[steven.coyne]

Ok here's the problem i own a 90mm celestron (C90 Mak) spotting scope which was originally bought just as a spotting scope and i'm attempting a little digiscoping with it.

I'm having difficulty getting high enough shutter speeds in apeture priority mode (aperture set at 5.1) to prevent the image from being too blurred (the tripod is not the problem its a manfrotto 055 with 701-rc2 head)

using a coolpix 4500 with a solid adapter setup.

anyways i was just wondering what the blame is here, it is a pretty miserable day today, very cloudy and so lighting is not the best so is it that or is it the scope.

the scope specs...

90mm lens at the front
focal length of 1200mm
f/13.3

so yeah just wondering what the specs are on other scopes especially focal length and f/ value so i can figure out if its just a bad day for digiscoping or is it a bad scope and its never going to be a good day.

Hope someone can help

-=Steve=-

[steven.coyne]

Well been out today again at around lunch time and its been fairly bright and I’ve been taking some readings of what shutter speeds I can achieve with different settings pointed at sun lit areas, the brightest I could find and so here are my results...

Firstly working with no camera zoom and so some vignetting at f/2.6 (aperture priority mode)

ISO 100 gave me speeds of 1/30
ISO 200 ......................... 1/60
ISO 400 ......................... 1/125
ISO 800 ......................... 1/250

Secondly I zoomed in enough to eliminate the vignetting and ended up at f/3.7

The highest shutter speed I was able to achieve at ISO 200 (what Andy recommended I switch to) was 1/60 and very rarely 1/125 (think I was pointing directly into the sky at that point)

SO the question still remains, is the scope simply not up to the job due to its large focal length and therefore its small f/ value (f/13.3)

I’ve calculated the f/ values (focal length/objective diameter) of some other scopes both astro types similar to my celestron c90 mak and some regular spotting scopes.

I’ve not been able to find out the full technical specs of the more popular digiscoping scopes such as the zeiss, swarovski, kowa etc but I’m assuming as they are all refractor scopes then it’ll be roughly equivalent to the scope body length.

Anyways most of the dedicated spotting scopes I’ve came across all come out at about at most f/6 and all astro type scopes and reflectors are all above f/9 with my own scope being a tiny f/13.3

Ok so basically this whole thing is just for someone to validate my thoughts that my scope will just never going to be able to achieve fast enough shutter speeds to take any decent wildlife photos through it.

[normjackson]

Hi Steven. If you haven’t done so recommend a check out of Jay Turberville's site:
http://www.jayandwanda.com/digicat/digicat.html
Nice photos there too.
You don’t say what eyepiece you use; for CATs tend to need a long fl eyepiece to get magnification down to the 20s or so which is more appropriate to successful digiscoping I understand. Adding a focal reducer to your scope might be another possibility…

[Neil]

Steve,
The problem may not be blurring due to the Shutter Speed. I was shooting at 1/8 sec today and getting pretty sharp results (it was raining here). Aperture of 5.1 on the CP4500 sounds a little high. Your camera is on a good tripod (most usual reason for camera shake) and so the shutter speed shouldn't be an issue. Have you tried using the Self Timer? Can you post a sample photo? Neil

[scampo]

Originally Posted by steven.coyne

Well been out today again at around lunch time and its been fairly bright and I’ve been taking some readings of what shutter speeds I can achieve with different settings pointed at sun lit areas, the brightest I could find and so here are my results...

Firstly working with no camera zoom and so some vignetting at f/2.6 (aperture priority mode)

ISO 100 gave me speeds of 1/30
ISO 200 ......................... 1/60
ISO 400 ......................... 1/125
ISO 800 ......................... 1/250

... ...

With such a high magnification, unless the tripod is rock steady (best to not use the centre column at all), you would need as fast a speed as you can achieve. Tripods should, of course, hold the camera/scope combination steady, but wind hitting the tripod or the scope shoulder strap as well as any movement such as walking nearby, can create sufficient movement to cause blur.

