EQ8 and Focal Length

[don4l]

I'm getting ready to build my observatory,  which will have two piers.

I was all set to get an EQ8, but I am now feeling a bit nervous - hence this post

My plan was to put my FSQ106 on a new EQ8 and use my EQ6 for both visual use and imaging with a TAL200.

However, I recently looked at some old images that I took with the TAL on the EQ6 and I realise that a 2000mm focal length is much more demanding than a 530mm f/l.  The TAL exposes even the smallest mount, or guiding problems.  My EQ6 is more than good enough at 500 but I would need to make some serious improvements to get any decent images at 2000mm.

 

I'm now thinking that I will have to keep the FSQ on the EQ6, and consider the new mount for the TAL. 

I know that the EQ8 will take a lot more weight, but is it any smoother?  Can anyone comment on how the two mounts compare?

 

Thanks

 

Don

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[alan potts]

Why not mount both on the EQ8, together they would be well within limits.

[michaelmorris]

10 minutes ago, alan potts said:

Why not mount both on the EQ8, together they would be well within limits.

What he said.

[don4l]

45 minutes ago, alan potts said:

Why not mount both on the EQ8, together they would be well within limits.

That thought has crossed my mind.  In fact, when I first decided on an observatory, I had planned to do exactly that. 

However, I would then be confined to imaging the same object with both OTA's.  The main object of interest is rarely at the centre of the field of view of the widefield shot.  (Think Elephant Trunk Nebula for example).  If I introduce any adjustment for either OTA, then there are likely to be autoguiding issues.

Another reason is that it is much quicker to  find your target at 500mm and start imaging.  Once that is running I would then get the longer f/l system going.

 

The layout of my garden means that if I don't have a second pier, then the space will just be wasted.

If I have a second pier, then I might as well make full use of it.  I might as well collect twice as much data if the opportunity is there.

Everything looked simple until I looked at my previous TAL images.

 

Don

 

 

 

 

 

 

[tooth_dr]

It could be a lot of faffing about getting both setup and guiding etc. I know it’s enough for me just getting everything talking, platesolved and guiding on one mount.  But it would be a nice option (if a little extravagant) to image different targets on the same night at different FL 

[don4l]

53 minutes ago, tooth_dr said:

It could be a lot of faffing about getting both setup and guiding etc. I know it’s enough for me just getting everything talking, platesolved and guiding on one mount.  But it would be a nice option (if a little extravagant) to image different targets on the same night at different FL 

I agree.

 

I'm very happy setting up at 500mm.  Finding the target is a doddle, and then autoguiding is also easy.  The only real issue is focusing. 

The second pier was going to be used for visual use.  Then I realised that I  have a spare camera, so I thought that I might as well give it a go.

 

If the EQ8 is an all round improvement on the EQ6, then I shall go ahead and get one.  I will need two mounts anyway.  However, I don't want to buy it if it won't last well into the future.  If I cannot afford the right mount now, then I will wait.  I've made the mistake of buying too soon in the past.  You just end up spending more.

 

 

 

 

[alan potts]

Don,

It was just a thought which even after posting full realised you would have already considered. I like your "Finding target is a doddle and auto guiding is easy ", I could write a book on the nights of hair tearing I had trying to auto guide. Most of the issues were of my making as I am nothing but a novice. I have a new guide camera now and have not used it because the mosquitoes are eating me alive this year, no matter what I spray on me. Weather is set to turn much colder by night which will slow them down so I will be able to get back out and have a go soon.

I fully intended to also buy an EQ8 but I just can't get on with using the observatory for imaging, I have tried 3 times now. I completely removed the LX200 12 inch and have now put it back. For some odd reason I am quite at home listerning to to the laptop talking the exposures away whilst I use the 18 inch Dob, good luck with what ever you do Don.

Alan

[tooth_dr]

15 hours ago, don4l said:

IFinding the target is a doddle, and then autoguiding is also easy.  The only real issue is focusing. 

