Advice on new telescope/accessories

[astro744]

Originally Posted by kkara4

Thanks for the reply. Is a T2 adapter the notation typically used for a wide T adapter as opposed to a normal one? Just trying to understand the jargon!

EdgeHD 9.25 comes with a 2" star diagonal which slips into the 2" visual back and accepts a 2" eyepiece so all good there then!

No, T2 does not refer to wide version. See history of 'T' mount at http://en.wikipedia.org/wiki/T-mount

I cannot remember who sells the wide body T2 adapter. If the barrel of the adapter is 2" then the only other restriction is inside the adapter and the T-ring. You may be able to get a special SCT wide T2 adapter from Baader that is larger than 2". I'm not familiar with what is available here.

[kkara4 (Krishan)]

thanks, I will take a look at that article and have a read! I got an email back from Televue from John Rhodes, managed to dig up his email address from google. He has been most helpful, but will fire back a few questions to confirm a few things, then should be able to complete the shopping list

[Camelopardalis (Dunk)]

OK, so I was curious about this so here's what I've found so far...

I measured my T-adapter for EOS...the clear aperture is 42mm...which coincidentally appears a lot in Celestron's white paper for Edge HD

Then I did some basic calculations... the 5D Mk III has a chip size of 36x24mm, which gives a diagonal of 43.26mm. So from that, I'd guess you might see a little vignetting in the corners

It might be a little worse than that because any scope->T or powermate->T has to have some additional incursion into that diameter, even if it's thinly constructed. I'm no optical path expert but from my basic understanding you might see some vignetting when you try to utilise the absolute full frame, but it'd be interesting to see by how much.

[astro744]

Originally Posted by Camelopardalis

OK, so I was curious about this so here's what I've found so far...

I measured my T-adapter for EOS...the clear aperture is 42mm...which coincidentally appears a lot in Celestron's white paper for Edge HD

Then I did some basic calculations... the 5D Mk III has a chip size of 36x24mm, which gives a diagonal of 43.26mm. So from that, I'd guess you might see a little vignetting in the corners

It might be a little worse than that because any scope->T or powermate->T has to have some additional incursion into that diameter, even if it's thinly constructed. I'm no optical path expert but from my basic understanding you might see some vignetting when you try to utilise the absolute full frame, but it'd be interesting to see by how much.

I found this one with a 47.5mm clear aperture and integral Canon EOS T-ring.

http://www.bluefireball.com/product-p/p-11.htm

Note this will be good for prime focus alone but you will need a separate Canon EOS T-ring to screw onto the Powermate T-ring adaptor when using Powermates.

Available at http://agenaastro.com/parts-accessor...anufacturer=87 but currently out of stock of EOS model.

[mithrandir (Andrew)]

Originally Posted by Camelopardalis

OK, so I was curious about this so here's what I've found so far...

I measured my T-adapter for EOS...the clear aperture is 42mm...which coincidentally appears a lot in Celestron's white paper for Edge HD

Then I did some basic calculations... the 5D Mk III has a chip size of 36x24mm, which gives a diagonal of 43.26mm. So from that, I'd guess you might see a little vignetting in the corners

It might be a little worse than that because any scope->T or powermate->T has to have some additional incursion into that diameter, even if it's thinly constructed. I'm no optical path expert but from my basic understanding you might see some vignetting when you try to utilise the absolute full frame, but it'd be interesting to see by how much.

http://www.telescopeadapters.com/ have a one-piece T-ring and 2" tube for Canon. 2" UltraWide Prime Focus Telescope Adapter They don't vignette like the ring and tube ones do. I have them for my Sony cameras and find the no-undercut version works better on my scopes.

[astro744]

Originally Posted by mithrandir

http://www.telescopeadapters.com/ have a one-piece T-ring and 2" tube for Canon. 2" UltraWide Prime Focus Telescope Adapter They don't vignette like the ring and tube ones do. I have them for my Sony cameras and find the no-undercut version works better on my scopes.

Thanks. That's the website I had in the back of my mind but couldn't find it and then found the Blue Fireball site which has a similar one piece T-adapter.

[kkara4 (Krishan)]

Thanks a lot for the replies guys. When I get some time tonight ill have a look at all the options/links you have all provided!

[kkara4 (Krishan)]

ok so something that is still confusing me is what exactly I will need for the guiding part. As I understand it, guiding with high precision is required for high magnification imaging? Can this be done relatively inexpensively? If so what components do I need?

The G11 I have bought has been tested to have periodic error less than +/- 2 arcsec, but as I understand it guiding must be used so as to minimise the periodic error and drive it to near-zero.

Also with polar scopes, does that alignment also have to be done extremely precisely? I believe the G11 comes with a polar scope but not sure at this stage how good it is or if it will suit my intended imaging targets

[Amaranthus (Barry)]

Polar scopes and software algorithms are great for EQ visual, but only get you so-so close. In the end, for AP, you must drift align. The most efficient way to do this, IMHO, is the DARV method or variants thereof.
http://www.cloudynights.com/item.php?item_id=2838

[astro744]

Originally Posted by kkara4

ok so something that is still confusing me is what exactly I will need for the guiding part. As I understand it, guiding with high precision is required for high magnification imaging? Can this be done relatively inexpensively? If so what components do I need?

