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Any EQ8 News?

RobinW

By RobinW
in Discussions - Mounts

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RobinW

RobinW

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Looking good, like the cup / beer tin holders on the top........................................................ :laugh:

Ha, I had to zip tie a pair of 2.5Kg dumbell weights on top to achieve balance.

It could do with a 5Kg counterweight. I'll have to drill out an eq6 weight.

They say the rain will clear for the week-end :)

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NickK

NickK

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I note that the encoder resolution is 1.2 arc minutes (according to the spec) - so that's about 50 pixels at 1.66 arc sec/pix. I'd like to know if it's resolution is 1.2 and what the error is associate with that - more in terms of just knowing rather than pulling holes in the mount.

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RobinW

RobinW

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Blooming typical, as soon as I say that the distributors are tight lipped, one of the uk suppliers has just posted the UK price of £2595 for just the mount head and no tripod. if you want the tripod it's a crazy £795....are they kidding lol,

Looks like, Sept-Oct time at the Earliest for the UK

Just saw that posted on an ozzie forum.

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chrisshillito

chrisshillito

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I note that the encoder resolution is 1.2 arc minutes (according to the spec) - so that's about 50 pixels at 1.66 arc sec/pix. I'd like to know if it's resolution is 1.2 and what the error is associate with that - more in terms of just knowing rather than pulling holes in the mount.

As far as I can see the hardware encoder resolution is much less than the acutal stepper resolution and associted software endcoders that are used by the synscan/EQMOD when commanding the mount to move or reading its position - perhaps the hardware encoders are only there for rough position feedback and/or home position indexing.

Chris.

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chrisshillito

chrisshillito

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Except the period is rather short. So the error per minute is actually not much different from my HEQ5!

NigelM

Well to be fair the worm period of the EQ8 is only a little over 3.3 minutes! - it has a 435 toothed worm gear so the worm period is roughtly 84600/435 = 198.6 seconds.

Chris.

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Jessun

Jessun

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If slow PE is good - easier to guide out - then why the 435 tooth worm? Is that linked to necessary gear ratio for the drive system they put on this mount? 'Direct drive' as it's been called, possibly against predominating view of what that should mean.

/Jesper

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chrisshillito

chrisshillito

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By the look of it there is a 1:2 ratio between the stepper pulley and worm pully and then 435 teeth on the worm gear.

So compared to the EQ6Pro:

they have removed the intermediate gear.

they have used a belt drive

they have more than doubled the number of teeth on the worm gear.

All of these changes will act to reduce the overall amplitude of the PE. However, reducing the worm period means that you could experience the full peak to peak movement of whatever PE is present in exposures of 1.67 minutes or greater. For an EQ6 that would only happen in exposures of 4 minutes or greater, and for a HEQ6 5 minutes or greater. Indeed you could take a PE capure of 1.67 minutes duration on an HEQ5 and see less deflection if you were lucky enough to start sampling as the mount approached the extrmities of its PE deflection. So playings devil's advocate one could argue that for unguided imaging a HEQ5 could out macth or even exceed the EQ8's performance - but only for short exposures where each exposure capture was started at just the right point on the PE curve - (no easy matter to do!)

Overall I'm pretty sure the significantly lower PE of the EQ8 will more than compensates for the highter repitition rate.

Chris

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dph1nm

dph1nm

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So playings devil's advocate one could argue that for unguided imaging a HEQ5 could out macth or even exceed the EQ8's performance - but only for short exposures where each exposure capture was started at just the right point on the PE curve - (no easy matter to do!)

Yes, this was my point really. At the moment I can only do ~30-45 secs unguided on my HEQ5 with 1" pixels. Given the relative price, I would have hoped that the EQ8 would allow 1-2 mins unguided, but it seems unlikely, unless the permanent PEC can bring the PE down reliably. Obviously if you are doing longer exposures at this scale autoguiding is essential (unless you go for a 10 Micron I guess!).

NigelM

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chrisshillito

chrisshillito

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Yes, this was my point really. At the moment I can only do ~30-45 secs unguided on my HEQ5 with 1" pixels. Given the relative price, I would have hoped that the EQ8 would allow 1-2 mins unguided, but it seems unlikely, unless the permanent PEC can bring the PE down reliably. Obviously if you are doing longer exposures at this scale autoguiding is essential (unless you go for a 10 Micron I guess!).

NigelM

Robin has used EQMOD's pec (nots the mounts PPEC) to reduce the PE down to 2.18 arcsecs pk to peak over the 3.3 minute worm period - buy you could so the same on your HEQ5 and typically this pulls the PE down to around +-5 arc sec pk to pk over the 10 minute worm period.

Lets do some theorectical number crunching for your HEQ5: if we assume that the PE is a sinusoide then a 2 minute exposure will occupy 360/5 = 72 degrees of the worm cycle. If we centre this around the 90 degree phase point where PE is at a maxium and moving hte least then the exposuse would have to start at a phase angle of 54 degrees (90-72/2). Sin(54) = 0.8 so during the exposure you would only movement equivalent to 20% of the total amplitude. If you use PEC and achieve a corrected ampilutude of 5 acrsec over the worm period this means that you only see movement of 1 arc sec during your exposure. Of course start you exposure at the "wrong point" (phase= 144 degrees) and you will see a mount movemnt of 6 arc seconds!

Now do the same sort of calculations for Robin's EQ8, where the exposure acounts for 218 degrees of worm phase, then the 2 arcsecond Pk-Pk corrected performance will yield a minimum mount movement of 3.2 movement ans a max movemen tof 4 arcseconds.

So in a way your HEQ5 wins, but only if you can start the exposures at just the right time or are prepared to throw a lot away. The EQ8 could be regarded as a more consistant performer at these shorter exposures whilst the HEQ5 is more hard work!

When considering the feasibility of unguided imaging we should also consider is that a 1 arcsec of mount movement will only ever translate to a 1 arcsec movement in image at Dec=0. Go higher and the observed movement is reduced by a factor of 1/cos(dec). So image say M51 up at dec=47 and the image will only expeience will 'see' 30% of the nominal PE.

Chris.

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