Different strokes! I can get focused on a bright star with a mask in about 1 minute, no issues. Plate solving and setting up PHD can take a little bit longer.

Its the usual story though, all is easy until something goes wrong, then an hour just vanishes.

I will look forward to hearing how you get on ?

[don4l]

7 hours ago, tooth_dr said:

Different strokes! I can get focused on a bright star with a mask in about 1 minute, no issues. Plate solving and setting up PHD can take a little bit longer.

Its the usual story though, all is easy until something goes wrong, then an hour just vanishes.

I will look forward to hearing how you get on ?

Your "hour just vanishes" made me smile.  I know that feeling so well.?

 

I've spent the summer just practicing with the equipment and I have been surprised at how well this has gone.   I used to spend ages "Finding and framing", but this bit almost happens automatically now.   It will be the opposite with the TAL.  Finding things and guiding will be difficult, but focusing will be much easier.

This is the reason that I asked my question.  Initially I thought to put the TAK on the EQ8, and the TAL on the EQ6.

 However, I then realised that my problem won't be weight capacity with either mount.  My big issue will be smooth tracking for the TAL.  Any errors will be magnified 4 times by the long focal length. 

BTW, I use Elbrus for plate solving.  It is difficult to get going with it, but once mastered it is brilliant.

 

 

 

 

 

[alan potts]

I am hoping I have had every possible problem thrown at me one way or another. First off I was provided the wrong lead for a camera that I have thinking it was the ST4 guide lead, not FLO I may add, wrong, that took a month to work out, then I bought a new but faulty camera. I did get some reasonable pleasing  (for me anyway) shots of a good few targets but only using the mount at 60s-90s subs. It is a good way to learn your setup though, even though I am now totally bald.

Alan

[alan potts]

On 19/09/2018 at 23:25, don4l said:

My big issue will be smooth tracking for the TAL.  Any errors will be magnified 4 times by the long focal length. 

Is there such a thing as a focal reducer for the Tal, I have one for my Meade but still gives me F/L of 1920mm. My original idear was the mount the Meade on an EQ8 but have the freedom to mount other scopes like the 115mm APO and the 190mm M/N, I even bought the mounting rail for the Meade which I doubt I will ever use now.

Alan

[don4l]

10 hours ago, alan potts said:

Is there such a thing as a focal reducer for the Tal, I have one for my Meade but still gives me F/L of 1920mm. My original idear was the mount the Meade on an EQ8 but have the freedom to mount other scopes like the 115mm APO and the 190mm M/N, I even bought the mounting rail for the Meade which I doubt I will ever use now.

Alan

About 10 years ago a few people installed focal reducers/flatteners. IIRC, it was a 0.9 reducer.   I was just beginning at the time and I didn't really know what an "imaging circle" was.  I have no idea what flattener they used.  I also think that they bought a special low profile focuser from a chap in Australia, who made a small batch.

There isn't a lot of back focus on the Tal, and the exit hole at the rear is too small to take a 2" crayford.  I did have a Heath Robinson affair bolted on the back, but it wasn't strong enough to take the weight of a camera and filter wheel.  I'm now looking into a low profile focuser.  I won't seriously consider a flattener unless I decide that I will use the scope for imaging in the long term.

 

It is a terrible pity that the Tal wasn't designed with photography in mind.  It would have been a wonderful instrument.

Some Tal 200's were produced with a ratio of F7.5.   I suspect that they might have had an even smaller imaging circle than the F10 version.

 

 

[ollypenrice]

I'd look at this from an entirely different point of view. (If I had a quid for every time I'd said weight/payload was over-rated I'd be on a yacht in the Bahamas and my pal Tom O'Donoghue would be slaving away at the mount!)

First: you need to know your imaging scale in arcsecnods per pixel.

Second: you need to know the RMS of your mount, under guiding, in arcseconds. This needs to be no more than half of your imaging resolution in the same units - arcseconds.