The G11 I have bought has been tested to have periodic error less than +/- 2 arcsec, but as I understand it guiding must be used so as to minimise the periodic error and drive it to near-zero.

Also with polar scopes, does that alignment also have to be done extremely precisely? I believe the G11 comes with a polar scope but not sure at this stage how good it is or if it will suit my intended imaging targets

The more accurately you align the less you will have to correct when guiding. Use the drift alignment method for polar aligning. You will get field rotation if you are doing long exposures and are correcting often due to poor polar alignment.

[kkara4 (Krishan)]

Originally Posted by Amaranthus

Polar scopes and software algorithms are great for EQ visual, but only get you so-so close. In the end, for AP, you must drift align. The most efficient way to do this, IMHO, is the DARV method or variants thereof.
http://www.cloudynights.com/item.php?item_id=2838

Thanks for the reply Barry.

the DARV method seems to be ideally suited to what I am doing since I am mostly going to be using a DSLR!

But just to clarify, is this DARV method to achieve correct polar alignment as opposed to contrinuous guiding? I have some more questions about the DARV method linked though, am confused by a few steps and what happens between some of the steps!

[kkara4 (Krishan)]

Originally Posted by astro744

The more accurately you align the less you will have to correct when guiding. Use the drift alignment method for polar aligning. You will get field rotation if you are doing long exposures and are correcting often due to poor polar alignment.

Thanks for the reply!

This makes sense, but isnt guiding still required because the periodic error must be near zero for high magnification photography?

Like even with 1 arcsecond period the image would still drift around enough to cause the image to be blurry if I took say a 2 minute video of Saturn at 200x power wouldnt it?

[Amaranthus (Barry)]

Accurate Polar alignment + AG are definitely required for long-exposure AP.

D.A.R.V. type corrections + PEC are sufficient for 1-2 min unguided, maybe more for your G11 which is a quality mount.

[Amaranthus (Barry)]

Here is a short summary, modified from another helpful post on IIS that I struggled to re-locate!

1. Point telescope east roughly 30 deg above horizon.

2. Start the webcam/CCD/DSLR with a 45 sec exposure

3. Wait 5 secs, at guiding speed, move mount using RA controls for ~20 secs, then reverse direction for the remaining 20 secs (no pause)

4, The image will show a star trail in a "V" pattern. The gap is how far out your Polar Alignment is.

5. Adjust your Alt manual adjustment a little. Then repeat the above exp/move again.

6. If you have adjusted the mount in the right direction the gap in the "V" should have gotten smaller, if bigger you've adjusted the mount in the wrong direction. Make appropriate adjustments again.

7. Continue until you get a straight line.

8. Then slew to about DEC zero (celestial equator) and near the Meridian, and repeat but this time adjust the azimuth and repeat until line is straight.

Go longer exposure for a better PA. The reason for the 5 sec pause means you can tell which line is which as on will have a blob at the start.

[astro744]

Precise polar aligning eliminates north or south drift (DEC axis).

Guiding corrections east or west may be needed to counteract slow/fast movements in RA drive. PEC should typically correct this depending on how good it is.

Note if you align so that there is no drift in DEC for say 10 minutes then you can photograph for 10 minutes without an issue. The PEC function of the drive should correct in RA axis. You don't have to align so accurately that you don't get drift for say 30 minutes if you don't intend on taking single exposures 30 minutes long.

[kkara4 (Krishan)]

Originally Posted by Amaranthus

Accurate Polar alignment + AG are definitely required for long-exposure AP.

D.A.R.V. type corrections + PEC are sufficient for 1-2 min unguided, maybe more for your G11 which is a quality mount.

Thanks Barry, what does PEC stand for?

[kkara4 (Krishan)]

thanks guys starting to make sense, and Barry that makes much more sense with the DARV procedure now! I still fail to see though how one would slew to true 0 declination without being in the middle of the ocean so you have a proper 0 horizon? and then do you find a star just as it comes above the horizon?

[Amaranthus (Barry)]

DEC zero, not ALT zero At my horizon, this is about 55 degrees altitude at the Meridan. Look up "Celestial Equator" for the theory.

PEC = Periodic Error Correction - corrects systematic anomalies in your worm drive, after a few cycles of training.

[kkara4 (Krishan)]

Originally Posted by Amaranthus

DEC zero, not ALT zero At my horizon, this is about 55 degrees altitude at the Meridan. Look up "Celestial Equator" for the theory.

PEC = Periodic Error Correction - corrects systematic anomalies in your worm drive, after a few cycles of training.

Ahhh thanks Barry, my bad! All makes perfect sense now. So DARV gets you pretty much perfect polar alignment, then only the periodic error which must be dealt with, either with PEC or true autoguiding.

That is awesome, means I wont have to worry about a polar scope at all, even if one ends up coming with the G11.

Now if I did want to do long exposure (lets say 30 minutes +), what would I need for the guiding side of things in terms of equipment?

[Amaranthus (Barry)]

You can do PEC + AG if you use pulse guiding. Then AG is only correcting for the stochasticity, leaving PEC to deal with the determinism

If you are working at a long FL you'll ideally want an off-axis guide port and sensitive CCD guidecam. If shorter FL (widefield), then a guidescope and CMOS guidecam are fine (this is what I currently use, even for my C8 @f/6.3)