Finderscope or ST80 for guiding?

[gnomus]

I have been 'getting into' astrophotography over the last few months, acquiring 'bits and pieces', as funds allowed.  

At the moment, I am using a ZWO ASI120 as my guide camera.  This is attached via an adapter to my 9 x 50 finderscope.  I started out attempting some DSO imaging with my C8 Edge.  However, since getting an ED80 all of my DSO efforts have been with this scope (and this is likely to continue for the foreseeable).  

The finderscope guider has been working.  I occasionally get a message that "star mass has been lost" (or words to that effect), but PHD is normally able to find the star again.  Picture-wise, I am not getting any obvious trailing (at least to these eyes).  On the other hand, because I am new to this I don't know what "good" looks like.  I am not really sure what I am seeing on the PHD graphs.  The lines do move around a bit.  Assuming I'm reading them correctly, most of the time they seem to stay within the +/- 2" range.  (Again, I don't know how good this is.)

The only other issue that I encounter is when PHD tells me that my mount is unable to move as PHD would like (again this is not the exact message - I haven't written it down).  I think I have worked out that this happens when the object moves across the meridian and the scope needs to be "flipped".

It seems to me that the majority of imagers are not using the finderscope method.  The ST-80 seems a popular and inexpensive option.  It also looks like it would be quite easy to bolt it securely on top of the ED-80 rings.

What would be the advantages (if any) of moving from my finderscope to an ST-80 for guiding?  Is this this the next logical step, or is finderscope guiding good enough?

Thanks in anticipation. 

[Leveye]

Are you using PHD2? Could you please post a picture of the PHD graph your getting and settings applied? It tells quite a story and will help evaluate your quality of guiding your getting. You can check the "flip calibration settings" box for a meridian flip.That will make that easier on everything.

[Tinker1947]

If you stack in DSS when the final images pops up hover the mouse over diffrent stars and check there pixel count in the top left corner, a ST-80 will show more star and there for give you better ones to guide on and while your at it, mount it on Guide Rings, this will allow you to centre a star in the PHD screen with the Bulls Eye selected and ajust it so the same star is in the centre of your camera sensor, this does save quite a bit of alignment time...Good Luck with what ever you choose....

[gnomus]

Are you using PHD2? Could you please post a picture of the PHD graph your getting and settings applied? It tells quite a story and will help evaluate your quality of guiding your getting. You can check the "flip calibration settings" box for a meridian flip.That will make that easier on everything.

Thank you for responding.  I am using PHD2.  I have Enable Guide Log and Enable Debug Log both set.  I have looked at these files - they are quite lengthy.  I don't know how (or if) I can load the data into the PHD graph, so I cannot post an image.  If you tell me what to do I will be more than happy to do so.

Looking at these logs did, however, allow me to find the error message that I was getting (which I thought related to meridian flip).  It was: "Your Max RA Duration setting is preventing PHD from making adequate corrections to keep the guide star locked. Increasing the Max RA Duration setting will allow PHD to make the needed corrections."

I'm not sure what this means exactly.  I'm not sure how I increase the Max RA duration...  

[halli]

Hi there

I use a ST80 with a 130pds and bolt the ST80 dovetail bar to the scope rings - this is quite a rigid mounting which helps with eliminating diff flexure.  I think the advantage of the ST80 is the larger aperture which enables guide stars to be found more easily although it is heavier than a finder guider,    Although I cannot adjust the ST80 direction I have never found this a disadvantage as I can always find a suitable guide star.

Wrt PHD2 I find if the graph remains in the +/- 2 range then this appears to be perfectly adequate to maintain round stars.   The max duration setting is 2000 in PHD2 (I think) so the software is simply informing you that your setting is below this although it may not be an issue because your setup doesn/t need to use the max duration setting - I guess there's no harm in using these settings though and it may give the software additional firepower,   The max duration setting is set  under the brain icon if you look at the mount TAB for PHD2.

[steppenwolf]

Your Max RA Duration setting is preventing PHD from making adequate corrections to keep the guide star locked. Increasing the Max RA Duration setting will allow PHD to make the needed corrections."

You can change (increase) this setting by clicking on the 'brain' icon to bring up a dialogue box. Currently (probably because of the short focal length of the finder-guider) the star guide star is not moving far enough during the calibration run to adequately evaluate the response to mount adjustments so you should increase this maximum value.

I am pleasantly surprised at how well a finder-guider works and with your ED80, the rather short focal length of your current guider should be fine so I wouldn't be in too much of a hurry to upgrade to an ST80 until you have explored getting PHD to work better for you. That said, I have also used a ST80 for guiding and was also excellent.

The great advantage of a finder guider is that its ultra short focal length gives a wide field of view from which to chose suitable guide stars and it is very lightweight which is never a bad thing as light weight and a firm mounting to your imaging telescope can help to avoid differential flexure. Differential flexure describes the situation where the guide camera/telescope and the imaging camera/telescope have a small amount of movement between them resulting in apparently good guiding but trailed stars in the image.

[gnomus]

I am worried slightly about flexure.  The ED-80, as you know, sits in a dovetail slot - held there by one thumbscrew.  Also, in terms of what settings I should use in PHD, I am hampered somewhat by not knowing the focal length of the finderscope - nor can I find this out from anywhere.  

I have a CGEM mount by the way.

My current settings in PHD2 are as follows:

Does this seem about right?

[Davey-T]

9x50 = 450f/length, the three numbers are related so 450 divided by 50 = 9 etc .