Focal length is not really the issue. Image scale in arcseconds per pixel is the issue because it is this which determines the theoretical resolution of your system. The REAL resolution of your system will then be constrained by your guiding and by your seeing. (Don't count your chickens before they're hatched!)

Olly

[don4l]

22 hours ago, ollypenrice said:

I'd look at this from an entirely different point of view. (If I had a quid for every time I'd said weight/payload was over-rated I'd be on a yacht in the Bahamas and my pal Tom O'Donoghue would be slaving away at the mount!)

First: you need to know your imaging scale in arcsecnods per pixel.

Second: you need to know the RMS of your mount, under guiding, in arcseconds. This needs to be no more than half of your imaging resolution in the same units - arcseconds.

Focal length is not really the issue. Image scale in arcseconds per pixel is the issue because it is this which determines the theoretical resolution of your system. The REAL resolution of your system will then be constrained by your guiding and by your seeing. (Don't count your chickens before they're hatched!)

Olly

Arc Seconds per pixel will be 0.922.  I'm new to PHD, and I cannot find any log files on my system, but I think that I have seen an RMS figure of 0.65 and a varying peak -  usually 1.6.

 

I agree with your figures.  However, I cannot visualise the effect unless I see it in practice.  When I did the stacking tests on data that I acquired myself, the impact of undersampling really jumped off the screen.   It was even worse when I tried to put together an 8 frame mosaic.  After a couple of tweaks the stars became very bloated.

 

 

I've seen some of the mounts that have absolute encoders, but they are out of my budget.  The 10Micron mounts look lovely!

 

Two things made me look for a bigger capacity mount.  The first is that I don't want to buy something that I might need to replace in the future.  I've done that before - and it is an expensive mistake.  The other reason is that I was recently offered the chance to test an odk12.  I had to turn it down because I wouldn't risk putting it onto the EQ6.

I think that it is becoming clear that I should take a little time before I take a decision.  I would be in a much better position if I could get the camera onto the Tal and take some photos with PHD running.

 

 

[sloz1664]

On 21/09/2018 at 21:19, don4l said:

About 10 years ago a few people installed focal reducers/flatteners. IIRC, it was a 0.9 reducer.   I was just beginning at the time and I didn't really know what an "imaging circle" was.  I have no idea what flattener they used.  I also think that they bought a special low profile focuser from a chap in Australia, who made a small batch.

There isn't a lot of back focus on the Tal, and the exit hole at the rear is too small to take a 2" crayford.  I did have a Heath Robinson affair bolted on the back, but it wasn't strong enough to take the weight of a camera and filter wheel.  I'm now looking into a low profile focuser.  I won't seriously consider a flattener unless I decide that I will use the scope for imaging in the long term.

 

It is a terrible pity that the Tal wasn't designed with photography in mind.  It would have been a wonderful instrument.

Some Tal 200's were produced with a ratio of F7.5.   I suspect that they might have had an even smaller imaging circle than the F10 version.

Hi Don,

The "newer" Tal 200K were made f8.5 and were equipped with a Crayford focusser with anM48 thread on the end of the focus tube so you attach a camera direct. I know that Tal did produce a prototype batch of focal reducers specifically for the f8.5 scope. I have yet to find one

Steve

 

On 21/09/2018 at 21:19, don4l said:

 

 

[ollypenrice]

53 minutes ago, don4l said:

The 10Micron mounts look lovely!

Perhaps they do, but the Mesu 200 is both better and cheaper. (Everybody has their preferences, of course! ?) I run two Mesu 200s myself and host two, soon to be three, others in our robotic shed. I also host two, and at one time three, 10 Microns but one of the 10 Micron owners got fed up with sending it back and switched to Avalon. Personally I think that the absolute encoders thing is a nice idea and a nice reality when you can get it to work. Lots of 10M owners end up guiding, though. For me guiding is an elegant and 'direct' system using real information in real time. I like it.

