once you start stacking lots of images together :)

Could be very good for narrowband...

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I didnt know you could get hyperstar for a 6SE, I would like to give it a go in my SE6. Where can you find the bits needed, I could only find hyperstar equipment for the larger celestron tubes.

*Edit

Found it http://starizona.com

Has anyone taken the secondary mirror off of the ota? was it difficult to get back in the right place?*

Edited by simon955
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I'm very surprised to see praise for the 3.3 reducer. It is , as said, for small chips and my experience was that it was lousy.

I would be a little careful over the Hyperstar. Sometimes the results are breathtaking (Greg and Noel, Greek Anthony) but sometimes they show signs of being off kilter. I suspect they are tricky to set up. What would go through my mind is that a small apo would cost less and be near as dammit certain to work. The narrowband I have seen with Hyperstars has been outstanding but SteveL knows his stuff. I wonder what the issue is. Interesting. Just in general, f2 is going to be critical on... everything!!

Olly

Edited by ollypenrice
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I`ll try to be as unbiased as I can here.

At f/2 the edges of the usable light cone come in at such an angle that interference based filters just cant cope. Almost all LP and NB filters are interference based, and their ability to work correctly are based on the light passing through it passing as near to perpendicular as possible. As the angle of incidence inctreases, the ability of the filter to work with tolerences reuces. Most filters state they work down to a certain "f" number, usuall f/3.5 or f/4.

For small chips, its not so much of a problem, but for larger chips, the light coming at the very edges/corners can experience some weird effects... it is possible that the wavelnegth of the light can actually get shifted by as much as 6nm, so for a 7nm Ha filter, the peak Ha response would be very near the limit of the filter, and in the part that doesnt work that well (i.e <10% passes through).

I always had issues getting my calibration frames to work correctly with my C8 Hyperstar system, it drove me crazy, and yet my images from William Optics 66 calibrated just fine. In hindsight, I am wondering if it was the UV/IR glass window on my QHY8 that was causing the odd flat field over/under calibration issues... i.e the flats and bias were fine, it was the UV/IR window thatw as causing some wavelength shift at the edges that my flats were unable to cope with. I was unable to test that theory as I had sold the kit when I thought of this possible scenario.

Its **VERY** senstive to collimation... silly crazy senstive! Not so bad in an observatory where you can tweak it to perfection and leave it, but I found it a chore to recollimate everytime I had to set up and strip down (secondary removal + hyperstar installation, plus reverse when stripping down).

And focus is even more sensitive... on the C8 hyperstar, the critical focus zone is 6nm. If you want to nail the focus accuartely everytime, you will probably require an electronic focuser. May work OK with a Bahtinov mask, but never tried one at the time.

The hyperstar system can grab a lot of light in a very short time, and with less accurate tracking required. The ability to grab many tens of subs in a session really helped with the signal to noise in the final stacked image. I learnt so much during the time I had my Hyperstar, I went from noob to half decent in one season.

A few things made me sell the Hyperstar system and move to a fast refractor for imaging.

#1 - I had a Nexstar 8 GPS on a wedge, which limited my upgrade path. I needed to move to a GEM of some sort.

#2 - The calibration file issue really did drive me mad.

#3 - I was getting to the point where I wanted to start using multiple night's data to combine into one final image, but getting the camera orientation (and more importantly, the cable locationb) the same for each session, so that the diffraction spikes matched up in both sets was a nightmare.

I hope that has helped a bit...

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Thank you, Helen, Olly & Steve, for your words of caution.

I must be honest and say that I have, from first seeing the advert for the hyperstar, had my doubts about such issues as collimation. The ad says it stays the same, but being as kak-handed as I am, I have had my doubts. The last thing I want to do is to completely screw up the operation of my 6SE for want of trying to get that bit more out of it.

My first priority has to be getting to grips with my new 'cool(-ed)' ccd camera. I'm sure this will throw up problems of its own until I get used to it. Then, as I already have a 3.3 reducer, I have nothing to lose (except, maybe, one of the few clear nights we seem to be getting recently) by giving that a try. At least I can use a filter wheel on that end of the scope, which would not be possible with a hyperstar.

Once I have gained confidence with what I already have, and the time comes to spend some more money, I can then decide if hyperstar is the way to go, or whether a short focal length apo is a better option. In which case I will probably be 'forced' to buy a solid EQ mount to go with it...

Either way: 'Wallet', you have been warned ...

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