Camera shake is, however, usually easy to spot, as the blurring is most often all in one direction.

[dahyon]

A Celestron 90Mak has a 32mm eyepiece giving 37.5x magnification.
This information along with CP4500 specifications were plugged into the "Digiscoping calculations" Excel spreadsheet by George Raiche (www.digibird.com). The CP4500 at f#5.1 has FL32mm which translates to digiscoping focal length 35mm equivalent of 5850mm. When digiscoping, the effective camera aperture is f#13.3. The same calculations have been made using a Zeiss85 with a 20x-60x zoom eyepiece and repeated with a Maxview 40mm eyepiece. For simplicity I have collated the data into a chart and have indicated were the Celestron appears when compared to the Zeiss85. Essentially, the Celestron is comparable to the Zeiss with a 35x eyepiece. The Table below shows behaviour of the Zeiss over a five stop light range with a rating of ISO100. The Celestron fits between the 30x & 40x columns. On a dull day the Celestron whould have a exposure of approximately 1/12 sec at ISO100 with f#5.1(FL32mm), on a bright day this equates to about 1/185sec at the same settings. You would gain an extra 1 stop at ISO200 and another with the CP4500 at f#3.7.



Quality of light...... Maxview...Zeiss....Zeiss....Zeiss....Zeiss...Zeiss
...........................12.5x......20x......30x ......40x.......50x.....60x

Bright over water...1/1000.....1/500...1/250...1/125....1/60....1/30

..........................1/500.......1/250...1/125...1/60.....1/30....1/15

..........................1/250.......1/125...1/60.....1/30.....1/15....1/8

..........................1/125........1/60....1/30.....1/15.....1/8

Overcast..............1/60.........1/30.....1/15.....1/8

Overcast in shade..1/30.........1/15.....1/8

My own view is that it should be possible to obtain good quality images within this range of exposures. If every image is blurred then you have a technical problem.

Hope this is of some use
Cheers Dave Nuttall

[Jay Turberville]

Originally Posted by steven.coyne

anyways i was just wondering what the blame is here, it is a pretty miserable day today, very cloudy and so lighting is not the best so is it that or is it the scope.

the scope specs...

90mm lens at the front
focal length of 1200mm
f/13.3

Hopefully you read my page that someone else already referred you to.

http://www.jayandwanda.com/digicat/digicat.html

As a general rule of thumb, for better images keep the overall magnification low. Three keys to doing that with a catadioptric are:

1) Shoot without an erecting finder. This will reduce the effective focal length of the scope some.

2) Us a relatively long focal length eyepiece. I'd suggest a an eyepiece between 35mm and 50mm for your scope. A Maxview 40 would be a good choice. Good quality optics matter. If TeleVue has a 40mm Plossl, I'd consider that also.

3) Don't zoom all the way out. Stay around 2x or 3x on a CP4500.

I just ran some numbers through my digiscoping calculator and if you do the above, your setup should give you range from around 2100mm equivalent at around f/5 to around 4000mm at f/10. The lower ranges will provide better quality.

http://www.jayandwanda.com/digiscope...cope_calc.html

[Jay Turberville]

Originally Posted by dahyon

On a dull day the Celestron whould have a exposure of approximately 1/12 sec at ISO100 with f#5.1(FL32mm), on a bright day this equates to about 1/185sec at the same settings. You would gain an extra 1 stop at ISO200 and another with the CP4500 at f#3.7.

At these magnifications over any significant distance, exposure time is the least of your worries. At an effective f/13 you are running into serious problems with diffraction - even if the combined optics are perfect - which they are definitely not.

Zoom the camera back to the 16mm or 24mm focal length range and enjoy the increased amount of light, shorter exposures and sharper images. Once you start going past 3000mm equivalent - especially over longer distances, you are really diminishing your likelihood of getting sharp images.