Regarding your PHD trace, if you plug in your FL and pixel size PHD will happily give you your RMS in arcseconds. You need to be around 0.45 for your resolution. Certainly our Mesus will deliver this. They run at about 0.3" or better under guiding.

Olly

[RayD]

I believe @swag72 uses an EQ8, obviously with incredible results, so would be well placed to answer the original question.  However, I'm not certain what FL she images at on this mount.

[swag72]

Mmm...... I use an EQ8 at a measly 330mm focal length and a imaging resolution of 3.37" pp. This doesn't trouble the EQ8 at all and I am very pleased with it. For me it has been a cheaper entry into a second more premium mount as I already had a Mesu 200. I generally get an RMS of about 0.7 and so for me it works perfectly.

Most of the time I run the Mesu and the EQ8 together. The Mesu has a 1200mm 0.63" pp instrument atop of it. When looking at the graphs side by side, the Mesu is better usually guiding at an RMS of 0.3-0.4". I am not about to put the big refractor on the EQ8 to test if it would cope at such a resolution, but my gut feeling tells me that it wouldn't cope as well as the Mesu. Would I recommend the EQ8 for a 0.93" pp (I think that was the scale mentioned further up in the thread) ? Not mine I wouldn't, I don't think it would work...... but that's not to say that there aren't EQ8's out there that wouldn't cope with it. I've heard people happily guide 11" SCT's without hitch.... I just don't think MINE would do it.

Am I pleased with my EQ8? You bet!!! I love it!!! I am pleased that my second hand mount that cost considerably less than my Mesu is happily able to guide my dual rig accurately enough for my purposes. I don't tax the EQ8 at all and it is flawless. It works for me and for my situation.

[don4l]

16 minutes ago, swag72 said:

Mmm...... I use an EQ8 at a measly 330mm focal length and a imaging resolution of 3.37" pp. This doesn't trouble the EQ8 at all and I am very pleased with it. For me it has been a cheaper entry into a second more premium mount as I already had a Mesu 200. I generally get an RMS of about 0.7 and so for me it works perfectly.

Most of the time I run the Mesu and the EQ8 together. The Mesu has a 1200mm 0.63" pp instrument atop of it. When looking at the graphs side by side, the Mesu is better usually guiding at an RMS of 0.3-0.4". I am not about to put the big refractor on the EQ8 to test if it would cope at such a resolution, but my gut feeling tells me that it wouldn't cope as well as the Mesu. Would I recommend the EQ8 for a 0.93" pp (I think that was the scale mentioned further up in the thread) ? Not mine I wouldn't, I don't think it would work...... but that's not to say that there aren't EQ8's out there that wouldn't cope with it. I've heard people happily guide 11" SCT's without hitch.... I just don't think MINE would do it.

Am I pleased with my EQ8? You bet!!! I love it!!! I am pleased that my second hand mount that cost considerably less than my Mesu is happily able to guide my dual rig accurately enough for my purposes. I don't tax the EQ8 at all and it is flawless. It works for me and for my situation.

Thank you Sarah.   That is exactly the sort of info that I need. 

 

You've also hinted at variability between EQ8's.  I have been very happy with my EQ6, but other people in our local club have "issues".

 

I clearly need to have a good think about this.

 

[dph1nm]

I have successfully imaged unguided for 2mins at 0.6"/pix with my EQ8 (with PPEC turned on).

NIgelM

[don4l]

I've decided to put the decision on hold for a couple of months. 

There is no point getting an expensive mount if my other skills/equipment won't allow me to get the benefit. 

At this point I cannot gain anything with the Tak/6303 combination by improving tracking performance.  This might change when I get a new camera with 6 micron pixels.

It looks like the new mount will initially be used with the Tal, so, before I  buy anything I will get the Tal onto the EQ6 and see how I get on.  I know that I won't have issues with weight, but I will have to see how well it tracks.