Dave

Had a question mark and I think  in original but seems to have lost a line in posting

[Astroboffin]

9x50 = 450f/length, the three numbers are related so 450 divided by 50 = 9 etc .

Dave

I am confused by this, are you saying that a 9x50 finderscope has a fl of 450mm ??

I find this hard to believe

Please explain, as I am a bit slow on the uptake

AB

[Astroboffin]

I have an 8x50 finder that has a fl of 165mm, so there is my confusion

[Astroboffin]

So my 50mm finder is approx an f3.2, as as far as I know you divide the fl by the diameter to get the f stop, so in your theory an 9x50 finder is an f9 !!!

I am really confused now

[gnomus]

So my 50mm finder is approx an f3.2, as as far as I know you divide the fl by the diameter to get the f stop, so in your theory an 9x50 finder is an f9 !!!

I am really confused now

I'm pretty sure that the formula given above is incorrect.

[Davey-T]

I'm pretty sure that the formula given above is incorrect.

Yes it did have a question mark and I think at the end but it seems to have disappeared in posting.

Sorry for the confusion the 8 in finderscopes like bino's refers to the magnification you can just measure the focal length 

Dave

[Astroboffin]

I'm pretty sure that the formula given above is incorrect.

No it's not

Well not for SCT scopes anyway, maybe others are different, but I have an 200mm SCT or 8" and it had a fl of 2000mm so you divide the 200 into the 2000 and get f10 which is what it is......

I also have a refractor an ST80 and that has a fl of 400mm and it is an f5 and 80mm diameter so work it out

400 divided by 80 gives 5 or f5, so it is the same for refractors too.

I am sure that is correct, at least for SCT scope, google it and you will see.

[Astroboffin]

Sorry were you referring to my formula or Daves ??

I might have misunderstood, sorry if I did

AB

[gnomus]

No it's not

Well not for SCT scopes anyway, maybe others are different, but I have an 200mm SCT or 8" and it had a fl of 2000mm so you divide the 200 into the 2000 and get f10 which is what it is......

I also have a refractor an ST80 and that has a fl of 400mm and it is an f5 and 80mm diameter so work it out

400 divided by 80 gives 5 or f5, so it is the same for refractors too.

I am sure that is correct, at least for SCT scope, google it and you will see.

Yes that is quite correct. However, the '9' in '9x50' is not the focal ratio, but the magnification. If you think about your 200mm scope you can produce a range of magnifications with different eyepieces. You cannot then multiply that magnification by the 200mm diameter to get the focal length. The focal length would change with each magnification.

[gnomus]

Sorry were you referring to my formula or Daves ??

I might have misunderstood, sorry if I did

AB

Sorry ... Dave's formula is incorrect. I've ended up posting in reverse now!!!,

[Astroboffin]

Yes that is quite correct. However, the '9' in '9x50' is not the focal ratio, but the magnification. If you think about your 200mm scope you can produce a range of magnifications with different eyepieces. You cannot then multiply that magnification by the 200mm diameter to get the focal length. The focal length would change with each magnification.

I really can't see how a 9x50 finder has a fl of 450mm I am sorry but can't see it, like I say mine is an 8x50 and is approx 165mm fl which gives approx f3.2, by you calculation my finder is 400mm focal length, the same as my ST80 ....I don't think so

AB

[Astroboffin]

Surely each and every scope has a native fl, to give f ratios?.

Like I say maybe someone with more knowledge than me can jump in and explain.....

AB

[gnomus]

I really can't see how a 9x50 finder has a fl of 450mm I am sorry but can't see it, like I say mine is an 8x50 and is approx 165mm fl which gives approx f3.2, by you calculation my finder is 400mm focal length, the same as my ST80 ....I don't think so

AB

We are in complete agreement. I think we got a bit 'crossed' in the timings of our posts. The joy of the Internet! Focal ratio is calculated as you say. You cannot work out focal length by knowing only the diameter of the objective and the magnification.

[steppenwolf]

The formula is incorrect because the 9 refers to magnification, not focal ratio. My guess would be that the focal length is of the order of 200mm (ish)

My current settings in PHD2 are as follows: .......

I'd be tempted to increase the calibration steps to 1500 to help ensure a good calibration.

[Astroboffin]

We are in complete agreement. I think we got a bit 'crossed' in the timings of our posts. The joy of the Internet! Focal ratio is calculated as you say. You cannot work out focal length by knowing only the diameter of the objective and the magnification.

Lol,

Yes and my internet is a particular pain in the proverbial ....

Glad we agree, especially as maths and physics are another language to me...but I was sure I had read that on the interest correctly, I don't claim to know much but I thought i had that right...

AB

[Astroboffin]

You see what you started now Dave.....lol

:)

AB

[Astroboffin]

The formula is incorrect because the 9 refers to magnification, not focal ratio. My guess would be that the focal length is of the order of 200mm (ish)

I'd be tempted to increase the calibration steps to 1500 to help ensure a good calibration.

Yes I would agree my 8x50 is around 165 to 175 fl so the 9x50 would be around 200mm fl, so divide that and you get approx f4

AB

[gnomus]

The formula is incorrect because the 9 refers to magnification, not focal ratio. My guess would be that the focal length is of the order of 200mm (ish)

I'd be tempted to increase the calibration steps to 1500 to help ensure a good calibration.

I will try your suggestion if the sky ever clears. Thank you for your assistance. I bought your book a few months back so much of this your fault!!!!